Weather and climate
What is precipitation?
• Precipitation is any form of moisture which falls
to the earth. This includes rain, snow, hail and
sleet. Precipitation occurs when water vapour
cools. When the air reaches saturation point
(also known as condensation point and dew
point) the water vapour condenses and forms
tiny droplets of water. These tiny droplets of
water from clouds.
• All rain is the same. It happens as the result of
warm, moist air being cooled, leading to
condensation and in turn rain.
Relief Rainfall
• Stage 1.
Warm wet air is forced to rise
over high land.
• Stage 2.
As the air rises it cools and
condenses. Clouds form and
precipitation occurs.
• Stage 3.
The drier air descends and
warms.
• Stage 4.
Any moisture in the air (e.g.
cloud) evaporates.
Convectional Rainfall
• Stage 1.
The sun heats the ground and warm
air rises.
• Stage 2
As the air rises it cools and water
vapour condenses to form clouds.
• Stage 3.
When the condensation point is
reached large cumulonimbus clouds
are formed.
• Stage 4.
Heavy rain storms occur. These
usually include thunder and lightening
due to the electrical charge created by
unstable conditions.
Frontal Rainfall
• Stage 1.
An area of warm air meets and
area of cold air.
• Stage 2.
The warm air is forced over the
cold air
• Stage 3.
Where the air meets the warm air is
cooled and water vapour
condenses.
• Stage 4.
Clouds form and precipitation
occurs
Climatic Zones
• The world has several climatic zones. These are summarised on the
map below.
• The classification is based on maximum and minimum temperatures
and the temperature range as well as the total and seasonal
distribution of precipitation.
Summary of climatic zones:
• Polar - very cold and dry all year
• Temperate - cold winters and mild summers
• Arid - dry, hot all year
• Tropical - hot and wet all year
• Mediterranean - mild winters, dry hot summers
• Mountains (tundra) very cold all year
What is pressure?
• Pressure is the weight of air pressing
down on the earth's surface. Pressure
varies from place to place and and results
in pressure systems.
What is low pressure?
• At times of low pressure the air is usually rising. As the
air rises, it cools, condenses and forms clouds. Areas of
low pressure are known as depressions.
• Depressions, or 'lows', bring rain, strong winds and
changeable conditions. Changeable weather is a feature
of British weather, depressions are responsible for much
of this.
• Strong winds blow in an anticlockwise direction.
Front
• Depressions occur when warm air meets
cold air. The point where warm air meets
cold air is called a front. Along the front
there is usually cloud and rainfall. This
occurs because the warm air cools and
condenses when it meets the cold air
What is high pressure?
• Areas with above average pressure are
called anticyclones. Anticyclones occur
when air is sinking. As a result there are
usually few clouds in the air. In the UK
anticyclones bring cold clear days in winter
and hot and sunny days in summer.
• Light winds blow in a clock wise direction.
Weather experienced during a
winter anticyclone
• In winter the skies are cloudless so heat is
allowed to escape. Therefore
temperatures are usually very cold. The
ground cools rapidly at night so frost often
forms. Fog can also form as the cold air
makes water vapour condense into tiny
droplets. Fog can last long into the day as
there is insufficient heat from the sun to
evaporate the water droplets away.
Weather experienced during a
summer anticyclone
• Summer anticyclones bring very different
weather. As the air descends it is heated
causing water in the air to evaporate.
Therefore there are few clouds in the air.
The skies are clear allowing the suns rays
to reach the surface of the earth. This
causes temperatures to rise. Heat waves
can occur if anticyclones remain over
Britain for a number of weeks.
Air mass:
• A large body of air that has similar
temperature, pressure and moisture
properties.
Anticyclone:
• High pressure system in which air
descends to give calm conditions and
clear skies. Associated with summer
heatwaves and winter frosts and fogs.
Atmosphere:
• The air surrounding the Earth and bound
to it by gravity.
Atmospheric pressure:
• Pressure produced by the atmosphere on
any surface by its weight.
Clear sky:
• Sky with a total cloud cover of less than
one okta.
Climate:
• long-term (30 year) weather averages.
Cold front:
• The "leading edge" of a relatively cold air
mass.
Continental climate:
• A climate with a high temperature range
away from the influence of the sea.
Winters will be colder and summers
warmer compared to a coastal location for
the same latitude.
Depression (cyclone, low-pressure):
• Area in the atmosphere in which the pressures
are lower than those of the surrounding region at
the same level. In its development a depression
usually has the following phases. A wave
(young) depression forms and moves along a
front. Mature depressions have well-developed
warm sectors and both cold and warm fronts. An
occluded depression is that within which there
has developed an occluded front.
Dew point (dew-point temperature):
• The temperature to which certain air must
be cooled in order for saturation to occur.
When this temperature is below 0 °C,
frosts form.
Evaporation:
• The physical process by which a liquid or
solid substance is transformed to a gas;
the opposite of condensation.
Fog:
• Saturated air with visibility below one
kilometre. Fog differs from cloud only in
that the base of fog is at the Earth's
surface while clouds are above the
surface.
Front:
• The meeting point between two air masses of
different density. Since the temperature is the
most important regulator of the atmosphere
density, a front almost invariably separates air
masses of different temperature. When warmer
air replaces the colder, it is a warm front, and a
front is a cold one when the opposite occurs.
Humidity:
• Water vapor content of the air.
Isobar:
• A line of equal or constant pressure.
Measured in millibars (mb).
Maritime climate:
• A climate with a low temperature range
influenced by proximity to the sea. Winters
will be warmer and summers cooler
compared to a continental location for the
same latitude.
Occluded front:
• A front that is formed as a cold front
overtakes a warm front and lifts the warm
air completely off the ground.
Okta:
• A measure of cloud cover (in fractions of
eight) on a synoptic chart.
Precipitation:
• Any of all of the forms of water particles,
whether liquid or solid, that fall from the
atmosphere and reach the ground. The
forms of precipitation are: rain, drizzle,
snow, hail, and ice pellets.
Prevailing wind
• Is the most common wind direction for a
particular location.
Relief rainfall:
• Formed when air is forced to rise over
relief features such as hills or mountains.
Cooling and condensation occurs as the
air rises.
Seasonality:
• Periodic fluctuations in the climate related
to seasons of the year e.g. wet winters,
drier summers.
Smog:
• A word currently used as a synonym for
general air pollution. It was originally
created by combining the words "smoke"
and "fog."
Synoptic chart:
• A weather chart reflecting the state of the
atmosphere over a large area at a given
moment.
Temperature:
• A physical quantity characterizing the
mean random motion of molecules in a
physical body. In other words, it is a
measure of the degree of hotness or
coldness of a substance.
Temperature range:
• Maximum minus the minimum temperature
for a particular location.
Warm front:
• The forward edge of an advancing warm
air mass that is rising over cooler air in its
path.
Warm sector:
• The zone of warm air within a depression.
Water vapour:
• Water substance in vapour (gaseous)
form; one of the most important of all
constituents of the atmosphere.
Weather:
• The state of the atmosphere, mainly with
respect to its effects upon life and human
activities. As distinguished from climate,
weather consists of the short-term
(minutes to about 15 days) variations of
the atmosphere state.
Wind:
• Movement of air caused by changes in
temperature and air pressure. Winds are
always identified by the compass direction
from which they blow.
Clouds
• A cloud is a visible aggregate of tiny water
droplets and/or ice crystals in the
atmosphere and can exist in a variety of
shapes and sizes. Some clouds are
accompanied by precipitation; rain, snow,
hail or sleet.
Introduction to Weather
• Meteorology is the study of weather. Weather is caused by
the movement or transfer of energy. Energy is transferred
wherever there is a temperature difference between two
objects. Many weather phenomena result from a transfer of
energy that occurs via the movement of air in the
atmosphere. This is known as convection.
• Air contains water vapour from the evaporation of liquid
water sources on the Earth's surface, including oceans,
lakes and rivers, and from evapotranspiration by plants.
When air is moved about the Earth, either vertically when
uplifted or horizontally as part of air masses, it may cool and
release water vapour as condensation in the form of clouds
and eventually rain and other forms of precipitation, which is
returned to Earth. This cycle of evaporation, condensation
and precipitation between the Earth and the atmosphere is
known as the water cycle.
Introduction to Weather
• The physical transfer of heat and moisture by convective processes is the
basis for the formation of many meteorological patterns and features,
including anticyclones, depressions, fronts, monsoons, thunderstorms,
hurricanes and tornadoes. Heat however, may also radiate directly from a
hot object to a colder one, without involving the movement of air. Many
small-scale weather phenomena are the result of this form of heat
transfer, including dew, frost and fog.
• Weather can be simply measured by observing and recording
temperature, rainfall, pressure, humidity, sunshine, wind and cloudiness.
It is also possible to identify and name different types of clouds, which are
associated with different patterns of weather. Commonly observed cloud
types include cirrus, cumulus, cumulonimbus and stratus. To make
predictions and forecasts about what the weather will do in the future
however, it helps to draw synoptic charts, composed of special weather
symbols and isobars that reveal patterns of weather. The use of
sophisticated technology such as weather radar and satellite imagery
also assist with weather forecasting.
Anticyclones
• The cloudy rainy weather of low-pressure depressions is due to rising air,
which is most pronounced near frontal regions. The anticyclone on the
other hand is produced by a large mass of descending air. This takes
place throughout a depth of the atmosphere up to 12km. This means that
the air is very stable and atmospheric pressure is high. In addition, winds
associated with an anticyclone are usually very light if present at all,
especially close to the centre of the high-pressure system.
• Subsidence warms the air by compression. Any clouds present quickly
evaporate as the temperature of the air rises above its dew point. For this
reason, anticyclones usually bring fine, dry and settled weather,
particularly in the summer.
• Sometimes, subsidence and compression of the air can produce a
temperature inversion at one or two thousand metres above the ground.
Such phenomena act as caps to rising air heated by the ground under
the influence of the Sun, preventing extensive air cooling and cloud
formation. Unfortunately, if the air is moist below the temperature
inversion, a dreary formless layer of cloud can form which becomes
difficult to disperse owing to the light winds.
Anticyclones
•
•
•
Winter anticyclones, if clear of cloud, bring with them further problems. A short
cloudless day is the forerunner of a long night with more radiation cooling than
a low-angle Sun can counteract the next day. The second night of cooling
therefore starts with a lower air temperature than the first. Such conditions, if
persistent, can lead to successive nights of frost, which become progressively
harder. When the air is particularly moist, cooling at night soon results in fog.
Britain in particular can experience episodes of anticyclonic fog from late
September through to May.
Anticyclones move, but not quite in the same purposeful way as travelling
depressions. They nudge their way into position and can be incredibly
stubborn about leaving, perhaps persisting for weeks, diverting depressions to
different routes. Such persistent anticyclones are known as "blocking highs". In
winter they can lead to long spells of very cold weather, especially if their
airflow comes from Russia and Siberia. In summer they can lead to long hot
spells and sometimes drought.
A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt
of high pressure. The weather associated with ridges is similar to that in an
anticyclone. In temperate latitudes as in the British Isles, ridges of high
pressure often occur between two depressions and move with them. They give
rise to intervals of fair weather between the cloud and rain of the low-pressure
systems.
Clouds
• A cloud is a visible aggregate of tiny water droplets and/or ice
crystals suspended in the atmosphere and can exist in a variety of
shapes and sizes. Some clouds are accompanied by precipitation;
rain, snow, hail or sleet.
• All clouds form as a consequence of rising air. Sometimes air is
forced to rise over mountains. More usually, warm air, being less
dense, will rise above cold air. At fronts for example, warm air
masses rise over cold air masses when they converge. At much
smaller scales, columns of rising warm air may be generated by
daytime heat from the Sun.
• When air rises, it expands, causing cooling and a drop in
temperature. As the temperature falls, the humidity (or water vapour
content) of air increases towards 100%. Finally, after sufficient
cooling, the air becomes saturated, and water vapour begins to
condense out as tiny water droplets, forming cloud.
Cold Fronts
• Cold fronts are usually associated with depressions. A cold front is
defined as the transition zone where a cold air mass is replacing a
warmer air mass. At a cold front cold air following warm air undercuts
the warm air, heaving it upwards. The air associated with a cold front is
usually unstable and conducive to cumulonimbus cloud formation.
Because the upthrust is delivered along a boundary between the two
air masses, the cumulonimbus form a well-defined line in contrast to
the well-spaced clouds forming during thermal convection. More rain
may fall in a few minutes as the cold front passes than during the
whole passage of a warm front. As the cold front passes, the clouds
roll by and the air temperature may become noticeably cooler, with
temperatures dropping by 5°C or more within the first hour.
• On synoptic (weather) charts a cold front is represented by a solid line
with triangles along the front pointing towards the warmer air and in the
direction of movement. On colored weather maps, a cold front is drawn
with a solid blue line.
Condensation
•
•
•
Condensation is the process whereby water vapour in the atmosphere is
returned to its original liquid state. In the atmosphere, condensation may appear
as clouds, fog, mist, dew or frost, depending upon the physical conditions of the
atmosphere. Condensation is not a matter of one particular temperature but of a
difference between two. Condensation of water vapour occurs when the
temperature of air is lowered to its dew point.
All air contains water vapour of varying quantities. The lower the air
temperature, the smaller the maximum possible capacity for vapour. When air is
cooled, relative humidity increases, until at a particular temperature, called the
dew point, the air becomes saturated. Further cooling below the dew point will
induce condensation of the excess water vapour.
The temperature of the dew point will depend upon the absolute content of
water vapour, that is the absolute humidity, measured in g/m3 (grams per cubic
metre). The dew point of humid air will be higher than the dew point of dry air.
Both air temperature and absolute humidity will determine what type of
condensation will occur when the air is cooled. If air in contact with the ground is
cooled to its dew point, dew or frost will form, dew if the point is above 0°C, or
frost if it is below 0°C. Cooling of a larger layer of air near to the ground may
produce mist of fog, which freezes if the dew point is below 0°C. Air that is
cooled to its dew point by rising and expansion will condense to form clouds.
Above 0°C, small droplets of water are formed.
Cooling Air
•
•
•
Many of the common phenomena of weather - clouds, frost, fog and rain - are
due to the cooling of air and the consequent condensation of excess water
vapour. Air is cooled by two main processes: cooling by contact (usually with the
Earth's surface); and cooling by uplift.
A common form of contact cooling is radiation cooling. When the Sun sets, the
Earth and any other radiating body upon it continue to radiate heat from their
supplies. With no heat from the Sun to replenish their stocks, their temperature
falls. Maximum loss of heat occurs under clear skies. As surface temperature
falls the air in closest contact with it begins to cool. Eventually, the surface air
will cool below its dew point temperature, and begin to condense water vapour
out as dew. When the dew point of the cooling air is below 0°C, hoarfrost results
from radiation cooling instead. Condensation then occurs directly as a crust of
white crystals. Sometimes a much thicker layer of moist air may be cooled.
Condensation then occurs throughout, giving rise to fog.
Usually wind serves to prevent or restrict the formation of dew, frost or fog. This
is because a steady flow of air over the cooling surface does not remain in
contact with the ground long enough to cool below its dew point for
condensation to occur. The one occasion when wind does not deter
condensation is when warm air from different sources passes over a much
colder surface. Advection cooling, as it is known, is a common source of sea fog
in coastal areas, when warmer sea air comes inland passing over colder land.
Cumulonimbus Clouds
• Cumulonimbus clouds are much larger and more
vertically developed than cumulus clouds which form in a
more stable atmosphere. They can exist as individual
towers or form a line of towers called a squall line often
present at cold fronts. Underneath they are dark. At a
distance they rise up like huge white mountains when
the Sun shines on them. Fuelled by vigorous convection
of air in an unstable atmosphere the tops of
cumulonimbus clouds can easily reach 12 km or higher.
Lower levels of cumulonimbus clouds consist mostly of
water droplets while at higher elevations, where
temperatures are well below 0°C, ice crystals dominate.
Cumulus Clouds
• Cumulus clouds look like white fluffy balls of
cotton wool and mark the vertical extent of
convection or thermal uplift of air taking
place in the in the atmosphere. The level at
which condensation and cloud formation
begins is defined by the flat cloud base, and
its height will depend upon the humidity of
the rising air. The more humid the air, the
lower the cloud base.
Depressions
•
•
•
Depressions, sometimes called mid-latitude cyclones, are areas of low pressure
located between 30° and 60° latitude. Depressions develop when warm air from
the sub-tropics meets cold air from the polar regions. There is a favourite
meeting place in the mid-Atlantic for cold polar air and warm sub-tropical air.
Depressions usually have well defined warm and cold fronts, as the warm air is
forced to rise above the cold air. Fronts and depressions have a birth, lifetime
and death; and according to the stage at which they are encountered, so does
the weather intensity vary.
A depression appears on a synoptic (weather) chart as a set of closed curved
isobars with winds circulating anticlockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere due to the rotation of the Earth. The warm
and cold fronts associated with depressions bring with them characteristically
unsettled weather. Depressions vary from between 200 and 2,000 miles in
diameter; they may be deep when pressure at their centre is very low and the
isobars are tightly packed, or shallow when less well developed.
A depression develops like the propagation of a wave in water. Initially, a uniform
boundary or front exists between cold air pushing southwards and warm air
pushing northwards (in the Northern Hemisphere). A wave-shaped distortion may
appear on the front, and a small low-pressure centre develops at the crest of the
wave. In the immediately surrounding area the pressure begins to fall. A
disturbance of this kind is called a wave depression. As the "wave" develops, a
warm sector of air forms bounded by the warm and cold fronts, which begins to
tie over the engulfing cold air. Both the warm and cold fronts originate from the
centre of the depression.
Dew
• When air is cooled the amount of water vapour that it can
hold decreases. At the dew point temperature, air is
saturated. A further fall in temperature will result in
condensation of excess water vapour in the form of water
droplets. On the ground this is known as dew.
• When air close to the ground cools at night through a loss
of radiation the temperature often falls below the dew point
temperature and dew may form. Dew forms most easily
over grass because the thin layer of air next to it is always
moist due to water transpiration by the grass blades. Wind
sometimes limits or prevents the formation of dew because
the cooling surface air is readily mixed with air above it. The
wind also increases the rate of evaporation, precluding the
formation of water droplets.
• When dew freezes it is known as frost.
Dew Point
• All air contains water vapour of varying quantities. The dew point
indicates the amount of moisture in the air. The higher the dew point,
the higher the moisture content of the air at a given temperature.
Conversely, the dew point of humid air will be higher than the dew
point of dry air.
• Dew point temperature is defined as the temperature to which the air
would have to cool (at constant pressure and constant water vapour
content) in order to reach saturation. A state of saturation exists
when the air is holding the maximum amount of water vapour
possible at the existing temperature and pressure.
• Condensation of water vapour begins when the temperature of air is
lowered to its dew point and beyond. The dew point, like other
measures of humidity, can be calculated from readings taken by a
hygrometer.
Energy
• Weather is caused by the movement or transfer of energy.
Energy is transferred wherever there is a temperature
difference between two objects. There are three main
ways energy can be transferred: radiation, conduction and
convection.
• The Earth receives and absorbs energy from the Sun in
the form of electromagnetic radiation (mostly light and
ultraviolet energy). The Earth also re-radiates a lot of heat
back to the atmosphere and into space (as infrared
radiation). Within the atmosphere however, a lot of energy
is transferred by convection, which drives much of the
world's weather. Convection involves the movement of air.
Evaporation
• Evaporation of water from the Earth’s surface forms one part of the
water cycle. At 100°C, the boiling point, all water will rapidly be turned
to vapour, for the energy supplied to the water is enough to break
apart all the molecular bonds in water. At temperatures between
100°C and 0°C, only some of the molecules in the water have enough
energy to escape to the atmosphere and the rate at which water is
converted to vapour is much slower.
• The rate of evaporation will depend upon a number of factors. Rates
increase when temperatures are higher. An increase of 10°C will
approximately double the rate of evaporation. The humidity of the
surrounding air will also influence evaporation. Drier air has a greater
"thirst" for water vapour than humid, moist air. It follows therefore, that
the presence of wind will also increase evaporation. On still days,
water evaporating to the air remains close to its source, increasing the
local humidity.
Fog
• When air is cooled the amount of water vapour that it can hold
decreases. At the dew point temperature, air is saturated. A further
fall in temperature will result in condensation of excess water vapour
in the form of water droplets. If a sufficiently thick layer of air is
moist, condensation can occur throughout giving rise to fog. Visibility
is usually reduced to below 1,000 metres.
• With no wind at all, fog will form first as shallow streaks near the
ground. More usually there is a little prevailing wind serving to
spread the fog evenly within one or two hundred metres of the
ground. The moister the air, the greater the likelihood of fog forming
under clear skies at night when radiation cooling is greatest. As with
dew and frost, fog formation is most likely in low-lying grounds and
hollows into which colder air sinks, and least likely on hilltops.
Forecasting
• Weather forecasts provide critical information about the weather to
come. There are many different techniques involved in weather
forecasting, from relatively simple observation of the sky to highly
complex mathematical models run on computers. Weather prediction
can be for the next day, next week, or next few months. The accuracy
of weather forecasts however, falls significantly beyond about 10 days.
Weather forecasting remains a complex business, because the
weather can be so chaotic and unpredictable.
• If weather patterns are relatively stable, the persistence method of
forecasting provides a relatively useful technique to predict the
weather for the next day. If it is hot and sunny on one day, it is likely to
be hot and sunny the next. Unfortunately, in many areas of the world
the weather is more unpredictable and changeable than that,
particularly in the mid-latitudes where depressions influence much of
the weather.
• With an understanding of how the air moves and how clouds and rain
form, some prediction can be made by simply observing the sky
overhead, observing wind direction and noting the temperature and
humidity of the air.
Fronts
• Fronts occur at the boundaries of converging air masses which come
together from different parts of the world. Since air masses usually
have different temperatures, they cannot mix together immediately
owing to their different densities. Instead, the lighter, warmer air mass
begins to rise above the cooler, denser one.
• Fronts are usually associated with depressions, regions of low
pressure centred on the rising air which develop as a result of the
Earth's rotation. As the sector of warm air is forced to rise, the cold air
begins to engulf it. The leading edge of the warm air is marked by the
warm front. The cold front marks the rear edge of the warm air and the
leading edge of the ensuing cold air. When the warm air is completely
uplifted off the ground, this may be marked on a synoptic chart by an
occluded front.
• Fronts are accompanied by clouds of all types, and very often by
precipitation. Precipitation is usually heavier although less prolonged at
cold fronts than at warm fronts, since the uplift of warm air there is
more vigorous due to the undercutting of cold air, resulting in increased
atmospheric instability.
Humidity
• Some water in the form of invisible vapour is intermixed with the air
throughout the atmosphere. It is the condensation of this vapour which
gives rise to most weather phenomena: clouds, rain, snow, dew, frost and
fog. There is a limit to how much water vapour the air can hold and this
limit varies with temperature. When the air contains the maximum amount
of vapour possible for a particular temperature, the air is said to be
saturated. Warm air can hold more vapour than cold air. In general the air
is not saturated, containing only a fraction of the possible water vapour.
• The amount of vapour in the air can be measured in a number of ways.
The humidity of a packet of air is usually denoted by the mass of vapour
contained within it, or the pressure that the water vapour exerts. This is
the absolute humidity of air. Relative humidity is measured by comparing
the actual mass of vapour in the air to the mass of vapour in saturated air
at the same temperature. For example, air at 10°C contains 9.4 g/m3
(grams per cubic metre) of water vapour when saturated. If air at this
temperature contains only 4.7 g/m3 of water vapour, then the relative
humidity is 50%.
• When unsaturated air is cooled, relative humidity increases. Eventually it
reaches a temperature at which it is saturated. Relative humidity is 100%.
Further cooling leads to condensation of the excess water vapour. The
temperature at which condensation sets in is called the dew point.
Isobars
• Isobars on a synoptic (weather) chart are
lines along which the atmospheric pressure
is the same. They are of the same nature as
height contours on a geographical map.
Usually they are drawn at intervals of 2 or 4
millibars. By definition, isobars can never
cross each other.
Measuring Weather
•
•
In everyday language, weather means such qualities as wet or fine, warm or
cold. For most people, such descriptive terms are adequate. However, many
industries today require more quantitative assessments of the weather, with
the use of standardised terms measured by suitably designed instruments.
The science of the study of weather is called meteorology. The meteorologist
measures temperature, rainfall, pressure, humidity, sunshine and cloudiness,
and makes predictions and forecasts about what the weather will do in the
future.
Meteorologists still use simple ground-based instruments to measure the
various elements of the weather, including thermometers, rain gauges and
barometers. However, to make really accurate weather forecasts it is useful to
know what the current weather is like over a large geographical area. Weather
radar and satellite photography can offer the meteorologist a snapshot of the
weather in a single image across an entire continent. Radar uses microwaves
to scan for raindrops. Wherever it is raining the raindrops bounce the signal
and by listening to the returning pulse, the radar can compute the location and
intensity of the rain. Satellites allow meteorologists to track the path and
development of weather systems. Satellites don't just "look" in the visible part
of the spectrum. They can also measure the temperature of the ground and
the clouds by "seeing" in infrared. Some satellites even measure the amount of
water vapour in the atmosphere.
Meteorology
• The science of the study of weather is called meteorology.
Meteorology is the study of the changes in temperature, air
pressure, moisture, and wind direction in the lowest part of
the atmosphere in which most of the observed weather
phenomena occur. Meteorologists investigate these dayby-day variations in the weather.
• Weather phenomena are governed by a set of physical
and chemical processes which are determined by simple
mathematical relationships. The way these processes
interact however, creates a much more complex system,
which is why the weather can be so unpredictable and
hard to forecast. Meteorology combines the disciplines of
mathematics, physics, chemistry and geography to try to
simplify the understanding of an inherently complex
atmospheric system.
Movement of Air
• Movement of air is caused by temperature or pressure differences and
is eperienced as wind. Where there are differences of pressure between
two places, a pressure gradient exists, across which air moves: from the
high-pressure region to the low-pressure region. This movement of air
however, does not follow the quickest straight-line path. In fact, the air
moving from high to low pressure follows a spiralling route, outwards
from high pressure and inwards towards low pressure. This is due to the
rotation of the Earth beneath the moving air. Consequently, air blows
anticlockwise around a low-pressure centre (depression) and clockwise
around a high-pressure centre (anticyclone) in the Northern
Hemisphere. This situation is reversed in the Southern Hemisphere.
• Air temperature is generally higher at ground level due to heating by the
Sun, and decreases with increasing altitude. This vertical temperature
difference creates a significant uplift of air, since warmer air nearer the
surface is lighter than colder air above it. This vertical uplift of air can
generate clouds and rain. Sometimes air from warmer regions of the
world collides with air from colder regions. This air mass convergence
occurs in the mid-latitudes, where the warm air is forced to rise above
the colder air, generating fronts and depressions.
Occluded Fronts
• Mid-latitude depressions are usually associated with warm
and cold fronts separating warm and cold sectors of air.
The lighter warm air rises above the heavier cold air, more
gently at a warm front but more vigorously at the cold front
following behind. Cold fronts usually travel faster than
warm fronts, and therefore at some stage of depression
development, the cold front catches up with the warm front.
In cross section, the warm air is lifted right off the ground,
so that the observer on the surface misses out the warm
sector stage. This is known as an occlusion or occluded
front.
• On synoptic (weather) charts an occluded front is
represented by a solid line with alternating triangles and
circles pointing the direction the front is moving. On
colored weather maps, an occluded front is drawn with a
solid purple line.
Precipitation
• All the forms of water that fall from the air to the Earth's surface are
called precipitation. Whether the precipitation is snow, rain, sleet or
hail depends on the temperature of the air that the water falls through.
If the air is above freezing, the precipitation will most likely be rain. If
the air is below freezing, the precipitation will most likely be snow.
When air temperature is only a few degrees above freezing,
precipitation may fall as sleet.
• Hail is most commonly formed within the cumulonimbus clouds of
thunderstorms. Large updrafts of air can throw rain droplets high up
into the tops of the cloud. Here, the temperature is well below
freezing, and the droplets freeze. The droplets then fall and can
become caught in further updrafts, adding a second coating of ice to
make the hailstones larger. This cycle continues until the hailstones
are too heavy to be lifted again. They then falls as hail.
• The amount of rain, sleet, snow or hail which falls in a specified time is
expressed as the depth of water it would produce on a large, level
impermeable surface.
Pressure
• Pressure is a force, or weight, exerted on a surface per unit area, and
is measured in Pascals (Pa). Usually, atmospheric pressure is quoted
in millibars (mb). Because pressure decreases with altitude, pressure
observed at various stations must be adjusted to the same level,
usually sea level.
• Atmospheric pressure is measured by a barometer.
• Air blows from regions of high atmosphere pressure ("highs" or
anticyclones) to regions of low atmospheric pressure. In a highpressure system, air pressure is greater than the surrounding areas.
This difference in air pressure results in wind, or moving air. In a highpressure area, air is denser than in areas of lower pressure. The result
is that air will move from the high-pressure area to an area of lower
density, or lower pressure..
• Air moving from high to low pressure does not however, follow a
straight-line path. In fact, the air moving from high to low pressure
follows a spiralling route due to the rotation of the Earth beneath the
moving air.
Sunshine
• The Sun is the Earth's only source of
radiative energy, heating the surface by
daytime. Greatest heating occurs under
cloudless skies, but even when the sky is
overcast, heating is usually sufficient to
raise the surface temperature above the
night-time minimum.
Synoptic Charts
• With an understanding of how the air moves and
how clouds and rain form, much prediction can be
made by simply observing the sky overhead,
observing wind direction and noting the
temperature and humidity of the air. But to be able
to predict and forecast weather it is necessary to
understand the development of weather systems
such as depressions and anticyclones by means
of isobar plots. Meteorologists plot isobaric
patterns on synoptic charts.
Temperature
• The hotness or coldness of a substance is called
its temperature and is measured with a
thermometer.
• Most temperature scales today are expressed in
degrees Celsius (°C), although one will sometime
see Fahrenheit (°F) in use, particularly in the
United States. The Celsius scale is fixed by two
points, the freezing and boiling point of water,
which at normal atmospheric pressure are 0°C
and 100°C respectively. The scale is then divided
into 100 units. 0°C is equivalent to 32°F and
100°C to 212°F.
Warm Fronts
• A warm front exists when warm air is rising over
cold air. In vertical cross-section, the boundary
takes the form of a gradual slope (roughly 1:100)
and lifting is slow but persistent. As the air lifts
into regions of lower pressure, it expands, cools
and condenses water vapour as flat sheet cloud
(altostratus), from which rain can start to fall once
cloud has thickened to about 2,500 metres from
the ground. On synoptic (weather) charts a warm
front is represented by a solid line with
semicircles pointing towards the colder air and in
the direction of movement. On colored weather
maps, a warm front is drawn with a solid red line.
Water Cycle
• Water covers 70% of the Earth's surface. Almost all of this is stored in
the oceans (97.5%) and in freshwater lakes, rivers and streams on
land (2%). The atmosphere holds less than .001% in the form of water
vapour. If all this water vapour was precipitated completely and evenly
over the whole Earth, it would yield only about 25mm or 1 inch of
rainfall. Water vapour in the atmosphere plays a very important role in
the weather.
• There is always water vapour present in the atmosphere. When the air
becomes saturated, excess water vapour is released as
condensation. This condensation is the source of all clouds and rain.
Water vapour enters the atmosphere by evaporation from surface
bodies of water. These include puddles, ponds, streams, rivers, lakes
and oceans. Water also enters the atmosphere by evapotranspiration
from plants and trees. The water vapour is returned to the Earth's
surface as precipitation (rain, hail, sleet or snow), and is received by
soil, vegetation, surface streams, rivers and lakes and ultimately the
sea. This cycle of evaporation, condensation and precipitation is
called the water cycle of the Earth and atmosphere.
Weather Symbols
• Weather symbols are plotted on a synoptic chart used for
weather forecasting, and show the position of weather
stations collecting information about the weather, and details
of the various weather elements. The position of a
meteorological station is marked by a small circle. The
weather report for each station is then plotted in and around
the circle. Elements like temperature and pressure are
entered as plain figures. Others, like the occurrence of rain,
snow, cloud and fog are plotted as internationally agreed
symbols.
• Temperature is measured in degrees Celsius to the nearest
whole degree. Pressure at a station is standardised to sealevel pressure measured in millibars. Sometimes the
hundreds figure for the pressure is omitted as being
understood since the pressure is almost always between
950 and 1050 millibars. For example, a pressure of 987.8
millibars would be written as 878; 1014.3 millibars as 143.
Typical weather symbols
Wind
• The air is nearly always in motion, and this is felt as wind.
Two factors are necessary to specify wind, its speed and
direction. The direction of wind is expressed as the point of
the compass from where the wind is blowing.
• Wind develops as a result of pressure or temperature
differences between two locations on the Earth's surface.
Sea breezes for example, develop due to the differential
heating of land and sea at the coast during warm sunny
days. Winds also blow out from high-pressure regions or
anticyclones and into low-pressure regions, for example
depressions. The wind however, does not blow in a straight
line, but follows a spiralling path.
• http://www.ace.mmu.ac.uk/eae/Weather/Ol
der/Weather_Introduction.html
• http://sixthsense.osfc.ac.uk/geography/pag
es/general_links/environment.asp