Aksum University
College of Agriculture
Department of Plant Sciences
Course: Agro-meteorology (HORT211)
Credit hours: 2 (2+0)
Teaching material
Compiled by: Abadi Berhane (Ph.D. in Agronomy)
February, 2024
Shire-Endaselassie, Tigray
Chapter 1: Introductory Concepts in Meteorology and Climatology
1. Components of climate and weather
1. Wind: is defined as the movement of air.
(A hemispherical cup anemometer of the type invented in 1846 by Thomas Romney Robinson)
An anemometer is a device for measuring wind speed, and is a common weather
station instrument. The term is derived from the Greek word ‘anemos’, meaning wind, and is
used to describe any airspeed measurement instrument used in meteorology or aerodynamics.
The first known description of an anemometer was given by Leon Battista Alberti around 1450.
Anemometers can be divided into two classes: those that measure the wind's speed, and those
that measure the wind's pressure; but as there is a close connection between the pressure and the
speed, an anemometer designed for one will give information about both.
A. Wind speed: wind speed is measure by an instrument known as anemometer.
B. Wind direction is reported by the direction from which it originates. It is measured
using an instrument called ‘wind vane’. For example, a northerly wind blows from
the north to the south wind vane
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2. Atmospheric humidity
Usually water in the form of invisible vapor is intermixed with the air throughout the lower part of the
atmosphere. However, there is a limit to how much water vapor the air can hold.
Humidity is thus a measure of the amount of water vapor in the air. When the air contains the maximum
amount of vapor possible for a particular temperature, the air is said to be saturated
The air’s capacity to hold vapor varies with temperature
Vapor content of the air increases dramatically as it gets warmer roughly doubling for every 10°C (18° F).
temperature increase,
The air around you has weight, and it presses against everything it touches. That pressure is called
atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls
it to Earth.
Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a
glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric
pressure by how high the mercury rises.
An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a
temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760
millimeters (29.92 inches) of mercury.
Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about
half that of Honolulu, Hawai'i. Honolulu is a city at sea level. Denali, also known as Mount McKinley, is
the highest peak in North America.
As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high
altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even
die.
Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used
to the altitude because quickly moving from higher pressure to lower pressure can cause decompression
sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to
the surface too quickly.
Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying. Atmospheric
pressure is an indicator of weather. When a low pressure system moves into an area, it usually leads
to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.
Example: If air at 10°C contains 9.4 g/m3 of water vapor when saturated, then the air can hold 18.8g/m3
of water vapor at temperature 20°C,
If the temperature of saturated air is raised its relative humidity (RH) will be reduced and hence can hold
more water vapor.
Thus, warm air can hold more vapor than cold air.
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3. Relative humidity (RH):
It is the moisture content of the air as a percentage of the moisture content when the air is saturated at that
temperature.
RH can be changed when there is changed: either in air temperature, or in the amount of water vapor of
the air.
The highest RH value is after midnight when the temperature is lowest, and lowest at noon, when the
temperature is highest
At saturation i.e. when RH is about 100%, the rate of evaporation would be very slow
RH gives an indication of the state of the air to a potential for condensation and cloud formation.
it is measured using hair hygrometer
4. Solar Radiation:
1. Bimetallic Actinograph, Robitzsch-Fuess Type 58dc
This bimetallic actinograph is a further development of model 58c, which has proven itself for years
in meteorology and climatology. Like the former, the new design records the varying intensity of
global radiation; i. e., the radiation received from sun and sky, on a horizontal surface. Operation of
the instrument is as easy as that of a simple thermograph so that it can even be operated by inexperienced
persons.
2. Belfort Pyranograph - Model 5-3850A
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Measures Direct Solar And Diffused Sky Radiation
Convenient And Simple Operation
Rugged Construction
Compact And Portable
Spring Or Battery Powered Chart Drive
Daily, Weekly, Monthly Recording Periods Available
No External Power Required
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The Model 5-3850A Pyranograph accurately measures and records variations in solar radiation without
external power. In addition, its compact size, portability, and sturdy construction, make it ideal for field studies
and remote installations.
The measuring element of the instrument consists of two bimetallic plated strips, one of which is blackened and
the other given a highly reflective plating. The blackened strip is exposed to both the ambient temperature and
the radiant energy of the sun while the polished strip is exposed to the ambient temperature but shielded from
the sun's radiant energy. These two strips are mounted in tandem through a long thermal path convector fin
coupling which prevents heat flow between the two strips. Mounting is such that only differential temperature
produces motion of the pen arm, while ambient temperature changes result in no motion. Thus, only the motion
of the blackened strip, which is caused by radiant energy, is recorded, while the motion resulting from ambient
temperature is compensated and has no effect. The elements are housed under a dome made of Borosilicate
glass. A table of transmission coefficients for this type of glass is included on the reverse side.
Time constant for this instrument is five minutes. Sensitivity is 0.1 gram calorie per square centimeter per
minute for 3/32" of chart width. The total range of the instrument is 3 gram calories per square centimeter
(Langleys) per minute. This provides adequate margin over the maximum solar radiation encountered and
brings the most usually recorded values at mid-scale, where the highest instrument accuracy exists. The speed
of response, accuracy, and precision of recording are entirely adequate for all practical use where extensive
area measurements are required and where convenience, simplicity, and sturdy construction are important
factors. No outside source of power is required.
Solar Energy Budget:
The greenhouse gases and the aerosols are central to the climate change and its variability. As energy from the
sun passes through the atmosphere, a number of things take place.
1. About 26% is reflected or scattered.
2. About 19% absorbed by clouds, gases and particles in the atmosphere
3. About 55% of the solar energy passing through the Earth’s atmosphere, 4% is reflected from the
surface back to space.
4. On average, about 51% of the Sun’s radiation reaches the surface of the Earth.
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3. Sunshine Autograph
According to Campell Stokes, for independent recording of
sunshine duration in northern and southern equatorial
zones from 0 to 40° and 25 to 60° respectively. The
instrument consists of corrosion-resistant gunmetal brass,
hammer dimple enamel finish, with high-grade polished
glass ball. Sun radiation onto the glass ball effects focus on
the opposite side. Due to high temperature at focus center,
the sun position is burnt unto heat-sensitive paper with
time graduation. A circular bubble is located on the base
plate allowing exact adjustment of the instrument.
Measures
Instrument approx. 205 x 185 x 245 mm
Glass ball 96 mm
diameter
Weight
5 kg
Accessories 1 set of recording strips (280 pieces for
summer and winter and 100 pieces for
spring/fall)
Order no.
Sunshine Autograph 0-40°
7.004
Spare recording strips (1 set has 380 pieces
7.004/2
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7.004/3
7.004/4
for 0-40°)
Sunshine Autograph 25-60°
Spare recording strips (1 set has 380 pieces
for 25-60°)
5. Precipitation
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The word precipitation in chemistry refers to material falling out of suspension.
Precipitation from a meteorological stand point is water in some form, falling out of the air, and
settling on the surface of the earth.
Precipitation is the most important phase in the hydrological cycle.
The first step in precipitation is condensation.
The process of condensation involves a change from water vapor to liquid, while the process of
precipitation involves the falling of water as: rain, drizzle, snow, hail, mist, dew, frosts
Clouds are formed by the condensation of water into extremely small droplets of liquid or ice.
Clouds are classified according to the height at which they form and their structure.
 High clouds above 7,000 m and are primarily composed of ice crystals.
 Mid-level clouds between 2,000 and 7,000 m
 Low clouds below 2,000 m. height
Rainfall is an important form of precipitation in many parts of the world especially in tropical and
subtropical regions.
It is measured in millimeters. A rainfall of 10 mm would cover 1m2 of perfectly level land to a depth
of 10 mm if there is no seepage, percolation or evaporation.
The yearly rainfall both in total amount as well as distribution has significant influence on:
 distribution of natural vegetation
 possible kind of agricultural production
 cropping seasons of an agro-ecological region
 even way of life of a given community,
Global precipitation distribution
 The amount of rainfall in semiarid tropics varies from less than 300-1500 mm.
 Equatorial regions are typically wet (contain more water vapor) while the subtropics and the Polar
Regions are relatively dry.
 The global distribution of precipitation is closely related to the:
o general circulation of the atmosphere and,
o distribution of mountain ranges and plateaus
 Precipitation is very high in the equatorial region where the air rises higher and create annual precipitation
of over 2000 mm
 It is least abundant where it sinks for instance, the subtropical deserts and the Polar Regions creating
below 250 mm.
 The mid latitude regions have intermediate values, being in general about 1000 mm per year.
a. The temporal precipitation variation
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o The temporal variation in global precipitation is directly linked to the seasonal changes in the heating of
the Earth
o This affects the movement of global pressure systems and air masses.
o Areas of high precipitation are usually moving north and south during the year following the migration
of global pressure and wind systems
o The migration of the ITCZ across the Equator keeps this region very moist
o The north - to - south shifting of ITCZ creates:
 summer dry climates pole-ward of the subtropical high but
 summer wet - to the equator ward side.
 However, into the mid-latitudes, frontal uplifts keep most of the seasons humid and moist.
 As one moves pole ward, the precipitation becomes more variable and seasonal as the drying effect of the
subtropical is experienced.
 In the cold Arctic or Antarctic very little moisture is available in the air for the formation of precipitation.
Generally:
 Precipitation decreases as one moves from the equator toward the subtropical regions and the poles.
 The largest annual precipitation totals straddle the equator while the driest regions on Earth lie near the
Tropic of Cancer.
 In addition, precipitation becomes more seasonal as one moves away from the equator primarily due to the
shifting locations of global wind and pressure systems.
 The ITCZ is the region where trade winds originating in the semi-permanent subtropical highs converge
causing air to rise.
 The combination of convergence and convection lifts, the air becomes cool and ultimately condenses its
moisture into clouds and precipitation.
Uplift Mechanisms of water vapor
1. Convective uplift
 When the air is warmed by the surface, it will expand and become less dense relative to air that
surrounds it.
 Then the air is uplifted from the surface causing the air to lose the heat which it initially gained from
the surface.
 The elevation above the surface where condensation begins is called the condensation level.
2. Convergent uplift
 It occurs when air enters a center of low pressure and converges into the center of a cyclone that forces
it to rise off the surface.
 As the air rises it expands, cools, and water vapor condenses.
 Convergent and convective uplift are the two most important uplift mechanisms for condensation in
the tropics.
3. Orographic uplift
 The forced ascent (rise) of air when it collides with a windward side of mountain in which it is
uplifted and cooled.
 Windward slopes of mountains usually tend to be the rainy sides while the leeward side is dry
4. Frontal uplift
o It occurs when greatly contrasting air masses meet along a weather front.
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o Usually when warm air collides with cool air, the warm air is forced to rise up over the cool air
Types of rainfall
Rains can be classified in to three on the bases of the driving forces:
B.
Convectional rains
– They are the result of vertical air motion which transfers heat from lower to higher levels of the
atmosphere.
– As water vapor having lower density than air layers near the ground, moves upwards to higher
altitudes they condense to form conventional rain.
– Conventional rains are usually received : during afternoons, in uniform pattern, over larger areas
C. Monsoon Rains/relief/ orographic
– Monsoon winds are generated by the seasonal intense surface heating during summer especially in
subtropics creating larger areas of relatively low pressure.
– Monsoon winds flow during two specific periods:
– Southwest monsoon during May-August
– Northeast Monsoon during December-February
– Monsoon winds usually cause wet (windward side), and warm and dry (leeward side) of mountains
– The amount of Monsoon rains depend on:
o The extent of water body exposed to the incoming wind
o The relative altitude of the place
o Geographical location of the country
D. Rains caused by atmospheric depressions
– Atmospheric depressions due to various reasons in the high sky cause the formation of vacuums.
– Then air gradient is formed and clouds are then attracted towards these vacuums
– The water vapor in the cloud becomes condensed and forms water due to very low temperature at high
altitudes.
– This brings heavy rain usually during unexpected times even in the dry seasons
E. Frontal rains: It occurs when greatly contrasting air masses meet along a weather front.
Usually when warm air collides with cool air, the warm air is forced to rise up over the cool air, and finally
form frontal rains.
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Chapter 2
2. Basic concepts of the atmosphere
2.1 Origin of the Earth's Atmosphere
Early Earth would have been very different and inhospitable compared to the Earth today.
Hot: at first the Earth was thought to be in a hot state of gaseous entity;
Why?
Primordial heat, collisions and compression during accretion, decay of short-lived radioactive elements.
Consequences: Constant volcanism, surface temperature too high for liquid water or life as we know it,
molten surface or thin, unstable basaltic crust are formed.
Atmosphere: early atmosphere probably completely different in composition (H2, He)
Cooling:
o Primordial heat dissipated to space;
o Condensation of water (rain), accumulation of surface water;
o Accumulation of new atmosphere due to volcanic out gassing;
o Conditions appropriate for evolution of life were formed;
Evolution of the Atmosphere
Atmosphere:
 Envelope of gases that surrounds the Earth. Used by life as a reservoir of chemical compounds used
in living systems. Atmosphere has no outer boundary, just fades into space. Dense part of
atmosphere (97% of mass) lies within 30 km of the Earth (so about same thickness as
continental crust).
 It envelopes the Earth up to 9,600 km above the Earth’s surface.
 Density of the atmosphere decreases with altitude; high variability at lower altitudes causing higher
temperature ranges;
 Is composed of gaseous molecules; dust particles, smoke, vehicle emissions, pollens, water vapor,
etc.
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Chemical Composition Today - Nitrogen (N2)- 78%, Oxygen (O2)- 21%, Carbon Dioxide (CO2) - 0.03
%, plus other miscellaneous gases (H2O vapor consist 1%).
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(Table from Lutgens and Tarbuck, The Atmosphere, 8th edition)
A. First Atmosphere
Composition: Probably H2 and He were the dominant gases at this stage:
 These gases are relatively rare on Earth compared to other places in the universe and were probably
lost to space early in Earth's history because;
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Earth's gravity is not strong enough to hold lighter gases;
Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's
magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds.
 Once the core differentiated the heavier gases could be retained
B. Second Atmosphere
 Produced by volcanic out gassing.
 Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2,
Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane).
 No free O2 at this time (not found in volcanic gases).
 Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid in the
Early Archean, allowing oceans to form.
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Evidence - pillow basalts, deep marine seds in greenstone belts.
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Addition of O2 to the Atmosphere
Today, the atmosphere contains about 21% free oxygen. How did oxygen reach these levels in the
atmosphere? Revisit the oxygen cycle:
Oxygen Production
o Photochemical dissociation - breakup of water molecules by ultraviolet
 Produced O2 levels approx. 1-2% current levels
 At these levels O3 (Ozone) can form to shield Earth surface from UV
o Photosynthesis - CO2 + H2O + sunlight = organic compounds + O2 - produced by
cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere. Thus plant
populations
Oxygen Consumers
o Chemical Weathering - through oxidation of surface materials (early consumer)
o Animal Respiration (much later)
o Burning of Fossil Fuels (much, much later)
Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What
little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the
surface were sufficiently oxidized, more oxygen could remain free in the atmosphere.
During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10 %. Most of this was released
by cyanobacteria, which increases in abundance in the fossil record 2.3 Ga. Present levels of O2 were
probably not achieved until ~400 Ma.
Evidence from the Rock Record
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Iron (Fe) is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record,
we can infer a great deal about atmospheric evolution.
Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean
sediments: Pyrite (Fools gold; FeS2), Uraninite (UO2). These minerals are easily dissolved out of rocks
under present atmospheric conditions.
Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate
with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate,
iron sulfide. BIF's are a major source of iron ore, b/c they contain magnetite (Fe3O4) which has a
higher iron-to-oxygen ratio than hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not
form today.
Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 B. y., but are
common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite
(Fe2O3) that probably forms secondarily by oxidation of other Fe minerals that have accumulated in
the sediment.
Conclusion - amount of O2 in the atmosphere has increased with time.
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Biological Evidence
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Chemical building blocks of life could not have formed in the presence of atmospheric oxygen.
Chemical reactions that yield amino acids are inhibited by presence of very small amounts of oxygen.
Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria,
methane-producing bacteria and bacteria that derive energy from fermentation. Conclusion, Since
today's most primitive life forms are anaerobic, the first forms of cellular life probably had similar
metabolisms.
Today these anaerobic life forms are restricted to anoxic (low oxygen) habitats such as swamps,
ponds, and lagoons.
Atmospheric oxygen built up in the early history of the Earth as the waste product of photosynthetic
organisms and by burial of organic matter away from surficial decay. This history is documented by
the geologic preservation of oxygen-sensitive minerals,
deposition banded iron formations, and development of continental "red beds" or BIFs. Figure from
the University of Michigan's Introduction to Global Change web site.
Atmospheric Structure
Not only does the atmosphere have a relatively stable composition, but it also has a structure to it.
Below are two figures from Lutgens and Tarbuck (The Atmosphere, 8th edition) showing vertical
profiles of the atmosphere. Note that the first major break, or boundary in the atmosphere as we
ascend is the tropopause.
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The Earth's atmosphere
The present atmosphere of the Earth is probably not its original atmosphere. Our current atmosphere is
what chemists would call an oxidizing atmosphere, while the original atmosphere was what chemists
would call a reducing atmosphere. In particular, it probably did not contain oxygen.
 Composition of the Atmosphere
 The original atmosphere may have been similar to the composition of the solar nebula and close to the
present composition of the Gas Giant planets, though this depends on the details of how the planets
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condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds out
gassed from the crust or (in some more recent theories) much of the atmosphere may have come
instead from the impacts of comets and other planetesimals rich in volatile materials.
 The oxygen so characteristic of our atmosphere was almost all produced by plants (cyanobacteria or,
more colloquially, blue-green algae). Thus, the present composition of the atmosphere is 79%
nitrogen, 20% oxygen, and 1% other gases.
Layers of the Atmosphere
 The atmosphere of the Earth may be divided into several distinct layers, as the following figure
indicates.
Layers of the Earth's atmosphere
Troposphere
 The troposphere is where all weather takes place; it is the region of rising and falling packets of air.
The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There
is a thin buffer zone between the troposphere and the next layer called the tropopause.
 The Stratosphere and Ozone Layer
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The word troposphere comes from tropein, meaning to turn or
change. All of the earth's weather occurs in the troposphere. The
troposphere has the following characteristics.
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It extends from the earth's surface to an average of 12 km
(7 miles).
The pressure ranges from 1,000 to 200 millibars (29.92
in. to 5.92 in.).
The temperature generally decreases with increasing
height up to the tropopause (top of the troposphere); this
is near 200 millibars or 36,000 ft.
The temperature averages 15°C (59°F) near the surface and
-57°C (-71°F) at the tropopause.
 The layer ends at the point where temperature no longer
varies with height. This area, known as the tropopause,
marks the transition to the stratosphere.
 Winds increase with height up to the jet stream.
The moisture concentration decreases with height up to the
Tropopause
The air is much drier above the tropopause, in the stratosphere.
The sun's heat that warms the earth's surface is transported
upwards largely by convection and is mixed by updrafts and
downdrafts.
The troposphere is 70%
and 21% . The lower density of
molecules higher up would not give us enough
to survive.
Stratosphere
Above the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in the
upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is
primarily responsible for absorbing the ultraviolet radiation from the Sun. The formation of this layer is a
delicate matter, since only when oxygen is produced in the atmosphere can an ozone layer form and prevent
an intense flux of ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution
of life. There is considerable recent concern that manmade flourocarbon compounds may be depleting the
ozone layer, with dire future consequences for life on the Earth.
The Mesosphere and Ionosphere
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Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many
atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very
thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from
the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.
The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind),
which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the
free electron density, which is an indicator of the degree of ionization. Here are electron density contour
maps of the ionosphere for months in 1957 to the present. Compare these simulations of the variation by
month of the ionosphere for the year 1990 (a period of high solar activity with many sunspots) and 1996 (a
period of low solar activity with few sunspots):
Atmospheric Processes
Interactions with Atmosphere and Ocean
In the Cycles overview, we learned that water is an
essential part of the earth's system. The oceans
cover nearly three-quarters of the earth's surface
and play an important role in exchanging and
transporting heat and moisture in the atmosphere.
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Most of the water vapor in the atmosphere
comes from the oceans.
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Most of the precipitation falling over land
finds its way back to oceans.
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About two-thirds returns to the atmosphere
via the water cycle.
You may have figured out by now that the oceans and atmosphere interact extensively. Oceans not only act as
an abundant moisture source for the atmosphere but also as a heat source and sink (storage).
The exchange of heat and moisture has profound effects on atmospheric processes near and over the
oceans. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the
northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the
development of many types of weather phenomena. They also warm the climate of nearby locations.
Conversely, cold southward flowing currents, such as the California current, cool the climate of nearby
locations.
Energy Heat Transfer
Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a
small part is directly absorbed, particularly by certain gases such as ozone and water vapor. Some energy is
also reflected back to space by clouds and the earth's surface.
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Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation.
Conduction is the process by which heat energy is transmitted through contact with neighboring molecules.
Some solids, such as metals, are good conductors of heat while others, such as wood, are poor conductors. Air and water
are relatively poor conductors.
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Since air is a poor conductor, most energy transfer by conduction occurs right at the earth's surface. At night, the ground
cools and the cold ground conducts heat away from the adjacent air. During the day, solar radiation heats the ground,
which heats the air next to it by conduction.
Convection transmits heat by transporting groups of molecules from place to place within a substance. Convection
occurs in fluids such as water and air, which move freely. In the atmosphere, convection includes large- and small-scale
rising and sinking of air masses and smaller air parcels. These vertical motions effectively distribute heat and moisture
throughout the atmospheric column and contribute to cloud and storm development (where rising motion occurs) and
dissipation (where sinking motion occurs).
To understand the convection cells that distribute heat over the whole earth, let's consider a simplified, smooth earth
with no land/sea interactions and a slow rotation. Under these conditions, the equator is warmed by the sun more than
the poles. The warm, light air at the equator rises and spreads northward and southward, and the cool dense air at the
poles sinks and spreads toward the equator. As a result, two convection cells are formed.
Meanwhile, the slow rotation of the earth toward the east causes the air to be deflected toward the right in the northern
hemisphere and toward the left in the southern hemisphere. This deflection of the wind by the earth's rotation is known
as the Coriolis effect.
Radiation is the transfer of heat energy without the involvement of a physical substance in the transmission. Radiation
can transmit heat through a vacuum.
Energy travels from the sun to the earth by means of electromagnetic waves. The shorter the wavelength, the higher the
energy associated with it. This is demonstrated in the animation below. As the drill's revolutions per minute (RPMs)
increase, the number of waves generated on the string increases, as does the oscillation rate. The same principle applies
to electromagnetic waves from the sun, where shorter wavelength radiation has higher energy than longer wavelength
radiation.
Most of the sun's radiant energy is concentrated in the visible and near-visible portions of the spectrum. Shorter-thanvisible wavelengths account for a small percentage of the total but are extremely important because they have much
higher energy. These are known as ultraviolet wavelengths.
Generally, the climate system of the Earth consists of five major components
A. Atmosphere
B. Biosphere
C. Hydrosphere
D. Cryosphere
E. Land sphere
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Chapter 3
Climate Classification
Approaches to classify climate
There are a number of climate classifications out of them:
1. Generic classification: based on the atmospheric processes forming the climate e.g. Equatorial, Tropical,
polar etc.
2. Empirical classification: based on the relationship between climate and observed distribution of natural
vegetation. E.g. Kopen’s classification
3. Hydrological classification: based on the annual water budget eg. Dry, wet, humid
4. Agro-climatic classification: based on soil, precipitation, length of growing period (LGP)
Factors Influencing the World Climatic Regions
The climate of a particular place is the function of a number of factors. These factors include:
1) Latitude and its influence on solar radiation received;
2) Air mass influences;
3) Location of global high and low pressure zones
4) Heat exchange from ocean currents;
5) Distribution of mountain barriers;
6) Pattern of prevailing winds;
7) Distribution of land and sea;
8) Altitude.
Köppen Climate Classification System
The Köppen Climate Classification System is the most widely used system for classifying the world's
climates. Its categories are based on the annual and monthly averages of temperature and precipitation. The
Köppen system recognizes five major climatic types; each type is designated by a capital letter;
 Climate classification is number advantageous to the society working on different aspects of the
natural environment, such as: soil, plant, animal life
 Classifying the climate into homogenous zones enables to predict various associated visible aspects of
the environment
 In this line W. Koppen’s model (1900) is the most widely known and accepted classification
 Koppen’s model takes temperature and precipitation values of months as its base since they can be
easily obtained and measured
 Koppen identified five different climates designated as: A,B,C,D,E;
19
A. Tropical Moist Climates: all months have average temperatures above 18° Celsius.
B. Dry Climates: with deficient precipitation during most of the year.
C. Moist Mid-latitude Climates with Mild Winters.
D. Moist Mid-Latitude Climates with Cold Winters.
E. Polar Climates: with extremely cold winters and summers.
E
Region
A
B
C
D
E
D
C
B
A
B
C
D
E
Characteristics
Humid tropical climate, winterless, hot all the seasons, all months have mean
temperature above 18oC
Dry climates, evapo-transpiration exceeds precipitation, there is a constant water
deficiency
Humid meso-thermal climate or warm temperate rainy climates. Mild winters, the
average temperature of the coldest months is below 18oC, but above - 3oC. The average
temperature of the warmest months is over 10oC. Has winter and summer seasons
Humid micro-thermal climates or cold snow-forest climates, severe winters, the average
temperature of the coldest months is below -3oC and that of the warmest month exceeds
10oC
Polar climates: Summers less, the warmest monthly mean temperature is below 10oC
20
Tropical Moist Climates (A)
Tropical moist climates extend northward and southward from the equator to about 15 to 25° of latitude. In
these climates all months have average temperatures greater than 18° Celsius. Annual precipitation is greater
than 1500 mm. Three minor Köppen climate types exist in the A group, and their designation is based on
seasonal distribution of rainfall.
a. Af or tropical wet is a tropical climate where precipitation occurs all year long. Monthly temperature
variations in this climate are less than 3°C. Because of intense surface heating and high
humidity, cumulus and cumulonimbus clouds form early in the afternoons almost every day.
Daily highs are about 32° Celsius, while night time temperatures average 22° Celsius.
b. Am (tropical monsoon climate): Annual rainfall is equal to or greater than Af, but most of the
precipitation falls in the 7 to 9 hottest months. During the dry season very little rainfall occurs.
c. (Aw) tropical wet and dry or savanna: has an extended dry season during winter. Precipitation
during the wet season is usually less than 1000 millimeters, and only during the summer season.
Dry Climates (B)
The most obvious climatic feature of this climate is that potential
evaporation and transpiration exceed precipitation. These climates extend from 20 - 35° North and South
of the equator and in large continental regions of the mid-latitudes often surrounded by mountains. Minor
types of this climate include:


BW - dry arid (desert) is a true desert climate. It covers 12% of the Earth's land surface and is
dominated by xerophytic vegetation. The additional letters h and k are used generally to distinguish
whether the dry arid climate is found in the subtropics or in the mid-latitudes, respectively.
BS - dry semiarid (steppe). Is a grassland climate that covers 14% of the Earth's land surface. It
receives more precipitation than the BW either from the intertropical convergence zone or
from mid-latitude cyclones. Once again, the additional letters h and k are used generally to
distinguish whether the dry semiarid climate is found in the subtropics or in the mid-latitudes,
respectively.
Moist Subtropical Mid-Latitude Climates (C)
This climate generally has warm and humid summers with mild winters. Its extent is from 30 to 50° of
latitude mainly on the eastern and western borders of most continents. During the winter, the main weather
feature is the mid-latitude cyclone. Convective thunderstorms dominate summer months. Three minor types
exist: Cfa - humid subtropical; Cs - Mediterranean; and Cfb - marine. The humid subtropical climate (Cfa)
has hot muggy summers and frequent thunderstorms. Winters are mild and precipitation during this season
comes from mid-latitude cyclones. A good example of a Cfa climate is the southeastern USA. Cfb marine
climates are found on the western coasts of continents. They have a humid climate with short dry summer.
Heavy precipitation occurs during the mild winters because of the continuous presence of mid-latitude
cyclones. Mediterranean climates (Cs) receive rain primarily during winter season from the mid-latitude
21
cyclone. Extreme summer aridity is caused by the sinking air of the subtropical highs and may exist for up to
5 months. Locations in North America are from Portland, Oregon to all of California.
Moist Continental Mid-latitude Climates (D)
Moist continental mid-latitude climates have warm to cool summers and cold winters. The location of these
climates is pole ward of the C climates. The average temperature of the warmest month is greater than 10°
Celsius, while the coldest month is less than -3° Celsius. Winters are severe with snowstorms, strong winds,
and bitter cold from Continental Polar or Arctic air masses. Like the C climates there are three minor
types: Dw - dry winters; Ds - dry summers; and Df - wet all seasons.
Polar Climates (E)
Polar climates have year-round cold temperatures with the warmest month less than 10° Celsius. Polar
climates are found on the northern coastal areas of North America, Europe, Asia, and on the landmasses of
Greenland and Antarctica. Two minor climate types exist. ET or polar tundra is a climate where the soil is
permanently frozen to depths of hundreds of meters, a condition known as permafrost. Vegetation is
dominated by mosses, lichens, dwarf trees and scattered woody shrubs. EF or polar ice caps has a surface that
is permanently covered with snow and ice.
Climatic Regions of Ethiopia
Ethiopia can be classified into different climatic regions on the basis of:
1. Koppens’s universal classification
2. Traditional methods
3. Moisture and Thermal regime combination methods
1. Koppen’s method
– Based on Kopen’s method, the climate of Ethiopia can be generally grouped in to 3 main categories:
I. Tropical Rainy Climate- A
II. Dry Climate- B
III. Warm Temperate Climate- C (Special) Cool highland climate- H
– Each of them is divided into three or more sub-groups making a total of 11 climatic types
– A and C climate groups are known as tree climates, since the combination of heat and moisture are
suitable for trees
– The differences between A and C are based on the temperature of the coldest month:
o if the temperature of the coldest month is less than 180C , then it is C
o if more than 180C, then it is A
2. Traditional agro-climatic zones
 Despite there are no clear distinction between them traditionally are known as:
 Kola (lowland)
 Woynadega (midland) and
 Dega (highland)
 However as population pressure increases and agricultural activities expanded, two additional zones at
the higher and the lower temperature extremes were included as:
22


Bereha,
Wurch,
Table 1: agro-ecological classification
Traditional
zone
Berha
Climate
Hot arid
Altitude
(m)
< 500
Annual temp.
Annual
( 0C)
rainfall (mm)
>27.5
<200
Kola
Warm semiarid
500-1500
27.5-20
200-800
Woyna- Dega
Dega
Cool sub-humid
Cool and humid
1500-2400
2400-3200
20-17.5
17.5-11.5
800-1200
1200-2200
Wurch
Cold and moist
> 3200
<11.5
>2200
3. Moisture and Thermal combination
– Characterization of agro-ecological region is based on the analysis of the relation between: - physical,
abiotic and biotic parameters with the climate such as: physiography, soils, vegetations, animals and
human activities.
– Especially precipitation and temperature are the two major climatic elements of a given region which
play important role in determining plant :
 biomass density
 productivity
 community composition and
 distribution of species
Thermal zones of Ethiopia
Thermal
zones
T1
T2
T3
T4
T5
T6
Climate
Hot
Warm
Tepid
Cool
Cold
Very cold
Temperature Regime
0
C
> 27.5
27.5 - 21.0
21.0 - 16.0
16.0 - 11.0
11.0 - 7.5
< 7.5
Elevation
(masl)
< 500
500 - 1600
1600 - 2400
2400 - 3200
3200 - 3800
> 3800
23
Moisture Regimes
Moisture Regime
Classes
A
Terminologies
Arid
Growing Period
(days)
< 45
B
C
D
Semiarid
Sub-moist
Moist
45 – 60
61 -120
121 – 180
E
F
Sub-humid
Humid
181 – 240
241 – 300
G
Super-humid
> 300
The combination of the Thermal and Moisture regimes
Moisture Regimes
Hot toWarm
>210C
Temperature Regime
Tepid to Cool
Cold to Very Cold
0
11-21 C
<110C
Arid (A)
A1
A2
No
Semiarid (B)
B1
B2
No
Sub-moist (C)
C1
C2
C3
Moist (D)
D1
D2
D3
Sub-humid (E)
E1
E2
E3
Humid (F)
F1
F2
F3
Super-humid (G)
G1
G2
No
24
Zones
Major Areas
within the zones
Central
Highlands
N.Shoa, E. Wollega
Gojjam & Gondar
Northern
Highlands
Tigray and Northrn
Wollo & Gondar
South western
Highlands
Highlands of
Wollega, Ilubabor
& Kafa
Eastern
Highlands
Sidamo, Bale, Arsi
& W.Hararge
Southern Rift
valley & the
lakes regions
E.Shoa, W.Arsi,
Sidamo, North and
South Omo
Western foot
hills & plains
Metekel, Asossa &
Gambella
Middle and
lower Awash
Wonji, Metehara,
Melka Worer,
Milka- Sedi, Asaita
& Dufti
Altitude
(m)
Annual
Rainfall (mm)
24003000
950-1500
20003000
450-950
15002400
18003000
1500-2000 without
real dry season
950-1500
Predominantly
Growing crops
tef, barley, wheat,
finger millet,
nigerseed, safflower,
linseed, chickpea,
lentil, field pea,
horse bean
wheat, barley, tef,
sorghum, cotton &
oil seeds including
groundnut
enset, maize, yams,
taro & specially
coffee
enset, barley, wheat,
cha´t, sorghum,
coffee & groundnut
12001600
High but the
distribution is
unreliable
sorghum, maize,
coffee & enset
Below
1000
150-1200
depending on the
altitude
cotton, sesame,
sorghum, groundnut
very low and
cropping depends
on irrigation
sorghum, cotton,
sugarcane and citrus
fruits
500-1500
25
Seasons
Meteorologically ‘season’ is a period, when an air mass is homogenous in climatic elements such as:
 Temperature
 Humidity
 Wind
 rainfall
 In the tropics seasons are classified as:
 wet
 dry
 However, in subtropics and temperate regions seasons are classified as:
 Winter
 Spring
 Summer
 Autumn (fall)
 Seasons are annual meteorological cycles caused by Earth’s rotational tilt relative to its orbit around
the Sun
 The rotational axis of Earth leans at an angle of 23.50 from the perpendicular position to its solar
orbit.
 Earth makes one complete orbit in one year (365.25 days)
 Depending on Earth’s orbital position, either the Southern or Northern hemisphere receives more
direct solar radiation.
 This differential positioning of the earth causes seasonal shifts expressed as Solistices and
Equinoxes
Solstices
 one hemisphere is positioned directly toward the sun and receives maximum sunlight
 the opposite hemisphere is as far away from the Sun and is mostly shadowed
 Solstices represents the beginning of summer and winter, in June and December respectively;
 June solstice:
• North Pole receives nearly continuous daylight, while the South Pole has uninterrupted
night
• summer in the northern and winter in the southern hemisphere,
• daylight is the longest in north and shortest south of the equator
 December solstice is the shortest day of the year in the Northern Hemisphere and the longest day in
the Southern Hemisphere
Equinoxes
 Sun rays strike the equator perpendicularly
 The positions itself halfway between solstices
 The poles have equal distance from the Sun
 Both hemispheres briefly receive equal sunlight
 Day length is the same everywhere
 The two equinox positions are:
26
 September Equinox beginning of:
 spring in the Southern Hemisphere
 autumn in the Northern Hemisphere
 March Equinox is a sign of:
 spring in the Northern Hemisphere
 autumn in the Southern Hemisphere
SUMMERY OF SEASONAL SHIFTS
Northern hemisphere
Southern
hemisphere
20 - March
21- June
22 - September
Spring Equinox
Summer Solstice
Autumnal Equinox
Autumnal Equinox
Winter Solstice
Spring Equinox
21- December
Winter Solstice
Summer Solstice
Date
Impact of ITCZ
The Inter-tropical Convergence Zone (ITCZ) is an area of low pressure that forms where the Northeast
Trade Winds meet the Southeast Trade Winds near the earth's equator. As these winds converge, moist air is
forced upward. This causes water vapor to condense, or be "squeezed" out, as the air cools and rises,
resulting in a band of heavy precipitation around the globe. This band moves seasonally, always being
drawn toward the area of most intense solar heating, or warmest surface temperatures. It moves toward the
Southern Hemisphere from September through February and reverses direction in preparation for Northern
Hemisphere Summer that occurs in the middle of the calendar year. The result of ITCZ is not only seasonal
wind patterns but also seasonal rainfall.
 ITCZ oscillates over Africa between:
 20oN in the northern hemisphere during June to August and
 17oS in the south during December to February
 The main factors influencing the position of ITCZ:
 apparent movement of the overhead sun
 relative strengthening of the trade winds
 locations of the maximum sea surface temperature
ITCZ and Ethiopian climate
 ITCZ is one of the important causes of inter-annual rainfall variability of tropical Africa including
Ethiopia.
 The ITCZ brings much rainfall to Ethiopia in July to early August despite differing in rainfall
duration and magnitude;
27
 Its south ward migration from the extreme northern parts of the country starts in late August and
reaches central Ethiopia in mid-September;
 It covers the southern and southwestern regions during October/November after which, it crosses the
equator and moves to the southern hemisphere
 By the end of November, anticyclones are developed over the Sahara and the Arabian Peninsula
and start shifting towards to the equator creating:
 Northeasterly winds, from the Arabian Peninsula and
 Northwesterly winds, from the Sahara anticyclone carrying dry winds;
 Thus during December to February, most parts of the country has:
 the clear sky, due to lower RH of the atmosphere;
 dry air mass, since it originates from drier land masses/deserts
 cooler weather, due to the winter season
 Sometimes however, moist air from the north Indian ocean and Gulf of Eden bring brief and
variable winter rain in:
 southeastern high lands
 central highlands and
 northwestern regions of the country
In general, Ethiopia is crossed by ITCZ twice in a year, following the apparent movement of the Sun.
The duration of ITCZ associated with the onset and cessation of the rainy season can be:
 largest in southern and southwestern (5-6 months),
 smallest in the northernmost parts of the country (2-3 months)
The major seasons of Ethiopia:
 Ethiopian seasons are unique and are classified mainly based on rainfall pattern and its distribution
 Rainfall Regimes and seasons of Ethiopia:
i. Two season type: Region B
- Mono-modal (single maxima)
ii. Three season type: Region A
- Bimodal Type-1(unequal– double maxima)
iii. Bi-two season type: Region C
- Bimodal Type two (equal double maxima)
iv. Undefined season: Region D
-Mono-modal type two
- Defused pattern of rainfall
I. Two seasons Type
Designated as region-B
 Mono-modal (single maxima)
 Dominated by wet and dry seasons and covers almost 50% of western Ethiopia
 The wet season gradually decreases northwards from about 10 months in the southwest to only about
2 months in the northwest creating three subdivisions as:
 b1 – The wet period:- February/March – September/October
 b2 – The wet period:- April/May – September/October
28
 b3 – The wet period:- June/July - August/September
 The major rainfall sources are ICTZ and monsoons
Region-B
350
300
250
200
Wolisso
Awassa
Gorgora
150
100
50
0
Jan
Feb
March
April
May
June
July
Aug
Sept
Oct
Nov
Dec
II. Three Seasons Type
Designated as Region A
Bimodal Type-1, with unequal double maxima
Characterized by three seasons as: Bega:- dry season covering the period: October – January, there may also be untimely rain
 Belg:- small rainy season in mid -February-mid-May, higher inter-annual and seasonal
variability
 Kiremt: - the main rainy season that covers the period: June – September
The main sources of kiremt rainfall are:
– Inter Tropical Convergence Zone- ITCZ
– Read Sea Convergence Zone- RSCZ
– Southwesterly Monsoons
29
Region A
200
180
160
140
120
Moyale
100
Yavelo
80
60
40
20
0
Jan
Feb
March
April
May
June
July
Aug
Sept
Oct
Nov
Dec
Bega
The country predominantly fall under the influence of northeasterly and northwesterly dry winds
originated from:
 the Sahara anticyclone and/or
 central Asia (Siberia) extending into Arabian Peninsula
Sometimes these winds may be interrupted by migratory low pressure system originated
in the Mediterranean area and moving eastwards resulting in rainfall over parts of central
Ethiopia.
The rainfall is about 400mm in the southwestern parts and generally decreases towards
north and northeast
The rainfall reaches up to 600mm decreasing towards north
mainly concentrated over the highlands of:
 central
 southwestern
 southern parts
Major sources for Belg season:
 the Arabian highs as it especially toward the north Arabian sea
 Development of low thermal over southern Sudan
 Generation and propagation of disturbance over Mediterranean some times coupled with
easterly waves
Occasional development of the Red Sea Convergence Zone (RSCZ) also affects coastal areas
Kremt:
The rainfall varies from 10mm to more than 1200mm
Constituting 50-90% of the annual precipitation of the major rainfall areas of the
country
30
The rainfall decreases towards northeast and it is relatively stable compared to the other
two seasons
The major rain producing systems:
 Northward migration of ITCZ
 Arabian and Sudan low thermal condition along 200N
 High pressure over south Atlantic and Indian Ocean
 The generation of low level “Somali Jet” enhancing low level southwesterly flow- SJ
III. Bi – two season type
 Designated as Region C
 Bimodal Type-two with equal-maxima and peaks during April and October
 The first wet season covers the period of:- March – May While the second extends:- September –
November
 Although the first is more prominent, both wet seasons are important
 Mainly covers the southern and southeastern parts of the country and generally, the annual rainfall
decreases towards east
 Main source of rainfall - ITCZ
Region C
140
120
100
80
Gewane
K/ridahar
60
40
20
0
Jan
Feb March April
May
June
July
Aug
Sept
Oct
Nov
Dec
IV. Undefined Season
 The area is designated D
 Mono-modal type -2
 Defused type of rainy season with no well defined seasonal type
 Characterized by irregular rainfall pattern without defined rainfall pattern
 However sporadic rain prevails from July – February
 Mostly warm and dry air associated with gusty winds
31
Region D
120
100
80
60
40
20
0
JAN
FeB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
Weather systems that affect Ethiopian precipitation
Ethiopia makes up the greater part of the East African ‘Horn of Africa’. At latitudes of 4 to 15°N,
Ethiopia’s climate is typically tropical in the south-eastern and north-eastern lowland regions, but much
cooler in the large central highland regions of the country. Mean annual temperatures are around 15-20°C in
these high altitude regions, whilst 25-30°C in the lowlands.
Seasonal rainfall in Ethiopia is driven mainly by the migration of the tropical rain belt, the Inter-Tropical
Convergence Zone (ITCZ). The exact position of the ITCZ changes over the course of the year, migrating
across the equator from its northern most position over northern Ethiopia in July and August, to its southern
most position over southern Kenya in January and February.
Most of Ethiopia experiences one main wet season, of up to 350mm per month in the wettest regions, when
the ITCZ is at its northernmost position. This season (known also as the ‘Kiremt’ rains or the ‘Meher’ rains)
occurs from mid-June, through July and August, to mid-September. Parts of northern and central Ethiopia
also have a secondary wet season of sporadic, and considerably lesser, rainfall from February, through to
May (called the ‘Belg’). The southern regions of Ethiopia experience two distinct wet seasons which occur
as the ITCZ passes through this more southern position The March to May ‘Belg’ season is the main rainfall
season yielding 100-200mm per month, followed by a lesser rainfall season (around 100mm per month) in
October to December called ‘Bega’. The eastern most corner of Ethiopia receives very little rainfall at any
time of year.
The movements of the ITCZ are sensitive to variations in Indian Ocean sea surface temperatures and vary
from year to year, hence the onset and duration of the rainfall seasons vary considerably interannually,
causing frequent drought. The most well documented cause of this variability is the El Niño Southern
Oscillation (ENSO). Warm phases of ENSO (El Niño) have been associated with reduced rainfall in the
32
main wet season in north and central Ethiopia, causing severe drought and famine, but also with enhanced
rainfalls in the earlier February to April rainfall season which mainly affects southern Ethiopia.
Ethiopian weather system is affected by several regional and global factors. Due to the effect of climate
change; flooding, drought and other health effects occur in the country. Some of the important factors that
have significant effect on Ethiopian precipitation are:
1. ITCZ,
2. ENSO,
3. Tropical Easterly Jet(TEJ),
4. South Atlantic Ocean or Somal Jet,
5. South West Indian Ocean Anticyclones,
1. Inter-tropical Convergence Zone(ITCZ)
The Inter-tropical Convergence Zone (ITCZ), is the region that circles the Earth, near the equator, where
the trade winds of the Northern and Southern Hemispheres come together. The intense Sun and warm water
of the equator heats the air in the ITCZ, raising its humidity and making it buoyant. Aided by the
convergence of the trade winds, the buoyant air rises. As the air rises it expands and cools, releasing the
accumulated moisture in an almost perpetual series of thunder-storms. In the Northern Hemisphere, the
winds that blow to the equatorial low pressure belt are called North East Trade Winds; and in the Southern
Hemisphere, the winds that blow to the equatorial low pressure belt are called South East Trade winds. So,
the line along which these winds converge is called the Inter Tropical Convergence Zone (ITCZ). These
ITCZ affects the seasonal variation across regions in the World. Seasonal rainfall in Ethiopia is driven
mainly by the migration of the Inter‐Tropical Convergence Zone (ITCZ). The exact position of the ITCZ
changes over the course of the year, oscillating across the equator from its northern most position over
northern Ethiopia in July and August, to its southern most position over Southern Kenya in January and
February. Most of Ethiopia experiences one main wet season (called ‘Kiremt’) from mid‐June to
mid‐September (up to 350mm per month in the wettest regions), when the ITCZ is at its northern‐most
position. Parts of northern and central Ethiopia also have a secondary wet season of sporadic, and
considerably lesser, rainfall from February to May (called the ‘Belg’). The southern regions of Ethiopia
experience two distinct wet seasons which occur as the ITCZ passes through this more southern position
The March to May ‘Belg’ season is the main rainfall season yielding 100‐200mm per month, followed by a
lesser rainfall season in October to December called ‘Bega’ (around 100mm per month). The eastern most
corner of Ethiopia receives very little rainfall at any time of year (McSweeney, New and Lizcano:
http://country-profiles.geog.ox.ac.uk ).
As Ethiopia is located within the tropical region, it is influenced by weather systems of various scales, from
mesoscale, such as thunderstorms, to large scale ENSO related phenomena. The major rain-bearing system
for the main rainy season (June to September) is the Inter Tropical Convergence Zone (ITCZ). On the other
hand, the eastward moving mid-latitude troughs will facilitate the interaction between the mid-latitude cold
air and the tropical warm air so that unstable conditions will be created for the moisture that comes into
Ethiopia from the Arabian Sea during the small rainy season (Feb. to May) (NMSA, 1996).
33
 The Belg rain in Ethiopia is affected by global and regional weather features like ITCZ, Subtropical
Westerly Jet (SWJ) stream, Arabian High, the frequency of tropical cyclones over the Southwest Indian
Ocean and ENSO (Abebe, 2010).
 During Bega (the dry season) the country predominantly falls under the influence of warm and cool
northeasterly winds. These dry air masses originate either from the Saharan anticyclone or from the ridge of
high pressure extending into Arabia from the large high over central Asia (Siberia). However, occasionally
the northeasterly winds are interrupted when migrating low pressure systems originating in the
Mediterranean area move southwards and interact with the tropical systems resulting into unseasonal rains
over central and northern Ethiopia. Occasionally the development of the red Sea Convergence Zone (RSCZ)
also produces rains over northeastern Ethiopia (Pedgley, 1966).
2. ENSO
The most well documented cause of this variability is the El Niño Southern Oscillation (ENSO). Warm
phases of ENSO (El Niño) have been associated with reduced rainfall in the main wet season in north and
central Ethiopia, causing severe drought and famine, but also with enhanced rainfalls in the earlier February
to April rainfall season which mainly affects southern Ethiopia (McSweeney, C. et al. (2008).
3. Tropical Easterly jet (TEJ)
The strength of the Tropical Easterly Jet (TEJ) at 200 hPa and higher, has been associated with enhanced
precipitation in Ethiopia, linked to upper-level divergence promoting convection (Hastenrath, 2000; Grist
and Nicholson, 2001; Nicholson and Grist, 2003; Segele et al., 2009 as cited inVistea and Sorteberga,
2011).
4. South Atlantic Ocean or Somali Jet
The circulation over the Horn of Africa is linked to the Indian summer monsoon (McGregor and Nieuwolt
1998). Below 3 km, the strongest winds are found in the Somali Jet, also called the East African low-level jet. This is
a system of low level jets, extending from east of Madagascar, across the flat lands of eastern Kenya, Ethiopia and
Somalia, across the Indian Ocean to India (Findlater, 1969, 1977). The jet induces considerable low-level divergence,
resulting in dry summers and arid land in Somalia and eastern Ethiopia (Flohn, 1987).
Pacific Ocean: Neutral to cooling conditions will continue in the central, north eastern and South Pacific Ocean, but
warming is expected over its northwestern, south central and central equatorial part. However, the set of dynamical
and statistical model forecasts of ENSO over Nino 3.4 domain (5°N – 5°S, 120°W – 170°W) indicated a spread of
possible SST anomalies maintaining neutral conditions throughout the forecast period..
Atlantic Ocean: A neutral to cooling condition is expected over southern Atlantic Ocean, while warming trend is
expected to continue over the rest of Atlantic.
5. South West Indian Ocean Anticyclones
The Western Indian Ocean is a critical factor in the formation of cyclones, which can bring severe flooding to coastal
Africa, as was the case in 1997. It is also the birthplace of the intra-seasonal or Madden Julian Oscillation (MJO),
which provides a major modification to tropical weather across the Indo-Pacific ‘Warm Pool’ on time-scales of 30–60
days, and which has been implicated in the initiation and amplification of El Ni˜no (e.g. McPhaden 1999; Lengaigne
et al . 2002, as cited in Solomon Yohannes, n.d).
Neutral to cooling condition is expected over south and northeastern Indian Ocean, while neutral to warming
condition will extend from northwestern up to southwestern around the South Africa coast.
The relationship between the Ethiopian rainfall during Belg and the tropical cyclones over the southwest Indian Ocean
indicates that low/high frequency of the cyclones resulted in excess/deficit rainfall (Shanko et al, 1998).
34
Warming sea surface temperatures (SSTs), especially in the southwest Indian Ocean (SWIO), may be linked to
decreasing rains across ESEA. This link was first identified in research performed by the Ethiopian National
Meteorological Services Agency (Shanko and Camberlin, 1998). Shanko and Camberlin found that tropical storms
over the SWIO led to lower than normal rains in Ethiopia, primarily during the Belg season, through reductions in
available moisture, upper level easterly wind anomalies, and a northward shift of the subtropical westerly jet. SWIO
cyclonic activity played a role in the devastating drought of 1984, and it’s terrible human toll of 400,000 to a million
deaths. (USAID, 2005).
Despite the northeasterly flux of moisture above Ethiopia, more attention has been given to the regions to the south.
Several studies have commented on the effect of SST and pressure anomalies in the Gulf of Guinea and the Indian
Ocean on the moisture flow toward Ethiopia (Korecha and Barnston, 2007; Segele et al., 2009; Diro et al., 2010 as
cited inVistea and Sorteberga, 2011).
References
1. McSweeney, C. et al. 2008. Climate change profile of Ethiopia.pp(2).
2. C. McSweeney, M. New1,2 and G. Lizcano. UNDP Climate Change Country Profiles. http://countryprofiles.geog.ox.ac.uk: retrieved on 22/05/2012
3. Robert McSweeney. 2010. Climate change profile: Ethiopia.
4. Solomon Yohannes(n.d). The use of satellite observation in the provision of climate monitoring over
Ethiopia.
5. USAID. 2005. Recent drought tendencies in Ethiopia and equatorial sub-tropical Africa. Report.
6. Ellen Vistea and Asgeir Sorteberga. 2011. Moisture transport into the Ethiopian highlands. International
Journal of Climatology. Royal Meteorological Society.
35
Chapter 5
5. Crop and climate
5.1 Responses of plants to environmental stress
Crop growth and development is affected with several factors;
1. Biological factors: insect pests, and diseases, genetic variation
2. climatic factors: maximum and minimum temperature, RH, radiation or Sunlight,
precipitation or rainfall, wind, CO2, O2 and N2 composition
3. soil factors: soil physical properties such as texture, soil structure and water holding capacity;
and soil chemical properties like pH, EC, CEC, N, macro and micro nutrients content
1. Initial response:
 Plant reaction, which occurs within hours or days in response to the negative impact of the drought
stress.
 The response may be associated with stomatal closure and slowdown of leaf expansion.
2. Acclimation:
 Morphological and physiological adjustment of plants in response to stress.
 This unilateral adjustment by the plant can be; changes in the activity or synthesis of new
biochemical constituents such as enzymes,
3. Adaptation
 Evolutionary response of plants to environmental stress that results in genetic changes leading to
morphological and physiological modifications.
 The mechanism of response is often similar to acclimation, except its inherent nature.
36
Plant Responses to Drought
Almost all plants have to deal at some point with limited water, and they have adapted ways to prevent
massive water loss.
fig. 1: Transport of Solutes across Plasma Membranes of Plant Cells
(a) As guard cells change shape; Stomata open and close. Greater bowing of the guard cells during turgor
increases the size of the stoma opening. As surrounding cells become flaccid, bowing decreases and the
stoma closes. Due to radial orientation of microfibrils in the guard cells, the guard cell length increases more
than its width during turgor. (b) Changes in guard cell turgor are regulated by the passage of potassium ions
across the plasma and vacuolar membranes. This passage is assisted by the uptake of anions such as malate
and chloride.
1. Extreme temperatures
 Air temperature is the most important climatic variable which affects plant life.
 The growth of higher plants is restricted to the temperature between 0 and 600C and crop plants
are further restricted to a narrower range of 10 to 400C.
 However each species and variety of plants has its own upper and lower temperature limits
known as cardinal temperature
F. Cardinal temperatures of plants
 Base temperature: Is a temperature below which no growth occurs.
 For typical cool season crops, it ranges between 0 and 5oC and for hot season crops between
15 and 18oC.
 Optimum temperature is the one at which the maximum plant growth occurs.
 For cool season crops optimum temperature ranges between 25 and 31oC, while for warm
season crops it ranges between 31 and 37oC.
 Maximum temperature above which the plant growth stops.
 For cool season crops it ranges between 31 and 37oC, while it ranges between 44 and 50oC
for the hot season crops
 Beyond the upper and lower limits:
 the plant gets considerably damaged or
 the growth process can be retarded and
37
 the plant goes in to dormancy despite remains alive.
 The amplitude of variations in temperature is thus more important to plant growth rather than the
mean values.
Effect of high temperature
 High temperature increases the saturation deficit of the plants and results in disturbs the balance between
photosynthesis and respiration desiccation and.

Higher the temperature causes greater respiration, which results in the rapid exhaustion of food
reserves of seeds

It also affects plant metabolism, especially high night temperature increases respiration
 In extremely high temperature, enzymes and other proteins are expected to be inactive.
 High temperature with high humidity favors the development of many plant diseases.
 High temperature also favors the growth of shoot and leaves at the expense of roots, stolon and fruits.
Low temperature
 Most crop plants are injured and may even be killed when night temperature is very low
 Tender leaves and flowers are very sensitive to low temperature and frost.
 Especially, plants which are rapidly growing and flowering are easily killed
 Frost interferes with plant metabolism since plant parts or the entire plant may be killed as a
result of actual freezing of tissues.
 Frost damage is caused by the formation of ice crystals:
 first in the intercellular spaces and
 then within the cells
 This causes more injury by mechanical damage disrupting the structure of the protoplasm and
plasma membrane
 Freezing of water in intercellular spaces also results in:
 withdrawal of water from cell sap and
 increasing dehydration causes cells to die
 Forecasts of these extremes are therefore, needed at farm level providing farmers with information
for possible adaptation strategies
G. Torrential rains, floods and hail
 Plants use water from rain, snow, hail and soil for translocation and transpiration.
 Extremely high rainfall however can be as damaging as the deficit
 Rains falling at high intensity have a considerable kinetic energy depending up on the number
and size of drops
 This energy is used in initiating soil erosion (splash erosion) by moving and breaking soil
aggregates.
 Heavy rain seals the soil surface, decreasing rate of infiltration and further increase runoff and
erosion
Plant Responses to Flood
Mangrove
Mangrove trees grow in coastal marshlands and have aerial roots that stick out of the water.
38
 When plants receive too much water, the air spaces around the roots are filled up and the plant
cannot get enough oxygen for cellular respiration in its roots.
 Some plants are adapted to very wet conditions. The mangrove, found in coastal marshes, has aerial
roots that are continuous with the submerged roots and supply oxygen to them.
 In less specialized plants, the lack of oxygen causes ethylene production, which
stimulates iapoptosis in the root cortex cells. This results in tubes, or "snorkels," that reach the
surface and send oxygen down to the roots.
 Marsh plants have evolved air spaces (ii"aerenchyma tissue") in their stems that allow oxygen to
move from the leaves (where it is produced in the process of photosynthesis) to the roots by
either diffusion or, in some cases, by pressure differences between the leaves and the roots.
 Other marsh plants are able to survive in low oxygen conditions by relying on anaerobic respiration.
These species apparently have altered their biochemical pathways to allow them to avoid the buildup of high concentrations of toxic ethanol (a waste product of anaerobic respiration).
 aerenchyma: a spongy, airy tissue spaces found especially in the roots of aquatic plants.
Wetland soils have been affected by the permanent cover of water. One problem faced by plants living in
marshes is the lack of oxygen in the soil. Oxygen is used by plants (and most other organisms) in the
process of cellular respiration, in which the energy from glucose (produced by photosynthesis) is released so
that the organisms can use the energy to do “biological work."
Plants lack a circulatory system, making them unable to “pump” oxygen from air (where the oxygen
concentration is about 21%) below ground to the roots. Thus, the roots of terrestrial plants rely on oxygen in
air spaces between soil particles. Oxygen is hard to get in hydrated soils for two reasons. First, water fills
the space between soil particles that are typically filled with air in terrestrial soils. Second, because the rate
of diffusion of oxygen is much slower in water than in air, the rate of movement from the water surface to
the root zone is extremely slow, so most hydric soils are either low in oxygen (hypoxic) or lack oxygen
altogether (anoxic) because the oxygen in the soil water is used up, often by the activity of decomposers
(bacteria).
So how do the roots of marsh plants obtain the oxygen that they need to survive? Marsh plants have evolved
air spaces ("aerenchyma tissue") in their stems that allow oxygen to move from the leaves (where oxygen is
produced in the process of photosynthesis) to the roots by either diffusion or, in some cases, by pressure
differences between the leaves and the roots. Other marsh plants are able to survive in low oxygen
conditions by relying on anaerobic respiration. These species apparently have altered their biochemical
pathways to allow them to avoid the build-up of high concentrations of toxic ethanol (a waste product of
anaerobic respiration).
Several plants have evolved other methods of thriving in extremely wet conditions. For example, mangrove
trees grow in coastal marshlands and have aerial roots that stick out of the water. The aerial roots are
continuous with the submerged roots and supply them with oxygen. In less specialized plants, ethylene
production occurs during flooding. The ethylene causes apoptosis (controlled cell death) within the root
cortex. When the cells die, the root becomes tube-like and can supply oxygen to roots from the surface of
the water.
39
Hail storms:
 Sometimes, cumulonimbus clouds with a very severe thunderstorms produce hail
 The hail suddenly affects standing crops and little can be done to prevent it.
 Hail storms:
 damage the foliage
 break branches and heads
 Shreds (cuts) the leaves and the stalks
 reduce or destroy photosynthetically active leaf area
 Floods on the other hand cause water logging problem, that hinders the availability of O2 to crop
plants
5.2 Dryness Ratio Index (DRI)
DRI is the ratio of the annual net radiant energy arriving at the place (Qn) to the heat required to
evaporate a year’s rainfall.
DRI = Qn/L.r
r = annual rainfall amount (mm)
L = latent heat for vaporization of water which is 59cal/gm
Qn = yearly total net radiant energy (cal/cm2)
 Qn is the Annual net radiant energy arriving at a place, that would be available for :
 sensible heat transfer
 evapotranspiration
 soil heat flux
 and to small extent for photosynthesis
 Qn is affected by:
 amount of reaching solar radiation
 surface albedo (α)
 soil moisture content
DRI: has no unit; low DRI values show wet while high values dry regions
 DRI = 1, shows between forest and savanna grass
 DRI < 1 shows rain forest
 DRI = 2 indicates that, the available energy can evaporate 2 years average rainfall
DRI estimation
 To estimate the DRI of Awassa, given are:
Latitude = 7.040N
Longitude =38.300E
Altitude =1750 m
Global Solar Radiation (Qs) = 6.965
 Yearly net radiation (Qn)= 93,908 cal/cm2
 Latent heat for vaporization of water = 59 cal/g
 Mean annual rainfall (r) = 944 mm
40
DRI
= Qn/L.r
= 93,908
59 * 944
= 93,908
55696
= 1.7
2. Fischer and Maurer stress resistance index (S)
 S evaluates genotypic performance between two environments ie. by comparing the yield under
stress as percent of yield under non-stress
 The ‘Fischer and Maurer stress resistance index’ (S) can be computed as:
S = (Gs/Gi)/ (Ms/Mi); where: Gs -genotype yield under stress
 Gi -genotype yield under non-stress
 Ms -mean yield of all genotypes under stress and
 Mi -mean yield of all genotypes under non-stress
 S > 1 indicates a relative resistance as compared with the mean of the population.
3. Moisture index (Im)
Im is an index of rainfall effectiveness that considers both water surplus and water deficiency
in combination
Im is expressed as:
Im = (100Ws – 60Wd)
PET
Ws = PET-AET, Annual water surp
Wd = ΣR-ΣPET, Annual water deficit (mm)
PET = Annual water need (mm) for transpiring
 Im values:
 positive indicates moist climate
 negative- dry climate
41
4. Humidity and Aridity Indices
Aridity index (AI) is a numerical indicator of the degree of dryness of the climate at a given location. A
number of aridity indices have been proposed; these indicators serve to identify, locate or delimit regions
that suffer from a deficit of available water, a condition that can severely affect the effective use of the land
for such activities as agriculture or stock-farming; or
Number indicating how much more precipitation could be lost by evapotranspiration if it were availablethan
is actually lost at a given location.
 Humidity Index (IH)
IH = 100 * Water surplus; S is surplus water
Annual PET
= 100*S / PET
 Aridity Index (Ia)
Ia = 100 x Water deficit
Annual PE
= 100D / PET; where, D is deficit water
Example: calculate the aridity and humidity index of an area with 100mm and 250mm of surplus and
deficit during the rainy and dry seasons respectively; with annual potential evapotranspiration of 1000 and
2000mm during the rainy and dry seasons respectively.
Table 2: Aridity and humidity index
Moist climates
Aridity Index (Ia)
S- small water deficit
m - moderate water deficit
0 – 16.7
16.8 – 33.3
L- large water deficit
> 33.4
Dry Climates
Humidity Index (Ih )
S- small water surplus
m- moderate water surplus
0 – 10
10 - 20
L- large water surplus
20
42
5. Coefficient of Rainfall Variability
 Coefficient of variability (cv) is a measure of year-to-year rainfall variation of a given location.
 The cv-value is essential for planning agricultural development and hydrological management
 It is estimated as:
The coefficient of variation (CV) is defined as the ratio of the standard deviation
to the mean
:
; where, S is standard deviation and X is mean rainfall
; where, X is mean rainfall, n is the frequency or period of measurement
; where, Xi is rainfall of the ith month, and X is the mean rainfall
cv - values
cv –values
%
< 20
20-30
> 30
Level of rainfall variability
stable
less variable
highly variable
Example: if the mean annual precipitation is 400mm and the estimated standard deviation is 10; calculate
the coefficient of variability of the rainfall in the year.
CV (%) = 10/400*100........ 2.5%; therefore, the rainfall in the given year is stable
Example: calculate the CV value using the following data.
Table: Rainfall data of mekelle University (2002-2010)
Month
Rainfall (mm)
January
1
February
8.02
March
24.2
April
5.45
May
20.2
June
57.88
July
158.20
August
221.89
September
42.08
October
3.38
43
November
December
5.3
2.81
0.08
Weather Prediction and forecasting
Weather forecasting: is the application of science and technology to predict the state of
the atmosphere for a given location. Human beings have attempted to predict the weather
informally for millennia, and formally since the nineteenth century. Weather forecasts are made by
collecting quantitative data about the current state of the atmosphere at a given place and
using scientific understanding of atmospheric processes to project how the atmosphere will
change.
44
Chapter 6
6.1 Weather and climate hazards
Extreme weather hazards and agriculture
Drought, flood, high temperature and low temperature ranges, high wind like hurricane, high rainfall, hail
and frost are caused as a result of climate change resulting in weather and climate hazards.
 In combination with other physical, socioeconomic and political factors, climate variability
contributes to vulnerability to economic loss, hunger, famine and dislocation.
 Agriculture is the largest employer in the world and is probably the most weather-dependent of all
human activities.
 Thus, climate variability is to be the principal source of irregularities in global food production
especially in the semiarid tropics.
 Throughout history, climatic extremes have been inflicting chaos on the agricultural systems.
1. Drought
Drought is a weather-related natural disaster. It affects vast regions for months or years. It has an impact on
food production and it reduces life expectancy and the economic performance of large regions or entire
countries.
Drought is a recurrent feature of the climate. It occurs in virtually all climatic zones, and its characteristics
vary significantly among regions. Drought differs from aridity in that drought is temporary; aridity is a
permanent characteristic of regions with low rainfall.
Drought is an insidious hazard of nature. It is related to a deficiency of precipitation over an extended period
of time, usually for a season or more. This deficiency results in a water shortage for some activity, group, or
environmental sector. Drought is also related to the timing of precipitation. Other climatic factors such as
high temperature, high wind, and low relative humidity are often associated with drought.
Drought is more than a physical phenomenon or natural event. Its impact results from the relation between a
natural event and demands on the water supply, and it is often exacerbated by human activities. The
experience from droughts has underscored the vulnerability of human societies to this natural hazard.
An operational definition for agriculture may compare daily precipitation to evapo-transpiration to
determine the rate of soil-moisture depletion, and express these relationships in terms of drought effects on
plant behavior. Operational definitions are used to analyze drought frequency, severity, and duration for a
given historical period. Such definitions, however, require weather data on hourly, daily, monthly, or other
time scales and, possibly, impact data (e.g., crop yield). A climatology of drought for a given region
provides a greater understanding of its characteristics and the probability of recurrence at various levels of
severity. Information of this type is beneficial in the formulation of mitigation strategies.
45
6.2
Types of droughts
A. Meteorological drought
Meteorological drought is defined on the basis of the degree of dryness, in comparison to a normal or
average amount, and the duration of the dry period. Definitions of meteorological drought must be regionspecific, since the atmospheric conditions that result in deficiencies of precipitation are highly regionspecific.
The variety of meteorological definitions in different countries illustrates why it is not possible to apply a
definition of drought developed in one part of the world to another. For instance, the following definitions
of drought have been reported:
 United States (1942): Less than 2.5 mm of rainfall in 48 hours.
 Great Britain (1936): Fifteen consecutive days with daily precipitation less than 0.25
mm.
 Libya (1964): When annual rainfall is less than 180 mm.
 Bali (1964): A period of six days without rain.
Data sets required to assess meteorological drought are daily rainfall information, temperature, humidity,
wind velocity and pressure, and evaporation.
B. Agricultural drought
Agricultural drought links various characteristics of meteorological drought to agricultural impacts, focusing
on precipitation shortages, differences between actual and potential evapo-transpiration, soil-water deficits,
reduced groundwater or reservoir levels, and so on. Plant water demand depends on prevailing weather
conditions, biological characteristics of the specific plant, its stage of growth, and the physical and
biological properties of the soil. A good definition of agricultural drought should account for the
susceptibility of crops during different stages of crop development. Deficient topsoil moisture at planting
may hinder germination, leading to low plant populations per hectare and a reduction of yield.
Data sets required to assess agricultural drought are soil texture, fertility and soil moisture, crop type and
area, crop water requirements, pests and climate.
C. Hydrological drought
Hydrological drought refers to a persistently low discharge and/or volume of water in streams and
reservoirs, lasting months or years. Hydrological drought is a natural phenomenon, but it may be
exacerbated by human activities. Hydrological droughts are usually related to meteorological droughts, and
46
their recurrence interval varies accordingly. Changes in land use and land degradation can affect the
magnitude and frequency of hydrological droughts.
Drought severity
Drought severity is generally judged by:
 the degree of moisture deficiency,
 its duration, and
 the size of the area affected.
When drought is severe, it can result in:
 drying-up of wells
 springs
 small streams and
 depletion of water stored in lakes and reservoirs
However if the drought is brief, it is known as a dry spell, or partial drought
A dry spell is defined as more than 14 days without sufficient rain, whereas a drought may last for
years
Excessive ET can also lead to drought even in the presence of rainfall
This condition can cause:
 shortage of potable water and
 power supply to all sectors of the economy over a wide space & time
Severity of drought varies with:
 Timing:- in relation to stages of crop growth, and
 Intensity:- duration of absence of rains
Other factors influencing crop yields during drought conditions:
 soil characteristics:
 texture, depth
 water retention capacity
 agronomic practices such as:
 weed control
 mulching
The damage is more sever when the period of drought coincides with the critical stages of crop
growth.
Catastrophic droughts generally occur at latitudes of about 15°-20°, in areas bordering the
permanently arid regions of the world.
Permanent aridity is a characteristic of those areas where warm, tropical air masses, in descending to
the earth, which become hotter and drier
A southward shifting westerlies caused most severe droughts over the Sahel regions of Africa for
several years;
D. Socio-economic drought
This type of drought is when some supply of some goods and services such as energy, food and drinking
water are reduced or threatened by changes in meteorological and hydrological conditions. Sometimes it is
even made worse by growing populations and excessive demands of such goods, to the point that it creates
47
stress on the little water available. It takes a very long time for this kind of drought to get into full gear, and
a long time to recover from it.
6.3
Drought Mitigation
Climate change adaptation and mitigation
The distinction between adaptation and mitigation still confuses many who are not so familiar with all
the climate change talk, particularly those coming from fields of work where these words are used to
mean different things.
In the field of climate change, mitigation refers to actions taken to lower the concentration of
greenhouse gasses in the atmosphere and thereby reduce the extent to which the global climate system
changes relative to how we have known it to be in the recent past. Mitigation involves:

reducing or eliminating emissions at the source (e.g. building a wind farm instead of a coalfired power plant, reducing the number of aeroplane/ship/car journeys, avoiding deforestation,
creating more energy efficient, energy neutral, or even energy producing buildings, appliances,
vehicles, etc.), and/or;

Sequestering greenhouse gases out of the atmosphere (e.g. through planting more trees that
absorb CO2, carbon capture and storage facilities at factories and power plants, stimulating
additional sequestration into the ocean via plankton growth, etc.).
Many mitigation methods are still in the research and development phase and for some there are
concerns over possible ‘secondary’ impacts across the system of such interventions, especially when it
comes to large scale geo-engineering options like pumping chemicals into the ocean and upper
atmosphere. National and sub-national governments around the world are in various stages of making
and passing rules on different mitigation targets, audits, standards, methods, etc. for their areas of
jurisdiction.
Adaptation, in the field of climate change, refers to actions taken to reduce the negative
consequences of changes in the climate (e.g. switching to drought resistant crops, creating / protecting
a coastal buffer zone, developing an effective early warning system, building flood barriers, extending
insurance, etc.), or if possible to leverage any positive consequences that may result from such
changes (e.g. growing climate sensitive crops like grapes in new areas, using land that was previously
water-logged, increased water availability in certain places and times of year, etc.). Most often the
evaluation of whether consequences are likely to be positive or negative is done with primary
reference to humans, rather than other species or the earth system as a whole. However, whether a
particular climatic change is positive or negative is rarely common to all people, it usually depends on
the scale considered, the locations affected, the income-generating or livelihood activities people are
involved in, the level of access people have to services (both public and private), etc.
Conclusion
The main difference between adaptation and mitigation lies in the objective that each option pursues.
While mitigation focuses on the causes of climate change by decreasing greenhouse gases in the
48
atmosphere or enhancing the sinks of greenhouse gases, adaptation addresses the impacts of climate
change through an ‘adjustment in natural or human systems in response to actual or expected climatic
stimuli or their effects, which moderates harm or exploits beneficial opportunities’ (IPCC 2001).
Drought is a natural hazard, it has a slow onset, and it evolves over months or even years. It may
affect a large region and causes little structural damage. The impacts of drought can be reduced
through preparedness and mitigation.
The components of a drought preparedness and mitigation plan are the following:




Prediction
Monitoring
Impact assessment
Response,
Prediction can benefit from climate studies which use coupled ocean/atmosphere models, survey of
snow packs, anomalous circulation patterns in the ocean and atmosphere, soil moisture, assimilation
of remotely sensed data into numerical prediction models, and knowledge of stored water available for
domestic, stock, and irrigation uses.
Monitoring exists in countries which use ground-based information such as rainfall, weather, crop
conditions and water availability. Satellite observations complement data collected by ground
systems. Satellites are necessary for the provision of synoptic, wide-area coverage.
Impact assessment is carried out on the basis of land-use type, persistence of stressed conditions,
demographics and existing infrastructure, intensity and areal extent, and its effect on agricultural
yield, public health, water quantity and quality, and building subsidence.
Response includes improved drought monitoring, better water and crop management, augmentation of
water supplies with groundwater, increased public awareness and education, intensified watershed and
local planning, reduction in water demand, and water conservation.
Drought preparedness and mitigation can be accomplished with the following practices: (1) soil and
water conservation, and (2) herd management.
A. Soil and Water Conservation
Conservation practices minimize the disruption of the soil's structure, composition and natural
biodiversity, thereby reducing erosion and soil degradation, surface runoff, and water pollution. The
following are established practices of soil and water conservation:


Crop rotation
Contoured row crops
49







Terracing
Tillage practices
Erosion-control structures
Water retention and detention structures
Windbreaks and shelterbelts
Litter management
Reclamation of salt-affected soil.
Soil and water conservation can be approached through agronomic and engineering
measures. Agronomic measures include contour farming, off-season tillage, deep tillage,
mulching and providing vegetative barriers on the contour. These measures prevent soil
erosion and increase soil moisture.
Engineering measures differ with location, slope of the land, soil type, and amount and
intensity of rainfall. Measures commonly used are the following:

Contour bunds, trenches and stone walls
These features prevent soil erosion and obstruct the flow of runoff. The retained
water increases soil moisture and recharges the groundwater.

Check dams and other gully-plugging structures
Check dams are temporary structures constructed with locally available materials.
Types of check dams are the brush-wood dam (Fig. 2 a), the loose-rock dam (Fig. 2
b) and the woven-wire dam.

Percolation ponds
These features store water for livestock and recharge the groundwater. They are
constructed by excavating a depression to form a small reservoir, or by constructing
an embankment in a natural ravine or gully to form an impoundment.
Water-supply projects can also be implemented for drought mitigation, with a view to
strengthen drought preparedness. Activities such as water-use planning, rain-water
harvesting, runoff collection using surface and underground structures, improved
management of channels and wells, exploration of additional water resources through
drilling and dam construction, are implemented as a part of a drought-mitigation plan.
To increase moisture availability, the following in-situ moisture-conservation
practices can be adopted:

For agricultural crops, measures include ridges and furrows, basins, and water
spreading.
50



For tree crops, measures include saucer basins (Fig. 3), semi-circular bunds,
crescent-shaped bunds, catch pits and deep pitting.
Rainwater harvesting collects rainfall or moisture for immediate or eventual
use in irrigation or domestic supplies. Part of the rainwater collected from
roofs can be stored in a cistern or tank for later use.
Landscape contouring is used to direct runoff into areas planted with trees,
shrubs, and turf.
Farmers can prepare for drought by developing plans which cover all aspects of farm
management and take into account variable climatic conditions. Sustainable strategies
include appropriate fencing to control overgrazing, pest-control measures, planting
drought-resistant crops and pasture, stabilization of eroded soils, pruning plants to
reduce leaf area, removing weak plants and thinning dense beds to reduce
competition, and the protection of native plant species.
B. Herd management
Herd management is an important strategy for drought mitigation. Factors to be
considered include the expected drought duration, the current water and feed supplies,
the composition and body condition of the herd, and the financial resources available.
Herd management practices include the following:

Reduction in herd numbers
When feed resources are getting short, one solution is to critically evaluate the
members of the herd and eliminate those that are less useful. Sale or agistment
(relocating herd to non-affected pastures) are the two options available to
reduce stock numbers.

Strategic weaning of calves
During a drought, the production of milk rapidly depletes a cow's body
reserves, while the calf derives little benefit. Weaning the calf gives the cow a
better chance of survival. However, the decision to wean must be made in
relation to the time of year and age of the calf. In normal years, the nutritive
value of pasture falls towards the end of autumn, at which time, beef cows may
be producing as little as 1 liter of milk per day. If the calf is 5 to 6 months of
age, weaning by the end of autumn will maintain or improve the cow's
condition.
In drought years, early weaning is recommended. However, calves should not
be weaned before 3 months of age unless absolutely necessary. Young calves
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need to be fed some true protein meal or preferably milk powder. Most calves
over three months of age will survive on grain and Lucerne hay or molasses
and protein meal diets.

Herd segregation
Segregating animals into classes gives the herd a better chance of getting
needed feed supplies. Segregation makes possible the preferential treatment of
vulnerable classes. The older dry cows can be moved to the poorer forage
fields. Pregnancy testing is a useful tool to identify heavily pregnant cows for
special feeding, especially young cows that are pregnant for a second time.

Parasite control
Cattle under nutritional and other stresses are less resistant to parasites than in
normal conditions. Worms can be a serious problem with young cattle. During
drought conditions, all cattle under 18 months of age should be treated for
worms.

Optimizing use of drought-affected paddock/land
Cattle do not graze well areas located far away from watering points. Use of a
drought-affected paddock can be encouraged by providing local water
facilities, with supplementary hand feeding as an attractant.

Attention to contaminated water supplies
Polluted surface waters represent a death trap for drought-weakened cattle.
Fencing may be necessary to separate cattle from undesirable water holes.
Salinity may increase with the depletion of the water table, with the water
becoming too salty for the herd. The upper limit of total soluble salts should
not exceed 8500 ppm. In addition, the sum of chlorides and sulphates of
calcium and magnesium should not exceed 1400 ppm.
Potential problems arising from drought conditions are:



Use of salt to limit feed intake may increase water intake 50 to 75%, or
approximately 50 gallons of additional water for each pound of salt. Water
must not be limited in any way or salt toxicity may result.
Over consumption of urea-containing supplements by cattle grazing on foragescarce ranges can result in urea toxicity. Generally, performance of cattle on
urea-type supplements is poor wherever forage is in short supply.
Hay cut under moisture-stress conditions, especially sorghum-type hays, may
52


contain high levels of nitrate. Tests for nitrate should be performed before
feeding these hays, especially before feeding large amounts. Farmers who cut
drought corn or sorghum for hay should check nitrate levels before feeding.
Prussic acid or cyanide poisoning can be a problem in grazing drought-stunted
plants such as Johnson grass, sorghum, sorghum hybrids, and sudan grass.
Prussic acid is generally not a problem when forage for hay is allowed to sun
cure for 3 to 5 days, in order to bleach out any bright green color.
Cattle that graze short or drought-stunted pasture are more likely to consume
toxic plants.
Climate change impact on insect pests occurrence
Changes in climate may result in changes in geographical distribution of species, changes in population
growth rates, increases in the number of generations, extension of the growing season, changes in crop-pest
synchrony, changes in interspecific interactions and increased risk of invasion by migrant pests (Porter et
al., 1991).
Insects are exothermic organisms; the temperature of their bodies is dependent on that of the environment.
Therefore, temperature is probably the single most important environmental factor influencing insect
behavior, distribution, development, survival, and reproduction (Kocmánková et al., 2009).
Impact of Temperature rises on insect pests
Higher average temperature might result in some crops being able to be grown in regions further north – it is
likely that at least some of the insect pests of those crops will follow the expanded crop areas. Insect species
diversity per area tends to decrease with higher latitude and altitude (Andrew & Hughes, 2005), meaning
that rising temperatures could result in more insect species attacking hosts in temperate climates. Based on
the evidence developed by studying the fossil record, some researchers (Bale et al., 2002) conclude that the
diversity of insect species and the intensity of their feeding have increased historically with increasing
temperature.
Insects are cold-blooded organisms - the temperature of their bodies is approximately the same as that of the
environment. Therefore, temperature is probably the single most important environmental factor influencing
insect behavior, distribution, development, survival, and reproduction. Insect life stage predictions are most
often calculated using accumulated degree days from a base temperature and biofix point. Some researchers
believe that the effect of temperature on insects largely overwhelms the effects of other environmental
factors (Bale et al., 2002).
There is likely to be an increase of the number of generations of pests and higher population density in
connection with the prolonged growing season. An increase in the number of generations means an increase
in the number of reproductive occasions per year. It has been estimated that with a 2°C temperature increase
in temperate climate zones insects might experience one to five additional life cycles per season (Yamamura
& Kiritani, 1998). If the mortality per generation does not change, the insect population will become
potentially larger under global warming. This fact could play an important role in the case of multivoltine
species, most of them are expected to wider their occurrence to higher latitudes and altitudes as was
recorded e.g. in many cases of butterflies. Warmer conditions may be expected to promote the poleward
extension of the range of species currently limited by low temperature or the altitude at which they can
53
survive. A 2°C rise in temperature, which is expected in northern temperate latitudes over the next century,
is equivalent to a shift of current conditions of 600 km latitude or 330 m in elevation. Some researchers
declare that the effect of temperature on insects largely overhangs the effects of other environmental factors
(Bale et al, 2002).
Insect life stage predictions are most often calculated using accumulated degree days from a base
temperature. Climate matching is a common model tool for the estimates of the impact of climate change on
the species’ extension. Climate matching identifies extralimital destinations that could be colonised by a
potential invasive species on the basis of similarity to climates found in the species’ native range (Rodda et
al., 2007). Model validation is carried out by comparing modeled and observed current pest’s occurrence,
obtained from field observations. A number of studies considering the climate change impact on various
organisms. For instance, according to one of the first studies focused on the impact of climate change on
European corn borer populations in Europe (Porter et al., 1991), a temperature increase associated with
ongoing climate change would lead to the shift of the ECB occurrence area to the north and possibly to the
occurrence of the second generation in the presently univoltine areas.
Other studies assess the climate change impacts on selected pests especially on the most important potato
pest, i.e. Colorado potato beetle and the most important pest of grain maize, i.e. European corn borer in the
Czech Republic. These two species are expected to wider their occurrence area and increase the infestation
pressure due to the climate change. The development of these species is closely related to climate conditions
and mainly to the course of temperature (Kocmánková et al, 2009).
Rising CO2 levels affect insects
Generally CO2 impacts on insects are thought to be indirect - impact on insect damage results from changes
in the host crop. Some researchers have found that rising CO2 can potentially have important effects on
insect pest problems. Recently, free air gas concentration enrichment (FACE) technology was used to create
an atmosphere with CO2 and O2 concentrations similar to what climate change models predict for the
middle of the 21st century. FACE allows for field testing of crop situations with fewer limitations than those
conducted in enclosed spaces. During the early season, soybeans grown in elevated CO2 atmosphere had
57% more damage from insects (primarily Japanese beetle, potato leafhopper, western corn rootworm and
Mexican bean beetle) than those grown in today’s atmosphere, and required an insecticide treatment in order
to continue the experiment. It is thought that measured increases in the levels of simple sugars in the
soybean leaves may have stimulated the additional insect feeding (Hamilton et al. 2005).
Although not observed in the FACE study, other researchers have observed that insects sometimes feed
more on leaves that have lowered nitrogen content in order to obtain sufficient nitrogen for their metabolism
(Hunter, 2001). Increased carbon to nitrogen ratios in plant tissue resulting from increased CO2 levels may
slow insect development and increase the length of life stages vulnerable to attack by parasitoids (Coviella
and Trumble, 1999).
Changes in precipitation will affect insects
There are fewer scientific studies on the effect of precipitation on insects than temperature. Some insects are
sensitive to precipitation and are killed or removed from crops by heavy rains - in some northeastern US
54
states, this consideration is important when choosing management options for onion thrips (Reiners and
Petzoldt 2005). For some insects that overwinter in soil, such as the cranberry fruitworm and other cranberry
insect pests, flooding the soil has been used as a control measure (Vincent et al 2003). One would expect the
predicted more frequent and intense precipitation events forecasted with climate change to negatively
impact these insects. Other insects such as pea aphids are not tolerant of drought (Macvean and Dixon
2001). As with temperature, precipitation changes can impact insect pest predators, parasites, and diseases
resulting in a complex dynamic.
Climate impact on the occurrence of plant diseases
The three legs of the triangle – host, pathogen, and environment – must be present and interact appropriately
for a plant disease to result. If any of the 3 factors is altered, changes in the progression of a disease
epidemic can occur. The major predicted results of climate change – increased temperature, moisture and
CO2 – can impact all three legs of the plant disease triangle in various ways (Coakley et al. 1999).
Impact of temperature rises
Temperature governs the rate of reproduction for many pathogens, longer seasons that result from higher
temperatures will allow more time for pathogen development. Pathogen development may also be more
rapid when large pathogen populations are present, so increased overwintering and over summering rates
will also contribute. Under climate change, pathogens, like plants, may potentially be unable to migrate or
adapt as rapidly as environmental conditions change. But most pathogens will have the advantage over
plants because of their shorter generation times and the ability to move readily through wind dispersal.
Research has shown that host plants such as wheat and oats become more susceptible to rust diseases with
increased temperature (Coakley et al. 1999). Generally, fungi that cause plant disease grow best in moderate
temperature ranges. For example, predictive models for potato late blight (caused by Phytophthora
infestans) show that the fungus infects and reproduces most successfully during periods of high moisture
that occur when temperatures are between 7.2°C and 26.8°C (Wallin & Waggoner 1950). Earlier onset of
warm temperatures could result in an earlier threat from Potato late blight (PLB) with the potential for more
severe epidemics and increases in the number of fungicide applications needed for control.
Changes in moisture will affect pathogens and disease
Moisture can impact both host plants and pathogen organisms in various ways. Some pathogens such as
apple scab, late blight, and several vegetable root pathogens are more likely to infect plants with increased
moisture – forecast models for these diseases are based on leaf wetness, relative humidity and precipitation
measurements. Other pathogens like the powdery mildew species tend to thrive in conditions with lower
(but not low) moisture. More frequent and extreme precipitation events that are predicted by some climate
change models could result in more and longer periods with favorable pathogen environments. Host crops
with canopy size limited by lack of moisture might no longer be so limited and may produce canopies that
hold moisture in the form of leaf wetness or high canopy relative humidity for longer periods, thus
increasing the risk from pathogen infection (Coakley et al, 1999). Some climate change models predict
55
higher atmospheric water vapor concentrations with increased temperature – this also would favor pathogen
and disease development.
Rising CO2 levels affect pathogens and disease
Increased CO2 levels can impact both the host and the pathogen in multiple ways. Some of the observed CO2
effects on disease may counteract others. Researchers have shown that higher growth rates of leaves and
stems observed for plants grown under high CO2 concentrations may result in denser canopies with higher
humidity that favor pathogens. Lower plant decomposition rates observed in high CO2 situations could
increase the crop residue on which disease organisms can overwinter, resulting in higher inoculum levels at
the beginning of the growing season, and earlier and faster disease epidemics. Pathogen growth can be
affected by higher CO2 concentrations resulting in greater fungal spore production. However, increased CO2
can result in physiological changes to the host plant that can increase host resistance to pathogens (Coakley
et al. 1999).
Lower plant decomposition rates observed in high CO2 situations could increase the crop residue on which
disease organisms can overwinter, resulting in higher inoculum levels at the beginning of the growing
season, and earlier and faster disease epidemics. Pathogen growth can be affected by higher CO2
concentrations resulting in greater fungal spore production. However, increased CO2 can result in
physiological changes to the host plant that can increase host resistance to pathogens (Kocmánková et al.,
2009).
This is neither published nor accredited. It is
just for teaching purpose.
i
ii
death of cells which occurs as a normal part of an organism’s development
a soft plant tissue containing air spaces, found especially in many aquatic pants
56