Weather and climate

1
Weather and climate
2
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.

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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.

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Convectional Rainfall

• Stage 1.The sun heats the ground and warm air
  rises.
• Stage 2As 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.

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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

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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.

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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

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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.

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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.

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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

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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.

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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.

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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.

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Air mass

• A large body of air that has similar temperature,
  pressure and moisture properties.

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Anticyclone

• High pressure system in which air descends to
  give calm conditions and clear skies. Associated
  with summer heatwaves and winter frosts and fogs.

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Atmosphere

• The air surrounding the Earth and bound to it by
  gravity.

17
Atmospheric pressure

• Pressure produced by the atmosphere on any
  surface by its weight.

18
Clear sky

• Sky with a total cloud cover of less than one
  okta.

19
Climate

• long-term (30 year) weather averages.

20
Cold front

• The "leading edge" of a relatively cold air
  mass.

21
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.

22
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.

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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.

24
Evaporation

• The physical process by which a liquid or solid
  substance is transformed to a gas the opposite
  of condensation.

25
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.

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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.

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Humidity

• Water vapor content of the air.

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Isobar

• A line of equal or constant pressure. Measured in
  millibars (mb).

29
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.

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Occluded front

• A front that is formed as a cold front overtakes
  a warm front and lifts the warm air completely
  off the ground.

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Okta

• A measure of cloud cover (in fractions of eight)
  on a synoptic chart.

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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.

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Prevailing wind

• Is the most common wind direction for a
  particular location.

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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.

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Seasonality

• Periodic fluctuations in the climate related to
  seasons of the year e.g. wet winters, drier
  summers.

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Smog

• A word currently used as a synonym for general
  air pollution. It was originally created by
  combining the words "smoke" and "fog."

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Synoptic chart

• A weather chart reflecting the state of the
  atmosphere over a large area at a given moment.

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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.

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Temperature range

• Maximum minus the minimum temperature for a
  particular location.

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Warm front

• The forward edge of an advancing warm air mass
  that is rising over cooler air in its path.

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Warm sector

• The zone of warm air within a depression.

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Water vapour

• Water substance in vapour (gaseous) form one of
  the most important of all constituents of the
  atmosphere.

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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.

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Wind

• Movement of air caused by changes in temperature
  and air pressure. Winds are always identified by
  the compass direction from which they blow.

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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.

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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.

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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.

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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.

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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.

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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.

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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 5C 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.

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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 0C, or frost if it is
  below 0C. Cooling of a larger layer of air near
  to the ground may produce mist of fog, which
  freezes if the dew point is below 0C. Air that
  is cooled to its dew point by rising and
  expansion will condense to form clouds. Above
  0C, small droplets of water are formed.

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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 0C, 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.

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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
  0C, ice crystals dominate.

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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.

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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.

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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.

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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.

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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.

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Evaporation

• Evaporation of water from the Earths surface
  forms one part of the water cycle. At 100C, 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 100C and 0C,
  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 10C 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.

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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.

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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.

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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.

64
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 10C 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.

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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.

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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.

67
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
  day-by-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.

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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.

69
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.

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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.

71
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 high-pressure
  system, air pressure is greater than the
  surrounding areas. This difference in air
  pressure results in wind, or moving air. In a
  high-pressure 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.

72
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.

73
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.

74
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 0C and
  100C respectively. The scale is then divided
  into 100 units. 0C is equivalent to 32F and
  100C to 212F.

75
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 1100)
  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.

76
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.

77
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 sea-level 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.

78
Typical weather symbols
79
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80
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.

81

• http//www.ace.mmu.ac.uk/eae/Weather/Older/Weather
  _Introduction.html
• http//sixthsense.osfc.ac.uk/geography/pages/gener
  al_links/environment.asp