The Concept of Weather

WEATHER
-Daily atmospheric conditions of a place at a particular time.
Elements of Weather

1. Temperature
2. Humidity precipitation
3. Precipitation
4. Atmospheric pressure
5. Wind
6. Sunshine
7. Cloud cover
   A Weather Station
   -A place where observation, measuring and recording of weather elements is done
   Factors to Be Taken Into Account When Sitting a Weather Station
8. An open place where there is little obstruction of weather elements.
9. Accessible place so that recording can be done easily.
10. A fairly level or gently sloping ground (5◦) so that it’s easy to position weather
    instruments.
11. The place should provide a wide view of the surrounding landscape and the sky.
12. The site should be free from flooding.
13. The place should have security.
    Instruments for Measuring Elements of Weather
14. Thermometer-temperature
15. Hygrometer-humidity
16. Rain gauge-rainfall
17. Barometer-air pressure
18. Sunshine recorder-sunshine duration and intensity
19. Wind vane –wind direction
20. Anemometer-wind speed
21. Evaporimeter-rate and amount of evaporation.
    The Stevenson Screen
    15
    -A white wooden box mounted on 4 legs used to house thermometers and hygrometers.
    The instruments which are found in it are:
22. Maximum thermometer
23. minimum thermometer
24. Six’s thermometer
25. hygrometer-wet bulb and dry bulb thermometer
    Importance
26. Provide shade conditions for accurate temperature recording.
27. Ensure safety of thermometers because they are delicate.
    Qualifications Which Make Stevenson Screen Suitable For Its Work
28. Painted white for little absorption of solar heat energy.
29. Made of wood which is a bad conductor of heat.
30. Well ventilated to allow easier flow of air inside it.
31. Raised to prevent contact with terrestrial radiation.
32. Has double roof which acts as an insulator to prevent direct heating from the sun.
    Recording and Calculating Weather Conditions
    Temperature
    -Degree of hotness of an object or a place.
    It’s measured using 3 types of thermometers namely:
33. Maximum thermometer
34. Minimum thermometer
35. Six’s thermometer
    Maximum Thermometer
    It’s used to measure the highest temperature reached in a day.
    It uses mercury.
    16
    How It’s Used/Works

• Temperature rises causing mercury to expand.
• Mercury pushes the index up.
• When temperature falls mercury contracts.
• The maximum temperature is read from the scale at the lower end of the index.
• Thermometer is reset by shaking it to force mercury back into the bulb.
  Minimum Thermometer
  It’s used to record the lowest temperature reached in a day.
  It uses alcohol.
  How it’s Used/Works
• Temperature falls causing alcohol to contract.
• Alcohol pulls the index down.
• When temperature rises alcohol expands and rises in the tube.
• The index remains where it was pulled.
• Minimum temperature reading is obtained from the scale at the lower end of the
  index.
  Calculating Temperature

1. Diurnal/daily Temperature range
   -Difference between the maximum and minimum temperature for any one day.
2. Mean Daily Temperature
   -Average of the maximum and the minimum daily temperatures.
3. Mean Monthly temperature
   -Sum of mean daily temperatures in a month divided by the number of days in that
   month.
4. Mean Monthly minimum Temperature
   -Sum of daily minimum temperatures divided by the number of days in that month.
5. Mean Monthly Maximum Temperature
   -Sum of daily maximum temperatures divided by the number of days in that month.
6. Mean Annual Temperature
   -Sum of mean monthly temperatures divided by 12.
7. Mean Annual Temperature Range
   -Difference between the highest and the lowest mean monthly temperatures in a year.
8. ◦k=◦c+273
9. ◦F=(◦c×1.8)+32 derive the rest from the formulas.
   Rainfall
   17
   Rain gauge is the instrument used to measure the amount of rainfall in a day.
   It should be made of impermeable material which can’t absorb water.
   How It’s Used/Works

• It’s taken to an open space to prevent water from dropping into the funnel.
• Its sunk into the ground to prevent evaporation
• The funnel top is left 30cm above the ground to prevent splashes of water and run
  off.
• After 24 hours water is emptied into the measuring cylinder.
• The reading of the amount of rainfall is got from the measuring cylinder in
  millimetres.
• The figure represents the millimetres of water falling on each square millimetre
  of the ground.
• It could be used to measure snow fall by melting it before the readings are gotten.
  Calculating Rainfall

1. Monthly Rainfall Total
   -Sum of rainfall recorded in a month.
2. Annual Rainfall Total
   -Sum of monthly rainfall totals for 12 months.
3. Mean Monthly Rainfall
   -Sum of rainfall totals for a particular month over several years divided by the number
   of the years of observation.
4. Mean Annual Rainfall
   -Sum of mean monthly rainfall for 12 months of the year.
   Sunshine
   18
   Duration of sunshine is measured using Campbell stokes sunshine recorder.
   How It Works

• Spherical lens focuses light on sensitized paper.
• The paper burns when the sun is shining.
• The total hours of sunshine is got by adding all the burnt sections from
  calibrations on the side of sensitized paper.
• The sensitized paper is changed every day.
  Humidity
  Humidity is the condition of atmosphere with reference to its water content.
  It’s measured with hygrometer or psychrometer which consists of wet and dry bulb
  thermometers kept in Stevenson screen.
  Dry bulb thermometer is a thermometer covered with muslin bag immersed in water
  while dry bulb thermometer has no muslin.
  How It Works
• When air is dry there is a lot of evaporation from the muslin.
• Evaporation cools the bulb of thermometer resulting in a low temperature
  reading.
• When humidity is high there is little evaporation from the muslin.
• The wet bulb thermometer is cooled at a slower rate and both thermometers show
  almost the same temperature reading.
• The difference in readings between the two thermometers is used to determine
  relative humidity.
  19
  Interpretation of Hygrometer Readings
• When the 2 readings are the same, relative humidity is 100% i.e. the air is
  saturated.
• If the difference is small, humidity is high.
• If the difference is big, humidity is very low.
  Calculating Humidity
  Absolute Humidity
  -Actual amount of water vapour a given volume of air can hold. It’s expressed in g/m3.
  Specific Humidity
  -Mass of water vapour in a given mass of air. It’s expressed in g/km.
  Relative Humidity
  -Ratio between the absolute humidity and the maximum amount of water the air can
  hold expressed in a percentage.
  R.H.=A.H/Maximum amount of water the air can hold at the same temperature.
  Example
• If the air at 20◦c contains 10g/m3 and given air can hold a maximum of
  20g/m3.calculate the relative humidity.
  10×100/20=50%
  Wind
  Direction
  Wind direction is determined using wind vane.
  How It Works
  20
• As the wind blows the arrow swings.
• The arrow points in the opposite direction of the wind flow.
• The direction is read from the cardinal compass points.
• The arrow will point in the direction from which the wind is blowing.
• For instance if it points S the wind is blowing from S towards N.
  Wind Sock
  -Used to indicate the general direction of wind flow.
  -Not kept in a weather station because it doesn’t give the accurate direction of wind
  flow.
  -Seen near airstrips for the benefit of pilots.
  How it Works
• When wind blows the bas stretches out in the direction that the wind is blowing.
  Wind speed/Velocity
  -Measured using anemometer.
  How It Works
• When wind bows hemispherical cups rotate.
• The number of rotations is obtained from the metre on the lower part of the
  anemometer.
• The units for measuring wind are called knots.
  Atmospheric Pressure
  21
  -The force exerted by gases in the atmosphere on some area or body on the earths
  surface.
  -Measured using barometers of three types namely mercury, aneroid and Fortin
  Barometers.
  Mercury Barometer
  How It Works
• Air exerts pressure on the mercury in the beaker.
• The height of mercury in the tube is proportional to the atmospheric pressure.
• The readings are taken in mmHg.
• Its 760mmHg at sea level
  Advantage
  Quite accurate
  Disadvantage

1. Cumbersome to carry around.
2. Can be damaged quite easily while being carried around.
   Aneroid Barometer
   -Measures changes in atmospheric pressure.
   How It Works

• Has air tight compartment (vacuum).
• Compartment expands when pressure decreases.
• It collapses when pressure increases.
• The movement is transmitted by lever to a pointer on a dial.
• The readings are in kg/cm3.
  Evaporation
  The rate and amount of evaporation is measured using piche and tank evaporimeters.
  Piche Evaporimeter
  22
• When there is a lot of sunshine water evaporates from the blotting paper.
• The level of water in the glass tube reduces.
• The rate and amount of evaporation is got by looking at the scale on the outside
  of the glass tube.
• The units are in mm.
  Tank Evaporimeter
  How It Works
• The tank with water is put in the open.
• Water evaporates when there is a lot of sunshine.
• Water in the tank reduces.
• The rate and amount of evaporation is got from calibrations in the inner side of
  the tank in mm.
  Cloud Cover
  The amount of cloud cover is observed using eyes.
  It’s given in oktas.
  Okta=approximately 1/8 of sky is covered with clouds.
  Weather Forecasting
  -Prediction of the conditions of the atmosphere for a given place for a certain period.
  Methods of Weather Forecasting
  Traditional Methods
  -Prediction of weather based on traditional beliefs and facts.
• Plants shedding leaves indicates period of drought.
• Safari ants indicate it will rain.
• Migration of butterflies also indicates it will rain.
  23
• Croaking of frogs during dry season indicate its going to rain.
• Flowering of certain plants indicates the onset of rainfall.
• Changes in the intensity of sunshine indicate its going to rain.
  Modern Methods
  -Prediction of weather using modern instruments and new technology of collecting,
  transmitting, processing and analysing weather data.
  Instruments Used

1. Satellites-electronic devices which orbit the earth which collect and transmit
   weather data which is interpreted by computers.
2. Radar-an instrument used to see cloud formation.
3. Sensors/radiosodes-instrument fixed on a balloon used to measure atmospheric
   pressure, temperature and humidity.
4. Computers-electronic device used to store, analyse and display weather
   information.
   Significance/Importance of Weather Forecasting
5. Helps us to be aware of natural calamities related to weather before they occur so
   as to take precautionary measures.
6. Guiding tourists on when to visit national parks.
7. Helps farmers to plan their activities such as planting, harvesting, etc.
8. Ensures air and water transport is carried out safely.
9. Helps sporting people to plan their training and competition schedules.
10. Helps people to plan many other activities such as mining, electricity generation,
    holiday events, etc.
11. Helps fishing communities to plan their activities.
    Factors Hindering Weather Forecasting
12. Lack of skilled man power due to limited training facilities.
13. Lack of modern equipment leading to wrong forecasts.
14. Natural calamities such as storms and earthquakes.
15. Extreme weather conditions which may damage or displace instruments.
16. Use of faulty instruments.
17. Human error.
18. Poor sitting of instruments.
    Factors Influencing Weather
    Temperature
    Factors influencing temperature
19. Altitude
    -Height above sea level.

• Temperature decreases with increase in height due to air at higher altitude being
  thinner and hence there is less particles e.g. gases, dust, smoke and water vapour
  to store heat so its rapidly lost to the outer space.

1. Latitude
   -Distance from the equator.
   Temperature decreases with increase in latitude.

• Places neat equator experience high temperature due to the rays of the sun
  travelling a shorter distance facing less interference from atmospheric conditions
  24
  hence more solar energy reaches the earths surface. Also the rays of the sun strike
  the earth at right angles hence solar energy is concentrated over a small area.
• At higher latitudes the rays of the sun travel a longer distance facing more
  interference from atmospheric conditions hence less solar energy reaches the
  earth’s surface. Also the rays of the sun strike the earth at an acute angle hence
  solar energy is spread over a large area.

1. Aspect
   -Direction of slope.

• At higher latitudes slopes facing the equator have higher temperature because
  they face the sun while those facing the poles have lower temperature have lower
  temperature because they face away from the sun.

1. Winds
   -Transfer heat from one place to another.

• When they blow from cool areas they take the cooling effect to the areas they
  blow over and when they blow from warm areas they take warming influence to
  the places they blow over.

1. Distance from a Large Water Body

• Areas near a large water body experience lower temperature during the hot
  season and higher temperature during the cool season due to sea breezes, warm
  and cold ocean currents and wind blowing over water which could be either
  warmer or cooler than the adjacent land.

1. Cloud Cover

• Clouds reduce the amount of solar energy reaching the surface by absorbing,
  scattering and reflecting solar radiation.
• When there are clear skies during the day the temperature is higher due to the
  earth receiving maximum solar insolation.
• During clear nights there are very low temperatures due to a lot of terrestrial
  radiation being lost to the outer space.
• Cloudy nights on the other hand are warmer due to clouds radiating to the earth
  heat absorbed during the day.

1. Length of Day

• The longer the period of solar insolation the greater the quantity of radiation a
  place receives and hence the more the heat that will be generated by the earth and
  vice versa.

1. Solar Altitude

• At equinox when the earth is farthest from the sun the temperature on the earth is
  lower due to less solar radiation reaching the earth’s surface due to travelling a
  longer distance and hence facing great interference from atmospheric conditions.
• At solstices the earth receives more solar energy due to travelling a shorter
  distance and hence facing less interference from atmospheric conditions.

1. Solar Input

• Sometimes the sun gives out more heat due to reactions being violent causing
  temperature on the earth to be higher.
• When it gives out less heat the temperature on the earth is lower.
  10.Surface Conditions
  25
• Light surfaces e.g. smooth surfaces reflect sunlight and hence less solar energy
  reaches the earth’s surface.
• Dark and irregular surfaces such as with vegetation absorb more heat leading to
  higher surface temperatures.
  Humidity
  Factors Influencing Humidity

1. Temperature

• Places with high temperature experience high humidity due to high evaporation
  and air having high capacity to hold moisture.
• Places with low temperature have low humidity due to low evaporation and air
  having low capacity to hold moisture.

1. Source of Moisture

• Areas near water bodies e.g. Kisumu and Mombasa experience high humidity
  due to evaporation of water from the water body.
• Places near thick vegetation also have high humidity due to evapotranspiration.
• Areas far away from water bodies such as the middle of deserts have low
  humidity.
• Areas receiving heavy rainfall also have high humidity.

1. Air Pressure

• There is high humidity at low altitudes because high pressure compresses air
  warming it increasing its capacity to hold moisture and also causes high
  evaporation.
• There is low pressure at high altitudes because air expands and cools thus
  reducing its capacity to hold moisture.

1. Latitude

• Low latitudes experience high humidity due to high temperatures resulting into
  high rates of evaporation and air having high capacity to hold moisture.
• High latitudes experience low humidity due to low temperatures resulting into
  low rates of evaporation and air having low capacity to hold moisture.
  Significance of Humidity/Moisture

1. Affects rain formation in such as way that places with high humidity are likely to
   experience higher rainfall than those with low humidity.
2. Regulates the heat loss from the earth’s surface by absorbing terrestrial radiation
   (process in which the earth gives off heat into the atmosphere).
3. It affects sensible temperature in that the higher the humidity the more we
   experience sensible temperature.
   Precipitation
   -The forms in which the earth’s surface receives moisture.
4. Snow
   Solid precipitation formed when tiny water droplets freeze and form ice crystals.
   The crystals may fuse to form flakes.
5. Sleet
   -Precipitation which is a mixture of rain and snow.
6. Hail
   Roughly spherical lumps of ice formed when super cooled cloud droplets mould
   themselves around ice crystals before cooling. It destroys crops life and house roofs.
   26
7. Dew
   -Precipitation consisting of water droplets formed on cold surfaces at night e.g. iron
   roofs and glass blades.
   How It’s Formed

• In a clear night there is a high ground radiation.
• Temperature of the earth’s surface fall below dew point (temperature at which air
  being cooled becomes saturated).
• Excess water condenses on cold surfaces.

1. rain
   -Precipitation consisting of water drops/droplets formed when tiny water droplets merge
   around particles of matter and become heavy and fall down to the earth.
   Condensation
   Turning of water vapour into tiny water droplets as cooling continues below dew point.
   The droplets join to form clouds.
   Causes of Condensation
2. Adiabatic cooling-cooling of moist air as it rises vertically.
3. Orographic cooling-cooling of moist air as it climbs a hill or mountain.
4. Frontal cooling-cooling of warm air mass when it blows towards a cold air mass.
5. Advection cooling-cooling as a result of moist air moving over a cool land or
   sea.
   How Condensation Takes Place/Cloud Formation

• Moist air rises to the condensation level (altitude where temperature is below dew
  point.
• It’s cooled below dew point.
• Tiny water droplets condense around tiny particles such as dust, smoke particles
  and pollen grains and salt particles (condensation nuclei).
• The droplets merge and eventually become bigger and fall as rain.
• If moisture rises to an altitude where temperature is below 0◦c the condensed
  water droplets freeze forming ice particles or super cooled water (water which
  has remained in a liquid state at temperatures below freezing point due to lack of
  sufficient condensation nuclei.
• Super cooled cloud droplets may mould themselves around ice crystals before
  freezing to form hail.
  Types of rainfall

1. Relief/Orographic/Mountain rainfall
   27
   Rain experienced on the windward slopes of mountains or hills formed when moist air
   is forced to rise over a mountain or a hill.
   How it Forms

• Moist air is forced to rise over a hill or mountain.
• The temperature and air pressure decreases making it to expand.
• Air cools due to decreased temperature and decreased pressure causing it to
  expand.
• Moisture condenses forming tiny water droplets (clouds).
• The tiny water droplets in clouds merge and become too heavy to be suspended
  in air and fall as rain.
• Air proceeds to the leeward side with low moisture content.
• Since its heavier due to being cool it descends over that side and gets warmed
  making it to hold onto the little moisture it had causing that side to receive low
  rainfall (rain shadow).

1. Convectional Rainfall
   Type of rainfall common near large water bodies formed as a result of convective rising
   and cooling of moist air.
   It’s accompanied by thunderstorms.
   How it forms

• Ground or water body is heated causing evaporation.
• There is convective rising and cooling of moist air.
• Condensation takes place forming tiny water droplets (clouds).
• The droplets merge and fall as rain.
  28
• The cooled dry air descends to the surface where its heated and its capacity to
  hold moisture is increased.
• The process is repeated.

1. Frontal/Cyclonic Rainfall
   Type of rainfall common in mid-latitudes formed when warm air blows towards a cold
   area or when warm air mass meets with a cold air mass.
   It’s accompanied by cyclones (violent winds).
   How it Forms

• Warm moist air mass meets with a cold air mass.
• The warm air is forced to rise as it’s less dense.
• It cools as it rises at the line of contact with cold air.
• The moisture condenses forming clouds resulting in frontal rain.
  Factors Influencing Rainfall Types and Amounts

1. Relief/Topography
   Relief features such as mountains and hills results in the rising and cooling of moist
   winds to form relief rainfall.
2. Aspect
   Windward slopes which are on the path of rain bearing winds receive heavier rainfall
   than leeward slopes which face away.
3. Forests and Water Bodies
   Areas near forests and large water bodies experience higher rainfall and more often due
   to high rate of evaporation.
4. air pressure
   High pressure areas receive low rainfall than low pressure areas due to pushing of air
   masses from high pressure to low pressure. The high pressure areas have descending
   dry air.
5. air masses
   When warm and cold air masses meet frontal rainfall is formed.
6. Ocean Currents

• It influences rainfall whereby coasts washed by warm ocean currents experience
  heavy rainfall when moist onshore winds are warmed by the current and made to
  hold on to moisture which they release on reaching the land.
• The coasts washed by cold ocean currents on the other hand experience low
  rainfall as a result of moist winds being cooled and moisture in them condensed
  resulting in rain falling over the ocean thereby bringing little or no rain to the
  29
  coastal areas. This is the cause of western margin deserts e.g. Kalahari and
  Namib deserts.
  Atmospheric pressure
  Factors Influencing Atmospheric Pressure

1. Altitude

• Pressure decreases with increase in altitude because the column of air becomes
  shorter hence it exerts less weight.

1. Temperature

• When air is heated it expands and exerts pressure over a large area resulting in
  reduced pressure.
• When it’s cooled it contracts and exerts pressure over a small area resulting in
  increased pressure.

1. Rotation of the earth

• Rotation pushes air masses from poles towards the equator causing air to spread
  out and occupy more space causing it to expand making pressure to decrease.
• When air from the equator moves towards the poles it occupies less space
  causing it to contract resulting into high pressure.
  Mist and Fog
  Mist and fog are a mass of tiny water droplets suspended in the lower layers of the
  atmosphere.
  Fog is denser than mist i.e. has more moisture.
  Both hinder visibility although fog reduces visibility to less than a kilometre.
  When fog mixes with smoke its called smog.
  How They Form
• Moist air cools below dew point.
• Condensation takes place.
• The resultant water droplets remain suspended in the air.
  Types of Fog

1. Radiation Fog

• Type formed when moist air is cooled below dew point as a result of intense
  radiation on the ground at night.

1. Advection Fog

• Type formed when moist air from the sea moves horizontally over a cold surface e.g.
  snow covered ground.

1. Orographic/Hill/Upslope Fog

• Type formed when moist air is cooled after climbing a hill or mountain.

1. Evaporation Fog

• Type formed when water vapour is added to cold air that is already near saturation
  causing excess water vapour to condense and form fog.

1. Frontal Fog

• Type formed when warm moist air is cooled from below as it rises over a cold air
  mass.

1. Steam Fog

• Type formed when moist air passes over the surface of a much warmer fresh
  water body.
  30
• The warm water is cooled from above and condensing water vapour forms fog. It
  appears to be steaming.

1. Ice Fog

• Type formed when water vapour is converted directly into ice crystals when
  temperatures are below freezing point.
  Clouds
  -Are a mass of tiny droplets or ice particles formed when water vapour condenses.
  Three Cloud Forms

1. Cirroform
   -Thin and wispy clouds composed of ice crystals.
2. stratiform
   -Appear as greyish sheets covering most of the sky and are rarely broken into units.
3. Cumuliform
   -Are massive rounded with a flat base and limited horizontal extent and billow upwards
   to great heights.
   Basic Cloud Types
4. Stratus Clouds
   -Are found in layers, are flat in shape and resemble fog.
5. Nimbus Clouds
   -are dark at the base and sometimes white at the sides and cause rain and thunderstorms.
6. Cirro-cumulus
   -Are white clouds consisting of white ice crystals.
7. Nimbostratus
   -A rain cloud which is dark grey and spreads over the sky in low uniform layers.
8. Cumulus Clouds
   -Clouds with a flat horizontal base, massive, rounded and less horizontal extent.
9. Alto cumulus
   -High clouds composed of ice crystals which indicate fair weather.
   World distribution of Pressure Zones and the Planetary wind System/World
   Prevailing Winds
   The Equatorial Low pressure Zone (ITCZ-low)

• Found between 23 ½ ◦N and 23 ◦N
• Experiences high temperatures.
• A zone of low pressure and doldrums (light and intermediate winds).
• Zone where S.E and N.E Trade Winds converge.
• Associated with convectional rain and thunderstorms.
• Migrates to the N and with the apparent movement of the overhead sun.
  The Sub-tropical High Pressure Zone
• Found within 30◦N and 30◦S.
• A zone of high pressure.
• A region of calm descending air.
• Source of Trade Winds and Westeries.
• Zone of divergence of T. Winds and Westeries.
  The Temperate Low Pressure Zone
• Found within 60◦N and 60◦S.
• A low pressure zone.
  31
• Zone of convergence of westeries and polar easteries.
  The Polar High Pressure Zone
• Found over the poles 90◦N and 90◦S.
• A high pressure Zone.
• Zone of descending calm air of low temperature.
• Source of polar easteries.
  The Worlds Prevailing Winds
  These are the major winds blowing over the earth frequently and consistently and which
  influence the world weather.

1. Trade Winds

• Blow from sub-tropical high pressure zone and blow to the equatorial low pressure
  belt.

1. Westeries

• Originate from sub-tropical high pressure zone and blow to the temperate low
  pressure belt.

1. The Polar Easteries

• Originate from polar high pressure zone and blow to temperate low pressure zone.
  Monsoon Winds
• Seasonal winds which reverse in the direction of flow.
• They blow towards the land during summer (onshore) and from the land during
  winter (off shore).
• Bring heavy rains when onshore which can cause severe flooding.
• Well developed in the Indian Sun-continent, china, Japan and S.E Asia.
  Air Masses
  -Distinct large parcels of air moving in one direction
  -Originate from areas of uniform weather and topography from where they derive their
  characteristics e.g. flat areas, forests, deserts, and snow covered areas.
  Characteristics of Air Masses
• A large volume of air.
  32
• Covers an extensive area.
• Has uniform temperature and humidity.
• Distinct from the surrounding air.
• Retains its characteristics when it moves away.
  Types of air Masses

1. Equatorial Air Mass

• Originate from equatorial oceans.
• It’s hot and unstable.

1. Sub-tropical Air Mass

• Forms near sub-tropical high pressure belt.

1. Polar Air Mass

• Forms near the poles or temperate low pressure zone.
• It’s cool.

1. Arctic and Antarctic air Masses

• Forms over the ice sheets of Greenland and Antarctica respectively.

1. Effect of air masses on Weather

• When warm moist air mass and cool air mass meet cyclonic rainfall is formed e.g.
  tropical maritime and polar maritime.
• Cool air masses take cooling effect to the areas they move to e.g. polar continental.
• If they are warm they take warming influence to the area they move to e.g. tropical
  continental.
  Pressure Systems in the World

1. Cyclone

• It’s a low pressure system characterised by low pressure at the centre and increases
  outwards.
• Starts in areas where air ascends from the ground to the atmosphere and descends at
  high altitude.
• It’s of two types. Tropical cyclones e.g. hurricane, typhoon and willy willies and
  depressions which are characterised by temperate latitudes.
• The movement of wind is anticlockwise in the N. hemisphere and clockwise in the
  S. hemisphere.

1. Anticyclone

• A high pressure system characterised by high pressure at the centre and decreases
  outwards.
  33
  •
• It starts in areas where air is descending from the atmosphere onto the ground and
  then blows outwards on the ground.
• The movement of wind is clockwise in the N. hemisphere and anticlockwise in the
  S. hemisphere.
  Local Winds
  -Which occur regularly for a short period of time affecting a limited area.
  -Modify the weather of the area they blow to.

1. Sea Breeze
   -A light and gentle wind which blows from the sea to the adjacent land.
   How it Forms

• During the day land is heated faster than the sea.
• Air over the land is warmed and rises.
• Air from the sea moves to the land to replace the rising air.
• The rising air from the land cools and descends over the sea at high altitude.
• Circulation continues until the pressure difference is reversed at night.
  Effects on weather
  It takes cooling effect on land on a hot afternoon.

1. Land Breeze
   -A light and gentle wind which blows from land to the sea during the night.
   34
   How it Forms

• At night land loses heat faster than the sea.
• Air over the sea is warmed and rises.
• Air from the land moves to the sea to replace the rising air.
• Rising air from the sea descends over land at high altitude.
• Circulation continues until pressure difference is reversed during the day.
  Effects on weather
  It causes early morning showers through moisture brought towards land at high altitude.

1. Anabatic winds (Valley Breeze)
   -Cool local winds which blow from the valley to the hill tops during summer
   afternoons.
   How it Forms

• During the day hill tops are heated more than valley bottoms.
• Air over the hill tops is warmed and rises.
• Cool air over the valley move up to the hill to replace the rising air.
  Effect on weather
  35
  -Cause afternoon showers on hilly grounds.

1. Katabatic/Descending Winds
   -Cold local winds which blow from hill tops to the valley during the night.

• During the night hill tops lose heat faster than the valley.
• Air over the valley is warmed and rises.
• Cool air over the hill tops move to the valley by gravity to replace the rising air.
  Effect on Weather
  -Takes chilly conditions on valley bottoms.

1. Harmattan Winds
   -N.E winds which originate from Sahara and blow across W. Africa between November
   and March taking dry conditions there.
2. Fohn Winds (Alps)
   -Local cold winds which slide down the leeward side of the mountain at high speed and
   are warmed producing a temperature rise.
   Due to the high speed and temperature they are associated with wild fires.
   They are known as Chinook in Rocky Mountains, Santa Anas in California and Mistral
   in France.
   Factors influencing Wind Flow (Speed and direction)
3. Pressure Gradient
   If the pressure difference between high and low pressure areas is high the winds blow at
   high speed (strong) but if it’s low they blow at high speed (are gentle).
4. distance between Places of High and Low Pressure
   if the high and low pressure areas are near each other winds blow at high speed but if
   distant from each other winds blow at low speed.
5. Rotation of the earth
   Rotation of the earth deflects winds to the right in the N. hemisphere and to the left in
   the S. hemisphere.
6. Frictional Force
   If the surface of the earth is rugged or has obstacles such as hills, mountains, valleys or
   vegetation the wind is blocked causing speed reduction and its direction of flow is also
   changed