Atmospheric Processes, Hazards and Management
Structure and Composition of the Atmosphere
1. Vertical Stratification
1.1 Troposphere
 Bottom most layer, 80% of atmosphere, most of water vapour and dust
 Vertical mixing of air, all weather occurs here
 Temperature decreases with height, 15C near ground, -57C at tropopause
 Due to lesser terrestrial radiation and less dense air
 Reaches 16 km in tropics and 9km in polar regions
1.2 Stratosphere
 19.9% of atmosphere
 Temperature constant until 20km, starts to increase until stratopause at 50km
 Due to ozone layer, which absorbs UV rays. Lesser UV with decreasing height.
 No precipitation – particulates and aerosols stay for long time
 Strong winds (jet streams) distribute aerosols all over the world
1.3 Mesosphere
 Temperature decreases with height because of decreasing density.
 At 80km, about -90C
1.4 Thermosphere
 Minimal mass of atmosphere, pressure about one-millionth of at sea level
 Temperature increases with height due to insolation
2. Atmosphere’s Gaseous Components
 Atmosphere composed of mixture of gases and microscopic solid particles and water
droplets
2.1 Permanent Gases
 Nitrogen and oxygen make up 99% of clean, dry air
2.2 Variable Gases
2.2.1 Water Vapour
 Source is evaporation from Earth’s surface, decreases rapidly with height
 Most found in lowest 5km of atmosphere
 Source of moisture to form clouds, also a greenhouse gas
2.2.2 Carbon Dioxide
 0.037% of the atmosphere
 Supplied via the carbon cycle – decomposition, respiration, photosynthesis
 Absorbs radiation, greenhouse gas
Earth’s Energy Budget and Regional Temperature Variations
1. Earth’s Energy Budget
 Variation in solar energy received from places results in movement of air from one place
to another, resulting in atmospheric circulation and oceanic circulation
1.1 Global Energy Balance
 Balance exists between insolation and outgoing terrestrial radiation so that the net
radiation is zero
 Insolation is scattered by gas and dust particles in the atmosphere, reflected by
clouds and Earth’s surface, and absorbed by atmosphere and Earth’s surface
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
Outgoing radiation is released from the atmosphere and Earth’s surface, and
removed by wind and condensation
1.1.1 Scattering
 Some insolation will be scattered in all directions until it reaches Earth’s
surface or returns to space
 This occurs because radiation travels in a straight line
1.1.2 Reflection
 Most important loss of insolation
 Light coloured and smooth, shiny surfaces will reflect more radiation
 Albedo – snow is about 90%, at low angles water is nearly 80%
 In general albedo of sea lower than land
1.1.3 Absorption
 Some gases in atmosphere absorb certain wavelengths of radiation, which
gain energy and heat up
 Insolation is converted to long-wave radiation to warm atmosphere
 Greenhouse gases, oxygen, ozone, carbon dioxide, water vapour
1.1.4 The Greenhouse Effect
 Terrestrial radiation has longer wavelengths due to being cooler that sun
 Atmosphere absorbs long wave easier than short wave – greenhouse effect,
gases absorb and re-emit radiation to warm the Earth
1.2 Poleward Heat Transfer
 Net radiation for Earth is zero, but not in all latitudes. Most energy received at
tropics.
 In poles, low angle of sun, greater albedo of snow and ice, thicker atmosphere for
sun to pass through, insolation is much smaller.
 Major equalising factor is transfer of heat by air movement, creating air currents,
winds and ocean currents and weather
1.2.1 Latitudinal Difference in Insolation amount
 Beam spreading: at tropics, sun directly overhead, insolation more
concentrated, more radiation per unit area. With increasing latitude, solar
radiation reaches surface at lower angle, spreading out more, reducing
insolation per unit area.
 Passing through atmosphere: at higher latitudes, lower angle of approach
means travelling through more atmosphere. More energy is absorbed,
scattered or reflected at polar areas than equatorial areas.
 Albedo: higher at poles due to snow, ice, and low angle over water
1.2.2 Latitudinal Radiation Balance
 Difference between incoming and outgoing radiation is radiation balance
 Positive budget in tropics (heat surplus) and negative budget in poles (heat
deficit)
 Redistribution of heat via advection of winds and ocean currents
2. Regional Temperature Variations
2.1 Proximity to Sea
 Maritime / oceanic areas have less seasonal contrasts, milder seasons
 Moderating effect of oceans – cooler summers and warmer winters
 Water has 5 times specific heat capacity than land, heats and cools slower
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2.2
2.3
2.4
2.5
 Radiation can penetrate water up to several meters, distributing throughout larger
mass. In land, restricted to a thin, opaque layer
 Warming of water reduced considerably due to energy used for evaporation
 Water can be mixed vertically and horizontally
Ocean Currents
 Warm currents move poleward in western portions of ocean basins near east coasts
of continents. e.g. Gulf Stream
 Eastern margins of oceans, cold ocean currents travel equatorwards. e.g Labrador
Current
Altitude
 Temperatures decrease with altitude – every 1km = 10C drop
 Low altitude – heat escapes slowly due to dense air, containing dust and water
vapour, retaining hear
 High altitude – thinner air, heat escapes rapidly
Cloud Cover
 Cloud cover decreases insolation reaching surface and amount leaving surface
 Due to reflection, high albedo of white clouds
Aspect
 Direction a place faces, noticeably in temperate latitudes.
 South facing slopes / adret slopes are warmer than north facing slopes / ubac slopes
in the Northern Hemisphere. Vice versa in Southern.
 The warmer slope receives midday sunlight at a more direct angle
Atmospheric/Oceanic Circulations and the Occurrence of Seasons
1. Global Atmospheric Circulation
 Unequal heating of Earth’s surface leads to thermally direct circulations
1.1 The Tri Cellular Model
 Proposed by William Ferrel, incorporating Hadley’s ideas.
1.2 Forces Influencing Air Movement
1.2.1 Pressure Gradient Effect
 In denser atmosphere, lateral pressure is great. Less dense, lesser pressure.
Horizontal pressure difference, air pushed from high pressure to lower
pressure, called the pressure gradient effect.
 Greater the difference, greater the wind speed. Unequal heating of land-sea
and different latitudes creates this difference.
 Magnitude determines by spacing of isobars. Direction always high to low
pressure at right angles to isobars.
1.2.2 Coriolis Effect
 Causes winds to appear to deflect to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere
 Greatest at poles, weakens equatorward where it becomes nonexistent
 Always deflected at right angles to direction of air flow
 Only affects direction, not speed
 Affected by wind speed – stronger the wind, greater the deflection
1.2.3 Friction
 Opposes pressure gradient to moderate and slow airflow
 Significant influence near surface, but negligible a few km above
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1.2.4
Geostrophic Wind
 Balance between Coriolis effect and pressure gradient
 Pressure gradient perpendicular to isobars at starting point. Coriolis no
effect because it is stationary.
 As air accelerates perpendicular to isobars, Coriolis intensifies since it
depends on wind speed. Deflects it until it is parallel to isobars, balance
 Geostrophic winds flow straight, parallel to isobars, velocity proportional to
pressure gradient
 An idealised model which approximates airflow aloft
1.2.5 Surface Wind
 At surface, friction slows movement of air, reducing Coriolis as well.
 Pressure gradient not affected by friction, so stronger than Coriolis
 Air moves at an angle across isobars
1.3 The Major Wind Belts
1.3.1 Hadley Cells and the Trade Winds
 Between equator and 30 latitude. Produced thermally, stronger in winter
when temperature gradients are stronger
 At equator, warm rising air releases latent heat, provides energy to drive
cell. At upper flow, air moves poleward, acquiring westerly movement due
to Coriolis.
 At subtropical highs, air subsides due to radiation cooling away from the
equator, increasing density
 Air is very dry, and adiabatic heating reduces relative humidity further. Dry
subsidence zones create the world’s subtropical deserts
 Surface flow splits into poleward and equatorward branches. Equatorward
turns right by Coriolis to from trade winds.
 Both Hadley Cells converge at the Intertropical Convergence Zone (ITCZ)
 ITCZ shifts north and south of the equator, but north more (30-40N
compared to 5S) due to more land in north hemisphere
1.3.2 Ferrel Cells and the Westerlies
 Poleward from subtropical highs is the Westerlies
 Prevailing westerlies, west only on average. Cyclones and anticyclones
disrupt the westerly movement
 Thermally indirect cells caused by turning of the other two
1.3.3 Polar Cells and the Polar Easterlies
 Subsidence near poles at polar highs produce surface flow which moves
equatorward and deflected into polar easterlies
 They meet the warmer westerly wind at the subpolar lows, or polar fronts
1.4 The Tri Cellular Model vs Reality
 The Hadley model provides a good account of low-latitude motions, but the Ferrel
and Polar cells not very well represented in reality, due to local factors
 With regards to upper-level motions, not realistic.
1.5 Observed Pressure and Wind Patterns
1.5.1 Zonal vs Cellular Distribution of Pressure
 Due to variations in land-sea surfaces, pressure systems rarely form
continuous belts around the earth.
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 In northern hemisphere, with higher proportion of land, the zonal pattern is
replaced by semi-permanent cells of high and low pressure.
 High pressure cells are anticyclones, low pressure cells are cyclones
 Air spirals clockwise out of anticyclones and anticlockwise into cyclones in
the northern hemisphere, vice versa for south.
 Changes in highs and lows affected by seasonal temperature changes
1.5.2 Seasonal Changes in Global Pressure/Wind Patterns
 Seasonal change in pressure more noticeable in northern hemisphere
 Subtropical anticyclones: centred between 20 and 35 latitude over larger
oceans, like Hawaiian and Bermuda-Azores highs
 More to east of oceans because of the cold currents
 Seasonal fluctuations: January, strong Siberian High. Subtropical
anticyclones weaken, and two intense cyclones (Aleutians Low and Icelandic
Low) dominate
 In summer, Siberian High replaced by Tibetan Low. Icelandic low weakens,
Bermuda-Azores High strengthens due to change in temperature contrasts
1.6 Monsoon Circulation
 Pronounced seasonal reversal in wind direction
 January – ITCZ in southern hemisphere, Siberian High produces cool, dry,
continental air
 July – ITCZ in northern hemisphere, Tibetan Low draws warm moist air from oceans
onto Asia, producing summer rains
2. Global Oceanic Circulation
 Accounts for ¼ of total heat transport
 Atmosphere and ocean in contact, energy passed from moving air to water
3. Seasonal Climatic Variations
 Latitudinal differences, tilt and revolution of Earth around sun causes seasons
3.1 Revolution of the Earth and Seasonal Climatic Changes
 Earth’s axis is tilted 23½ from perpendicular in relation to orbit
 Change in orientation throughout orbit causes the subsolar point to migrate
annually from Tropic of Cancer in N to Tropic of Capricorn in S.
 Causes angle, and solar radiation to vary within a year, causing seasons in midlatitudes
3.2 Solstices and Equinoxes
 June 21/22 and December 21/22 are solstices, when the subsolar point is at Tropic
of Cancer or Capricorn
 March 21/22 and September 22/23 are equinoxes which occur halfway between the
solstices. the subsolar point hits the equator.
3.3 Seasonal Variation in Daylight Hours
 Length of daylight vs darkness at a latitude depends on the position of Earth in orbit
 June solstice, all locations in northern hemisphere experience longer periods of
daylight than darkness, further north = longer daylight. Vice versa in December
solstice. Southern hemisphere is other way round.
 Equinox – all latitudes experience 12 hours of daylight and darkness
Upper Atmosphere Wind and Jet Streams
1. The Westerlies
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 Airflow in the middle and upper troposphere is westerly. As cold air is more dense and
compact, air pressure decreases more rapidly when cold than warm
 At same altitude over Earth’s surface, higher pressure exists over tropics than at poles
 Pressure gradient directs air from equator to poles, turned by Coriolis to generate
westerly winds (geostrophic winds)
 Pressure gradient increases with altitude, and decreasing friction also contributes to fast
wind speed
2. Jet Streams
 Narrow ribbons of high-speed winds
2.1 Origin of Jet Streams
 Greater temperature contrasts at surface produce greater pressure gradients aloft
and faster upper air winds
 Large wintertime temperature contrasts occur along fronts, resulting in very fast
westerly winds
2.2 The Polar Jet Stream
 The polar jet stream often meanders
 Travels at 125km/h in winter and about 60km/h in summer, due to stronger
temperature contrasts in winter
 Position also changes – during winter, the jet stream may extend south till 30N
 Polar jet migrates between latitudes of 30 and 79, mid latitude jet stream
2.3 Jet Stream and Weather
2.3.1 Jet Streams and Seasonal Cyclonic Activities
 As polar jet shifts northwards from winter to summer, regions where
thunderstorms and tornadoes also shift northwards
 More cyclones are generated in late summer and early autumn when
temperature contrasts are greater
 Jet streams supply energy to circulation of surface storms
2.3.2 Jet Streams and Short Term Cyclonic Activities
 Can change positions on shorter time scales as well
 When jet stream does not meander, relatively mild temperatures occur and
few disturbance south of jet stream
 When meander, large amplitude waves form and a general north-south
flow occurs, allowing cold air to advance southward, intensifying
temperature gradient and strengthening flow aloft
 Cyclonic activities redistribute heat, resulting in weaker temperature
gradient, weaker flow and less intense weather at surface
 Cycles of calm and stormy weather can last 1 to 6 weeks
 When air moves out of trough, accelerates, drawing air from lower
atmosphere causing reduction in surface pressure. Air converges at ground
within depression
 If divergence exceeds convergence = low pressure, if convergence exceeds
divergence = high pressure
 In N hemisphere, troughs and ridges tend to favour certain locations
2.3.3 Jet Streams and Temperature Variations
 If jet stream south of location, temperature will be colder and stormier
 If north, warmer and drier
 Wave development affects local temperatures
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 If position of jet stream remains fixed for long periods, weather extremes
can result
3. Rossby Waves
 Westerlies follow wavy paths with long wavelengths and varying amplitudes
 A wave effect as a result of the polar jet stream
 Air flows eastward, but occasionally remain stationary or even slowly from west to east
 Flow aloft is not perfectly circular, amplitudes distribute air temperature
 Surface features also play a role in the development of Rossby Waves
Precipitation Formation
1. Evaporation
 Energy required to excite water molecules is latent heat of vaporisation
 Depends on the vapour pressure deficit, the difference between saturation and actual
vapour pressure
 When air is saturated, no evaporation can occur
2. Condensation
 Air should be saturated for condensation to take place
 There generally must be a surface on which water vapour condenses
2.1 Saturation of an Air Parcel
 Air needs to reach saturation point, either by cooling below dew point, or add water
to air parcel
 Cooling of air by expansion (adiabatic) to form clouds is more common. Radiative
cooling at a cold ground surface forms fog.
2.2 Role of the Condensation Nuclei
 At dew point, condensation occurs around particles in the air known as
condensation nuclei
 Sources are dust, volcanic material, smoke and sea salt
 Some nuclei particularly attract water, such as sea salt. Hygroscopic nuclei. Air can
condense even when not saturated
3. Environmental Lapse Rate
 Normally, air temperature falls with increasing height in the troposphere, since heating
is greatest at ground surface
 Rate at which temperature falls is the environmental lapse rate (ELR)
 ELR depends on atmospheric and surface conditions
 Strong winds can mix air and lower ELR. Strong surface heating can cause ELR to be
steeper.
 ELR influences air movement and determines if atmosphere is stable
 Inversions exist where temperature increases with altitude, and are thus extremely
stable and resist vertical mixing
4. Atmospheric Stability/Instability
 Parcel of air, Dry Adiabatic Lapse Rate (DALR) and Wet Adiabatic Lapse Rate (SALR)
 Stable ELR falls slower than DALR and SALR
 Unstable if ELR falls faster than DALR and SALR
 Conditionally stable if ELR falls slower than DALR but faster than SALR
5. Air Lifting Mechanisms
 Rising air expands and cools adiabatically, reaches lifting condensation level, cools to
dew point and condenses into clouds
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5.1 Convective Lifting
 When air heated from below rises. Unstable air found in areas with strong surface
heating such as the humid tropics
 Solar heating and irregular surfaces cause irregular heating on ground and some
areas to heat up more than others. These warm, unstable parcels of air are buoyed
upward. If reach condensation level clouds form
 Height of clouds produced is limited, as instability of surface heating is confined to
first few km of atmosphere
5.2 Orographic Lifting
 Air mass forced to ascend elevated terrains such as mountains
 Adiabatic cooling encourages condensation, generating clouds and precipitation on
the windward slope
 On leeward side, most moisture has been lost, and with further adiabatic heating
upon descending, condensation and precipitation is less likely, and can cause
rainshadow deserts
5.3 Frontal Lifting
 Transition zones where great temperature differences occur across short distances
are called fronts
 Cold air advances towards warm air, displacing it up
 Warm air advances towards cold air, forced upwards
5.4 Convergence
 When low pressure cell is at surface winds converge on centre of low from all
directions, rising and cooling
Climatic Zones of Tropical Africa and Asia
1. The Koeppen Classification System
 Based on vegetation, monthly mean temperature and precipitation
1.1 The Main Climatic Groups
 A – Tropical. Climates in which temperature for all months is greater than 18C
 Almost confined between Tropics of Cancer and Capricorn
 B – Dry
 Potential evaporation exceeds precipitation
 Only B considers precipitation, designating deserts and semi-deserts. The rest are
classified by temperature.
1.2 The Sub-Climatic Groups
 Af, Am, Aw, and BWh
2. Features of Climatic Zones in Africa and Asia
 A climates are warm year round with minor variations in temperature. They are
distinguished by their degrees of precipitation seasonality. Af has no dry season, Am
are monsoonal with short dry season of 1-3 months, Aw have a distinct dry season
coinciding with subtropical high pressure of the Hadley cell
 B are divided into BW (true deserts) and BS (semi-deserts)
2.1 Tropical Wet/Rainforest Climate (Af)
 Found in equatorial regions, around 10 N and S. Constant influence of ITCZ, with a
lot of convectional rain due to strong solar heating
2.1.1 Precipitation Characteristics
 High total annual rainfall, distributed nearly uniformly
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2.1.2
Temperature Characteristics
 Uniformly high temperatures, but not the highest on earth, due to moisture
available for evaporation and cumulus clouds reflecting and scattering
insolation
 Diurnal range is lower due to high humidity and cloud cover. Diurnal range
normally exceeds annual range
2.1.3 Natural Vegetation
 Prevalence of tropical rainforests, with dense tree cover and species
diversity
2.2 Tropical Monsoon Climate (Am)
 Transition between Af and Aw. Occur along tropical, coastal areas and predominant
onshore winds supply warm, moist air to region.
2.2.1 Precipitation Characteristics
 Not very far inland due to offshore air required. Rainfall enhanced by
orographic uplift. Surface heating is less a factor. Hurricanes can also bring
heavy rainfall
 Precipitation not as steady throughout year. Annual precipitation are
among highest in world due to very heavy rainfall during wet months
2.2.2 Temperature Characteristics
 All months warm, little variation. Variation associated with timing of
precipitation. Warmest months just before precipitation season due to
lesser cloud cover
2.2.3 Natural Vegetation
 Dense forests, lush vegetation. Diversity not as abundant as Af, but much
more living matter
2.3 Tropical Wet and Dry/Savanna Climate (Aw)
 Along poleward margins of the tropics, most extensive in southern Africa.
 Farther from Equator, more seasonality in precipitation and temperature
2.3.1 Precipitation Characteristics
 When ITCZ is near in summer, convectional rain is favoured
 When ITCZ shifts south in winter, subtropical high brings descending air,
stable atmosphere and lack of precipitation.
 Being further from Equator means being closer to the subtropical high.
Occasional hurricanes also brings precipitation
 Annually, lesser precipitation than Af or Am
 Considerable year-to-year variability in precipitation. Droughts or flooding
possible
2.3.2 Temperature Characteristics
 Monthly mean temperatures more variable, about 3-10C
 Diurnal variations are greater as well, especially during dry season due to
lack of cloud cover
2.3.3 Natural Vegetation
 Savanna – grasses with widely separated trees
 Recurrent fire, water logged soil and hard layers developing in soil
2.4 Subtropical Deserts (BWh)
 Subtropical deserts – subsidence of the Hadley cell at the subtropical highs
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 Adiabatic heating with descending air increases air temperature, reduces relative
humidity and a stable atmosphere which impedes condensation and cloud
formation
 Some coastal deserts such as the Atacama Desert in Chile occur, along cold ocean
currents on west coast of continents. Air flows out of subtropical high pressure
system, cooling over the Peruvian Current, bringing air temperature down to dew
point and creating fog via contact cooling, lowering the ELR and causing stability,
which suppresses uplift and cloud and precipitation formation
2.4.1 Precipitation Characteristics
 Very little precipitation over the year – any rainfall occurs during the winter
months
 Greatest amount of inter-annual variability of rainfall – annual average is
poor indicator of rainfall in a single year
2.4.2 Temperature Characteristics
 Diurnal temperature range can be very large, due to low humidity, high sun
and clear skies allowing solar radiation to be relatively unhindered
 Low soil moisture means little heat is lost in evaporation
 Similarly, no cloud cover and low humidity means heat is lost quickly at
night
 The same applies to the annual temperature range
3. Delineating the Boundaries of the Climatic Zones
 A strength of the Koeppen system is that boundaries are determined easily, but these
boundaries should not be seen as fixed and unchanging
 All boundaries shift their positions constantly – current boundaries are only averages
based on data collected. Thus, they should be viewed as broad transitions zones instead
of sharp lines
Adverse Weather Conditions: Tropical Cyclones
1. Formation and Characteristics of Tropical Cyclones
1.1 Formation of Tropical Cyclones
 Strong winds of at least 119km/h and a rotary circulation
 Requires very warm, moist air – most occur in late summer when ocean
temperatures reach 27C or higher, so they often do not occur in colder waters of the
S Atlantic or E S Pacific, or beyond 20 degrees of latitude
 Sufficient spin or twist provided by the Coriolis effect, so usually do not form within
5 degrees of Equator
 Warm sea heats air above, warm moist air rises creating low pressure
 Wind rushes into low pressure zone spiralling upwards, releasing heat and moisture
 Rising air cools, creating cumulonimbus clouds which release latent heat upon
condensation, rising higher. Self-sustaining heat energy increases speed of the
system
1.2 Characteristics of Tropical Cyclones
1.2.1 Location of Cyclone Occurrences
 Most occur between 5 and 20 latitude over tropical oceans except South
Atlantic and Eastern South Pacific due to cold currents
 N hemisphere – occur between June and December in Atlantic Pacific and
Indian Oceans.
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 S hemisphere – occur between January and March in Pacific and Indian
oceans
1.2.2 Cyclone Paths
 Initially move west due to trade winds, but easterly influence diminishes
after intensity increases
 Upper level winds and spatial distribution of water temperature are key
factors determining their speed and direction. Tend to move towards
warmer seas
 Once developed, they are likely to move poleward and move into influence
of westerlies, thus moving eastward
 Irregular paths make movement unpredictable
1.2.3 Features of a Tropical Cyclone
 Average about 600km across, the eye about 25km across
 The eye is a zone where precipitation stops and winds subside
 Air within eye descends and heats by compression, making it warm.
However, the subsidence is often not strong enough to inhibit cloud
formation
 Beyond the eye is the eye wall, a zone of intense convective activity where
the greatest wind speeds, thickest cloud cover and heaviest rainfall occur
 Consists of a large number of thunderstorms arranged in a pinwheel
formation
2. Dissipation of Tropical Cyclones
 When move over ocean waters that cannot support warm moist tropical air
 When reaching a location where large-scale flow aloft is unfavourable
 When moving onto land, due to depletion of water source, cooler air and increased
surface roughness and friction
3. Impacts of Tropical Cyclones
3.1 Storm Surge
 In coastal zone, 90% of deaths and majority of damage is caused by storm surge
 A dome of water 65-80km long which sweeps across the coast
 Due to piling up of water when heavy winds drag surface waters landward, bringing
heavy surf. Also, low atmospheric pressure causes water level to rise by 1cm every
millibar
 Drowning, inundation, erosion, saline intrusion, infrastructure damange
 Bangladesh – most land less than 2m above sea level. Nov 13 1970, official death toll
200k, but unofficial about 500k
3.2 Wind Damage
 Crop destruction, flying debris, spreading of fires in urban and forested areas
 US$20billion of damage in southern Florida and Louisiana due to Hurricane Andrew
3.3 Heavy Rainfall
 Can affect places 100s of km from the coast for several days after the storm’s winds
have slowed
 Loss of life, property damage, crop destruction from flooding. Contamination of
water can lead to disease outbreaks. Landslides or mudflows. Saline intrusion.
 Hurricane Agnes in 1972 – US$2 billion in damage and 122 lives due to flooding
3.4 Cyclone Damages – A Comparison
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 In coastal zones, storm surge is usually the cause of extensive damage and loss of
lives
 Wind damage not as catastrophic, but affects a larger area, especially when building
codes are inadequate. Also, most wind damage occurs 200km of coast since
cyclones weaken inland
 Far from coast, rainfall and flooding can still cause damages, sometimes greater
than storm surge
4. Management of Cyclonic Hazards
 No atmospheric conditions can be measured to predict cyclone development, only
forecasting its path is possible
4.1 Use of the Saffir-Simpson Scale
 Ranks relative intensities of tropical cyclones
 Advantage of being related to property damage, can be used to predict
 Disaster potential can be monitored and precautions can be planned and
implemented
4.2 Predicting and Monitoring Tropical Cyclones
4.2.1 The Atlantic Tropical Cyclone Prediction Model
 Forecasters use atmospheric models to predict tracks of tropical cyclones to
provide storm warnings or evacuations. Colorado State University attempt
to predict degree of activity up to eleven months in advance of hurricane
season in North Atlantic
 Forecasts based on series of indices which show statistical relationships
with cyclonic activity, such as the El Nino Southern Oscillation which
reduces hurricane activity, as well as ocean surface temperatures
4.2.2 The Role of Satellites
 Detect and track cyclones even before cyclonic flow is developed
 Improved detection of tropical storms and monitoring
 However, by itself is not entirely accurate. Errors of up to 10s of km in
position or wind speed estimates
4.2.3 Aircraft Reconnaissance
 Accurately measure details of position and state of development. Higher
accuracy than remote satellites
4.2.4 Radar
 Since 1960s, radar system in place covering Gulf of Mexico and Atlantic
regions.
4.3 Mitigating the Effects of Tropical Cyclones
4.3.1 Early Warning Systems
 Hurricane watch: announcement for coastal areas that are potentially
threatened within 36 hours
 Hurricane warning: when sustained winds of 119km/h or higher are
expected within 24 hours
 So preparations can be taken
 Adequate lead time is required to protect life and property
 Over-warning must be minimised because it is costly and diminishes
credibility. Delicate balance to issue warning
 Reduced deaths, but increased property damage due to rapid population
growth and development in cyclone prone areas
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4.3.2
Evacuation Exercises and First Aid Strategies
 Reduce casualties – DCs are more capable of doing this than LDCs
4.4 Case Studies for Comparison
 Effects depend in development of economy, transport network and infrastructure
 United States: focus on early warning and monitoring though satellites. Coastal
protection works, cloud seeding. However, evacuation takes too long, and warning
time may not be sufficient. Furthermore, complacency in population has set in due
to no serious cyclones so far. No evacuation experience.
 Bangladesh: ineffective warning from Pakistani Meteorological Bureau – detected 3
days in advance but not issued until the evening it hit, and even then most people
were sleeping. Looting occurs. Evacuation impossible because of illiterate
population with low education who do not understand risks of hazard.
Past Climate Change
1. Climatic Changes in the Past
1.1 Ice Ages
 Most of the time, planet has usually been warmer, only punctuated by about 7 short
ice ages
 Most recent is the Pleistocene Ice Age
1.2 Glacials and Interglacials within the Pleistocene Ice Age
 Climate not uniform within ice age, it oscillates in cycles between glacials and
interglacials
 Glaciation involves a general cooling of temperature, and ample snowfall must have
persisted
 Deglaciation involves shrinkage of ice sheets in depth and volume
 Currently we are in a warm interglacial
1.3 The Last Glacial Maximum – the Wisconsinian Glaciation
 18000 years ago, thick ice sheets covered North America and Europe and Northern
Asia and Southern South America.
2. Evidence of Climatic Change
2.1 Glacier Evidence
 Glacial advance and retreat leave erosional and depositional features
 Alpine glaciers erode valleys, transforming V-shaped valleys into U-shaped valleys
 They also leave striae on rocks, smoothing them out
 Ice sheets are also capable of moving very large sized sediment, depositing these
tillites when they melt, which contain a wide assortment of sediment sizes
 Poorly sorted deposits indicate past ice sheets and glaciations
 Most glaciers in the N have been decreasing in size in the 20th century, probably as a
result of rising temperatures. However, exceptions like the Hubbard Glacier in
Alaska have been increasing, which shows that they respond to local climatic
conditions as well.
2.2 Sediment Evidence
 Ocean drilling programs collect cores of deep sea sediments
 Remains of organisms settle on floor, such as shells. The numbers and types of such
organisms change with climate changes.
 Also, oxygen isotope analysis can be used to analyse calcium carbonate in shells of
organic life, such as foraminifera
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 O16 is lighter than O18, making it evaporate more readily, so when glaciers are
expanding, more O16 is removed from ocean and deposited as snow, so O18:16
ratios will be higher in the ocean and also in the shells
2.3 Ice Core Evidence
 Oxygen isotope analysis can also be applied to ice cores taken from ice sheets. The
ratio of O18:16 in ice cores is influenced by temperature since more O18 is
evaporated with higher temperatures
 Also, air bubbles trapped in ice give an indication of the components of past
atmosphere. Higher temperature coincided with higher concentrations of carbon
dioxide
 Also, dust from volcanic activity can settle on glaciers, which show up in core
analysis. Can be valuable in determining the importance of volcanic activity in
climate change
2.4 Pollen Analysis
 Climate influences distribution of vegetation communities
 Spores and pollen can be preserved in lake beds or bogs, indicating the type of
vegetation which occupied that region, which can then be subject to radiocarbon
dating to figure out the kind of climate at the time
 Not all pollen can be identified to species level, and not all species produce the same
amount of pollen, resulting in underrepresentation of some species in palynology
2.5 Tree Ring Analysis
 Each year trees increase width of trunk by growth of concentric rings
 Width depends on climate – if favourable, grow more. If climatic stress conditions
from lack of moisture or too much warmth, growth is less
3. Natural Causes of Climatic Change
3.1 Astronomical Theory
3.1.1 Variation in Eccentricity
 Minor significance – elliptical orbit of Earth around sun
 Earth is 3% closer to sun during perihelion than aphelion, meaning Earth
receives about 6% more solar energy during perihelion
 This depends on eccentricity – when longer ellipse, radiation received at
perihelion may be 23-30% more than at aphelion
3.1.2 Changes in Obliquity
 Change in tilt of Earth’s axis – between 22.1 and 24.5. Currently 23.5
 Smaller tilt = smaller temperature difference between summer and winter
 Reduced seasonal contrast promotes growth of ice sheets
 Warmer winter and cooler summer allow for more precipitation to fall and
lesser melt
3.1.3 Variation in Precession of Equinoxes
 Axis points between Polaris and Vega – currently Polaris
 When tilting towards Vega, the times when winter and summer solstices
occur will be reversed, resulting in warmer summers at perihelion and
colder winters at aphelion
3.1.4 Credibility of the Astronomical Theory
 Climatic changes occur because they change the degree of contrast
between seasons, not because of distance from sun
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 Statistical correlation concludes that astronomical theory is the
fundamental cause behind the Quaternary Ice Ages
3.2 Solar Variability and Climate
 Hypothesis – Sun is a variable star which changes output over time
 However, no major long term variations in total intensity of solar radiation have
been measured outside atmosphere
 Cycle of sunspots, places where huge magnetic storms occur, of 11 years, increasing
and decreasing regularly
 Prolonged periods of absent sunspots corresponded with cold periods in Europe and
N America. Also, plentiful sunspots correlated with warmer times.
 However, global data does not show significant correlation, and no actual physical
mechanism exists to explain the effect
3.3 Volcanic Activity and Climate
 Eruptions emit gases and debris into atmosphere, which remain in the stratosphere
for long periods of time
 Sulphur in the form of sulphur dioxide and hydrogen sulphide reforms into an
aerosol of sulphuric acid droplets, which absorb and scatter solar radiation, cooling
 Also, they serve as condensation nuclei, increasing surface area of clouds to reflect
 More extensive, longer-lived, brighter clouds
 However, impact is small and short-lived – significant changes can only happen with
many eruptions within a short time
3.4 Plate Tectonics and Climatic Changes
 Glacial features in Africa, Australia and India indicate the movement of plates
 Continental drift accounted for many dramatic climate changes as landmasses
shifted to different latitudes
 However, plate movement is very slow – significant changes can only occur over a
great period of time. Not useful for explaining climatic variations on a shorter time
scale.
Climate of the Cities
1. Changes in Temperature
1.1 The Urban Heat Island Effect
 Urbanised areas have higher temperatures than countrysides
1.1.1 Determinant of the Magnitude and Effect
 Size of the city and density of population. Greater size and density have
larger hear island effect
1.1.2 Spatial Variation of the Effect
 Highest temperatures occur at greatest building densities and industrial
areas
 Greatest over downtown regions, decreasing over less populated areas,
and even more over green spaces
 Urban-rural differences greatest in winter months due to heat generation
 Tropical heat island less pronounced than temperate heat islands due to
lesser human-generated heat sources, but in NICs quite close to DCs
1.2 Causes of the Urban Heat Island Effect
 Modifications to the energy balance when natural surfaces are paved and built upon
and as a result of human activity
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1.2.1
Changes in Thermal Capacity
 Materials used in buildings (concrete, asphalt) have a far higher heat
capacity than natural surfaces, resulting in more stored heat available for
transfer to atmosphere at night
 Surface of city cools at slower rate
1.2.2 Sensible and Latent Heat Transfer
 Urbanisation favours sensible heat transfer over latent heat transfer, since
concrete is impervious to water, and runoff is diverted by drains and canals
 Reduction in water available for evaporation reduces latent heat and
increases sensible heat
1.2.3 The Urban Structure
 Buildings change the surface albedo, as urban surfaces have lower albedos
than natural ones
 Also, 3D structure of city increases amount of heat absorbed via multiple
reflections
 Vertical walls do not allow radiation to escape as easily as flat surfaces –
diffuse radiation is easily absorbed by adjacent walls, increasing total
absorption
 Similar processes occur at night, slowing rate of cooling and increasing
minimum temperatures
1.2.4 Heat Production by Combustion
 Waste heat from home heating, power generation, industry and transport
 Magnitude of human energy very significant – 1/3 of solar radiation in
Sheffield, England in a year
 Most pronounced in winter due to heating – 2.5 times greater than solar
energy in Manhattan in winter
1.2.5 Presence of Pollutants
 Particulate matter, water vapour, carbon dioxide absorbs long-wave
radiation and reemits it, leading to a localised greenhouse effect
1.2.6 Alteration of Wind Speed
 Tall buildings retard the flow of air due to rougher surfaces
 Decrease ventilation by inhibiting movement of cooler air into centre
 If regional winds are strong enough, structures are not enough to block
ventilation, and the heat island cannot be detected
2. Urban Induced Precipitation
2.1 Causes for Higher Urban Precipitation
 Heat Island effect creates thermally induced upward air, reducing stability
 Adding of condensation nuclei and freezing nuclei from industrial discharge
 Rougher city surface leads to low-level convergence and upward air motion
 Rain producing processes may linger over urban area due to impeding progress of
weather systems, increasing precipitation
 Condensation can set in at 78% relative humidity due to hygroscopic nuclei
2.2 Case Studies – St. Louis and Paris
 Precipitation over city is about 10% greater than rural
 St. Louis – increases in precipitation downwind that grew along with industrial
 Precipitation was significantly greater on weekdays than weekends in both places
3. Changes in Other Aspects of the Urban Climate
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3.1 Reduction in Solar Radiation
 Urban pollution adds aerosols like carbon, lead and aluminium, causing a pollution
dome to occur, reducing insolation depending on ventilation and angle of sun (lower
angle reflects more)
 c.p., solar energy will be reduced more at higher-latitude cities during winter
3.2 The Lowered Relative Humidity
 Temperatures are higher and evaporation is reduced due to lack of large water
bodies and vegetation for evapotranspiration
 Fogs and clouds are greater mainly because of increased condensation nuclei
3.3 The Generation of Country Breeze
 Heat of city creates upward air motion, low pressure, causing air to blow into city
from countryside
The Enhanced Greenhouse Effect and Global Warming
1. The Enhanced Greenhouse Effect
1.1 The Phenomenon
 Greenhouse effect is natural – long wave terrestrial radiation absorbed by
greenhouse gases, reemitting and reabsorbing in a cycle
 Keeps Earth 35C warmer than it would be without greenhouse gases
1.2 Greenhouse Gases
 Human activities lead to the enhanced greenhouse effect by emitting more gases
 Collectively important since they absorb different spectrums of energy
1.2.1 Carbon Dioxide
 Burning of fossil fuels and clearing forested areas disturb the carbon cycle
by adding more into atmosphere
 Exponential increase since mid 19th century, by 1980 about 20kmil tonnes
annually from fossil fuels, with most coming from DCs
 Deforestation – lesser photosynthesis, burning of plants. About 20% of Co2
anthropogenically added each year from deforestation
1.2.2 Methane
 Byproduct of energy consumption and agricultural activity
1.2.3 Other Trace Gases
 Include nitrous oxide and CFCs, absorbing different wavelengths from the
other 2
2. Trend and Projection of Global Warming
2.1 Trend of Global Warming
 Global temperature is increasing, with warming occurring since late 1970s
2.1.1 Long Term Carbon Dioxide and Temperature Variations
 Fluctuations in CO2 content and temperature over the last 160000 years
appear to be correlated
 Solar energy variations does not seem to account for this since
temperatures have been steadily going up without a corresponding increase
in solar activity
2.1.2 Recent Measurement of Carbon Dioxide Content
 At Mauna Loa Observatory in Hawaii, CO2 concentration has been
measured to have increased from 315ppm to 370ppm
2.1.3 The Relationship between Carbon Dioxide Level and Temperature
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 Most believe effects of CO2 increase are not yet large enough to be clearly
measured
 Also, although there is a correlation, it does not imply causation, nor show
which one causes which
2.2 Difficulties in Future Climate Change Projection
 Various climatic feedback mechanisms make it difficult and complicate modelling
efforts for climate change
 Positive feedback mechanisms magnify and reinforce temperature rise
 Negative feedback mechanisms produce opposite results and tend to offset rises
2.2.1 Positive Feedback Mechanism – Evaporation Rates
 Greater greenhouse gases -> higher surface temperatures -> greater
evaporation rates -> more water vapour -> more terrestrial radiation
absorbed
2.2.2 Positive Feedback Mechanism – Albedo Changes
 Increasing temperatures -> melting ice sheets -> reduced albedo ->
increased insolation absorbed -> higher temperatures especially at higher
latitudes
2.2.3 Negative Feedback Mechanism – Cloud Cover
 Higher moisture -> increased cloud cover -> reflect more insolation (higher
albedo more dominant than reflecting terrestrial radiation) -> lower
temperature. However, magnitude is less great than positive feedbacks,
ultimately still temperature rise
2.2.4 Negative Feedback Mechanism – Ocean Sink
 Increase temperature -> increased marine life -> removal of carbon dioxide
via photosynthesis -> carbon sink -> lesser carbon -> cooler. However,
warm water absorbs less carbon dioxide than cold water.
2.2.5 Global Dimming
 Pollution caused by humans cause pollutant layer and forms condensation
nuclei, which cuts insolation, producing a cooling effect
 Models predicting global warming may have been affected by this cooling
effect, being under sensitive to temperature changes
 Reducing insolation has also caused droughts due to reducing heat
changing the positions of the ITCZ
 Solving global dimming can cause global warming to worsen
2.3 Predicted Future Temperature Changes
 Consensus is that increasing CO2 and trace gases will lead to warmer planet, but
more so in polar regions, due to stable air preventing heat from rising, and melt in
sea ice increasing temperatures
 Depends on how much humans emit in future years, affected by economic
conditions, population growth and alternative energy sources
 Even if no gas was added, temperatures would still increase due to heat being
released from oceans
2.4 Debate on Causes of Warming – Natural or Anthropogenic
 Impossible to conclude for sure either way, but generally accepted that currently
largely the result of increases in greenhouse gases
3. Consequences of Global Warming
3.1 Sea Level Changes
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3.1.1
Thermal Expansion
 Warmer atmosphere cause increase in ocean volume due to expansion,
causing water level to rise
 Any rise in gently sloping shorelines will result in significant erosion and
shoreline retreat
3.1.2 Glacial Melt
 On a large scale, lead to much greater rise in sea level. However, significant
melting not expected this century
3.1.3 Different Extent of Sea Rise
 Shoreline shift is minor when coastal land is steep. Major when gentle.
3.1.4 Impact of Sea Level Rise
 Drastic, especially in tropics and warm temperate regions. Low-lying areas
are also susceptible
 Maldives – 1m rise would cover 85% of capital Malé
 Delta environments, such as Egypt and Nile, losing prime agricultural land,
infrastructure and tourist revenue
 Bangladesh and India
 Economical solution – seawater diverted into depressions such as Dead Sea
3.2 Changes in Agricultural Productivity
 Some regions experience significant changes in precipitation and runoff, affecting
distribution of world’s water resources and productivity of agricultural regions
 May be offset by gains elsewhere – expansion of agriculture into areas currently not
suited to crop production
 However, it improves in wealthy countries like Canada but declines in poorer
tropical countries that feed majority of the world
3.3 Effects on Plant Migration
 Plant communities migrate north and south in the past due to climate change
 The rate at which they migrate is important. Rapid changes in climate may affect it,
but not by much since they migrate rather fast
 However, human intervention may have fragmented plant communities, hindering
their ability to migrate
3.4 Other Possible Changes
 Higher frequency of tropical storms
 Shift in paths of cyclonic storms, affecting distribution of precipitation and storms
 Increases in intensity of heat waves and droughts
 Actual impact is largely unknown due to many factors and lack of understanding of
the complex interplay of feedback mechanisms. Unlikely to accurately predict
climate change
4. Mitigation of Climate Change
4.1 Reduction of Energy Consumption
 Reduce per capita consumption of energy through conservation and efficiency
 Inexpensive and simple e.g. insulation for buildings to minimise heat loss, mass
public transportation
4.2 Alternative Energies
 Key issue is cost – currently coal and natural gas are the cheapest fuels, while
alternative energy sources are still quite expensive, but the prices are reducing
4.2.1 Wind Power
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
Cost of wind power has dropped since 1990s. Once built, cost of providing
energy is zero. No pollutants.
 However, can kill animals, unsightly, and cost of cables to transport energy
is quite high.
4.2.2 Solar Energy
 Effective but expensive to manufacture silicon solar panels
 New thin-film solar materials are much cheaper to manufacture
4.2.3 Nuclear Energy
 Japan, western European countries generate significant nuclear energy,
about 16% of worldwide energy supply
 However, limited deposits of uranium fuel can only last about 50 more
years
 Power plant disasters cause bad reputation, such as Chernobyl, Three Mile
Island and Fukushima Dai Ichi Plant
4.3 Carbon Trading
 Cap-and-trade law to limit carbon emissions. Encourage companies to reduce
greenhouse gases without hindering growth
 However, does not distinguish between different levels of green technology being
used
4.4 Global Cooperation
4.4.1 The Kyoto Protocol
 Meaningful reductions in greenhouse gas emissions can only be achieved
with global cooperation
 111 countries signed in 1997, with Russia signing in 2004
 Plant forests, change agricultural practices, clean technology, buy carbon
credits to keep to the limit
4.4.2 Problems Associated with the Kyoto Protocol
 Bush argued that it would hurt US economically, despite being the largest
contributor at the time
 LDCs argue that DCs should reduce their emissions because they can afford
the costs, while LDCs are poorer and cannot purchase more efficient fuels
4.4.3 Other International Cooperation Efforts
 July 2009, the G8 committed to reducing carbon emission by 80% by 2050
 India and China pledged to reduce the rate of increase, not absolute level
 However, no short term goals, not binding
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El Nino
Large scale sea surface temperature changes across the tropical Pacific. The El Nino event is when a warm
countercurrent flows southward along the coasts of Ecuador and Peru in contrast to the normal cold
Peruvian current. A strong change occurs every 3 to 7 years, but milder ones can occur at the end of
every year.
1. The El Nino Event
1.1 The El Nino/Southern Oscillation (ENSO)
 Sir Gilbert Walker identified an irregular oscillation between atmospheric pressures
in Tahiti in central equatorial Pacific and Darwin
 Tahiti normally has higher atmospheric pressure than Darwin, but during El Nino
events, barometric pressure drops over large parts of southeastern Pacific, while in
the western side pressure rises. When the event ends, pressure reverses
 The seesaw pattern is the ENSO
1.2 The Walker Circulation
 The difference in air pressure causes air to move from subtropical high pressure cell
over the southeastern Pacific to a low pressure area over the western Pacific
 The ENSO changes the strength of the Pacific trade winds and the circulation of the
oceanic currents
1.2.1 The Walker Circulation During Normal Period
 The northeast and southeast tradewinds blow towards the west, creating a
frictional drag on ocean currents, resulting in a westward surface current
 Pooling of warm surface water occurs around the west equatorial Pacific,
about 100m deep. Piling causes water level to rise 40cm higher than in the
eastern Pacific
 Pooling causes the thermocline (boundary between warm surface water
and cold subsurface water) to be much deeper in the western Pacific than
the eastern Pacific
 Warm water leads to higher temperatures and lower surface air pressure,
intensifying convective rainfall in western Pacific.
 On the eastern Pacific, the warm water migrating westward is replaced by
cold water from below. The southeasterly trade winds cause the cold
Peruvian current rich in plankton and fish. This combination results in a
shallow thermocline (40m) and a shallow inversion, providing positive
feedback, strengthening the trade winds
 Western counterflow at 200mb results in subsidence over South America
1.2.2 The Walker Circulation during an El Nino Event
 Barometric pressure rises in Indonesia and subsides in South America,
causing the pressure gradient to weaken or reverse
 The weakening of the trade winds causes the warm pool to move gradually
eastward, also due to the higher water level, causing Kelvin waves
 Warm water influx covers the nutrient rich cold water, preventing upwelling,
cutting off nutrients to plankton, causing a biological starvation chain
 A south flowing warm current replaces the north flowing Peruvian current.
This countercurrent is termed the El Nino, and increasing temperatures of
about 1-4oC further weakens the trade winds, increasing warm water
movement and decreasing cold water upwelling. Positive feedback.
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 Thermocline is depressed, generating a positive feedback loop as cold water
is prevented from upwelling
 El Nino takes 3-6 months to reach local peak intensity. Stronger Hadley Cell
activity when pressure situations return to normal eventually terminates
the El Nino
2. Past Occurrences of El Nino and Future Prediction
 Development and decay of El Nino takes place over 12-18 months. El Nino itself occurs
roughly every 3-4 years
 Identifying development of ENSO is normally done by monitoring surface temperatures
across the tropical Pacific. Unusually warm waters over the eastern Pacific indicate a
strong El Nino, while colder than normal waters signify a La Nina.
 Computer forecasting models based on sea temperature have been created to attempt
to predict ENSO and its effects. One more successful one is the Lamont-Doherty
Geological Observatory at Columbia University in New York. The ENSO of 1982-83, 198687, 1989-90 and 1991 were predicted 8 months ahead
 However, ENSOs are largely unpredictable, especially with regards to magnitude, since
the 1990 ENSO peaked in early 1992, weakened suddenly and strengthened again
 Consequences are even harder to predict. Regardless, efforts are important as we need
to take steps to manage climate change. Global warming could intensify ENSO events.
3. Impacts of El Nino
3.1 Economic Impact on Peru and Ecuador
 The cold upwelling Peruvian current encourages nutrients which are food source for
plankton and millions of fish, especially anchovies. These fish support birds whose
guano are mined for fertiliser. The cessation of cold water during an El Nino harms
the local fishing industry due to the loss of fish, as well as the fertiliser industry.
 Some inland areas, normally arid, receive unusual amounts of rain, and pastures and
cotton flourish, yielding far more than normal
3.2 Worldwide Weather Anomalies
3.2.1 Effects of the 1992-1993 El Nino
 Heavy rains and flooding occurred in normally dry parts of Ecuador and
Peru, up to 350cm from 10-13cm of rain per year
 A warm winter and a very wet spring occurred for the United States.
California was struck by storms, causing landslides, floods and coastal
erosion. Heavy snowing in Sierra Nevada and mountains caused mudflows
and flooding in Utah, Nevada and Colorado River
 This ENSO caused up to 2000 deaths and $13 billion worth of damage
3.2.2 Effects of the 1997 El Nino
 Strongest since 1983. Severe droughts in Indonesia, worst in 50 years.
Associated with widespread forest burning in Sumatra and Kalimantan,
causing large amounts of smoke pollution. KL and Kuching airports closed,
schools closed. 50000 people ill due to smog. 2 ships collided in Straits of
Malacca due to poor visibility, 29 died. Garuda Airlines plane crashed near
Medan in Sumatra, killing 234.
 Hurricane Pauline on 9 October in Acapulco (south Mexico). Severity of
storm attributed to El Nino peak causing instability and high sea temp
 Southern California: winter storms and mudslides
3.2.3 Teleconnections
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 Weather anomalies can be induced worldwide due to teleconnections:
linkages over great distances of atmospheric and oceanic variables
 Linkage between eastern and western Pacific: typical teleconnection
 ENSO related with lower monsoon rain over South Asia. Parts of India
experience more droughts during El Nino e.g. 1888: 1.5 million people died
due to famines caused by droughts
 Relations between ENSO and extreme weather regimes around the globe
 Widespread and variable – ENSO has effects on global weather
Adverse Weather Conditions: Droughts
When an area experiences a prolonged period of below average rainfall, drought conditions can prevail.
1. Types of Droughts
 Generally refers to an extended period of rainfall deficit when agricultural biomass is
severely curtailed
 Meteorological Drought – shortage of rainfall. Britain – used to be 14 consecutive days
without rain. No direct ecological/economic impact, no effective human response
 Climatic Drought – longer period of little or no rainfall when rainfall is expected
 Hydrological Drought – water levels in rivers and ground are well below expected levels a period of low flows due to meteorological drought and lack of groundwater recharge.
Responses come from authorities, involve managing of water
 Agricultural Drought – impact of reduced precipitation results in crop failure. Direct
responses at a national level, involving compensation and other measures
 Famine Drought – most severe, deaths from starvation, response at international level
2. Causes of Droughts
 Some climatic regimes have less predictable rainfall, high variability. Agricultrual,
industrial and domestic water supply planning becomes more difficult. Increasing
demand may cause supply problems during below average/average conditions
 El Nino can have major effects in rainfall in Australia, Indonesia, parts of west USA
3. Managing Droughts
3.1 Drought Prediction and Forecasting
 Many strategies depend on preparedness. Climate studies to predict possibility of
droughts relies on data from precipitation, temperature, evapotranspiration, soil
moisture, stream flow, groundwater, reservoir and lake levels.
 Assessment of probability and recurrence of droughts, intensity of droughts,
duration, spatial extent, risks and potential effects on agriculture
 Provide information enabling and persuading people and organisation to maximise
successful crop production, minimise damage to crops and other assets
 Monitoring El Nino and Sea Surface Temperature anomalies – 1988 British predicted
rain in Sahel due to alteration in ocean currents
3.2 Drought Mitigations
 Short and long term actions, programs or policies implemented during and in
advance of droughts, reducing degree of risk to lives, property and production
 Mitigated by increasing water supply – drought reserve dams, large house and
garden storage for stock – ideally provide for stock and domestic use.
 Changing practices that can worsen drought effect – efficient use of water
 Conservation of soil and water and managing livestock – herd reduction
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 Mitigating agricultural impact – low tech: stone bunds to reduce overland flows,
small earth dams to store water. High tech: drip and sprinkle irrigation, fertiliser use,
mechanisation of farming
3.3 Response to Drought
 Actions taken once drought has occurred – address impacts, expedite recovery.
Resettlement, food, hygiene, medical care
3.4 Drought Management in DCs/LDCs
3.4.1 Management in DCs
 Droughts disrupt lifestyle and economic activity, but access to capital
makes in mere inconveniences in comparison
 Reservoirs can be enlarged, extracted from rivers, improved distribution
system by reducing leaks in pipes or linking supplies in different areas
 Desalination plants – Saudi Arabia
 1986-1993 drought in California – 55-75% of normal rainfalls, tensions
between conservation, agriculture and urban land use
 Water authorities ordered 25% reduction in water use in SF
 Pasadena city – free installation of water saving devices
3.4.2 Management in LDCs
 Resources are less readily available. Improving water supplies only possible
through international action e.g. Ethiopia.
 UN Disaster Relief Office (UNDRO) and Red Cross
 UN Education, Scientific and Cultural Organisation (UNESCO) involved in
dam construction financed by World Bank
 Short term measures – pump groundwater, which can lead to concentration
of population and grazing animals around the new supply
 Lake Kariba on Zambezi and Lake Nasser on Nile - damming
4. Case Study of Droughts
4.1 Drought in the Sahel
4.1.1 Natural Causes
 Semi-arid area, rainfall has high seasonality and variability. Possible reasons
include El Nino (2003, International Institute for Climate Prediction found
evidence of a link), also global dimming (air pollution from Western
countries driving tropical rain belt in Sahara further south, 50% decline in
rainfall in Sahel) causing cooling sea surface temperatures. Affected 19681974, Mauritania and Ethiopia
 Feedback mechanisms – decreasing precipitation reduced plant growth,
reducing evapotranspiration, decreasing moisture content in the
atmosphere, reducing cloud cover, soil surface dries out and vegetation
reduced, increasing ground heating.
4.1.2 Anthropogenic Causes
 Drought occurred concomitantly with increase in population and
deleterious economic conditions. 1970s – fuel prices soared, higher demand
for wood for cooking, rapid harvesting of trees. Diminishing crop yields, less
land used for crops, reducing soil moisture. Plowing caused surface crusts,
preventing infiltration.
 Droughts are getting more frequent – now every 5 years or less in Ethiopia.
1984-85 killed a million Ethiopians
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 LDC governments are more inactive, not economically able to develop
mechanisms for drought mitigation and alleviation. International relief plays
a more vital role
 UN Office for Emergency Operations in Africa (UNOEOA) channelled US$4.5
billion from 35 countries, 47 NGOs to northern Africa, saving 35 million
 Supplying aid: increase in volume of goods may not be efficiently handled
due to inefficient or non-existing transport infrastructure. Grain to Ethiopia
in 1980s. Isolated area = no transport network.
 LDCs may be corrupt or inefficient, siphoning aid for export or black market
 Inefficient administration slow relief aid with paperwork
4.2 Drought in Australia
 Farmers rely on technology and efficient land management. Maximise return,
minimise permanent damage
 Land management -> moisture conservation and crop protection, mulching, tillage,
contour banks, minimise runoff. Catchments cleared and compacted to maximise
runoff into farm dams used for irrigation during droughts
 Pasture is set aside in spring to be used for hay when dry. Grain is held over to
supplement feed. When persistent drought, stocks are culled or shipped overseas
 Technological advances – aerial spraying of insecticides, aerial seed sowing,
monitoring, computer management models to maximise crop yields
5. Droughts and Desertification
 Desertification refers to land degradation, loss of soil fertility. Often caused by droughts.
Semi-arid and sub-humid areas face highest risk.
 Sahel – desertification led to expansion of the south edge of the Sahara, largely
attributed to overexploitation of resources by an increasing population – accelerated soil
erosion and deforestation
 Removal of vegetation cover causes loss of fertile topsoil and surface crusting, reducing
productivity and degradation. Sahel, also human activity like nomads and herds, built
near water, exacerbates overgrazing and compaction of soil
 Irrigation from groundwater causes water table to rise – Murray Valley in SE Australia,
rose by nearly 10m from 1962-1974. Once saline groundwater within 1-3m of ground
surface, salt crusts accumulate, degrading the land.
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