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Weather - Geography Form 1

Weather forecasting

Gadgets such as a radiosonde carries a balloon up to 12 km above the ground with weather instruments that records temperature and pressure.

Satellites are also used recording and transmitting weather records and photographs of clouds and movement of cyclones.

Traditional weather prediction is done by studying vegetation and the behaviour of animals and insect.

By the end of the session you should be able to ;

a) define weather
b) state weather elements

c) expain the conditions necessary for setting up a weather station.

d) use instruments to measure weather elements.

e) analyse and interprete data on weather conditions

f) describe the structure and composition of the atmosphere

g)explain the factors influencing weather


Weather Elements

Various weather elements combine to produce certain weather conditions. these inlude;

Sunshine which is light and heat from the sun

Temperature - Hotness or coolness of a place

Humidity - Moisture particles

Precipitation - All forms of moisture, such as rain, snow, or hail.

Atmospheric pressure - weight of the atmosphere upon the earth

Winds - Moving air

Measurement of weather elements

Different instruments are used to measure and record elements of weather.The table below shows the elements of weather and the corresponding measuring instrument.

A Weather station

By the end of this lesson you should be able to;

  • describe a weather station
    explain the conditions for setting up an ideal weather station
  • describe the Stevenson's screen
  • identify the instruments kept in the Stevenson's screen
  • Weather station

A weather station is an area set aside and fitted with the necessary instruments for measuring, reading, observing and recording weather elements.

Conditions necessary for setting up a weather station

The space must be open to allow free flow of air

  • The space (area) should provide a wide view of the surrounding environment.
  • The ground should be near level
  • It should be away from tall trees and other tall structures to avoid the obstruction of wind, rain and sun rays
  • The grass should be kept low
  • The area should be free from floods or heavy surface run off
  • The area/space should be fenced and secured with a lockable gate.
  • The altitude should be established preferably from a topographical map
  • General geographical directions (Orientation) should be established
    • The success of the weather station depends on proper management and maintenance of records. There must be a trained person to maintain the following records
      Daily records
      Monthly summary
      A graphical record book
      Within the station a container for keeping some weather measuring instruments is built. This is the Stevenson screen.

      The Stevenson Screen
      By the end of the session you should be able to;

      describe the Stevenson Screen

      identify the instruments kept in the Stevenson Screen.

      The Stevenson screen

      The Stevenson screen is a wooden box raised on four stands.

      It houses thermometers and the hygrometer

      The screen has louvered sides to allow the free air circulation.

      Has open slots for air circulation at the base.

      The roof has double boards to reduce the direct absorption of heat from the sun

      It is painted white both inside and outside to reflect heat and light avoiding undue influence on air temperature.

      The stands are usually metallic to withstand rotting or attack by termites

      The box should be raised 1.2metres so that the heat from the ground does not influence air Temperature.

      Provides shelter for delicate instruments for reliable and accurate records.

      For further understanding of weather elements, more information is provided on each of the elements

      Learning objectives

      By the end of the session you should be able to explain factors that influence different weather elements.


      Factor influencing Temperature

      Temperature over the earth's surface will differ due to the following factors that influence atmospheric insolation and radiation.

        • Latitude
        • Length of day
        • Altitude
        • Aspect
        • Winds
        • Distance from the water bodies
        • Ocean currents
        • Cloud cover


      Latitudes are imaginary lines on the earth's surface. They are parallel to the Equator. It is the angular distance of any point on the earths surface North or South of the equator which is 0 degrees.

      There are 180 degrees lines of Latitudes, 90 degrees N and 90 degrees S of the equator. The temperature decreases with increasing latitude. this means that as you move towards the poles the temperature becomes lower.At 90 degrees N and 90 degrees S there is permanent Ice.

      This variation can be explained using this diagram which stresses these facts.

      The angle at which the rays strike the earth's surface determines the temperature. At the equator the rays are at/near right angles and therefore hotter than areas further away where the angle of the suns rays become acute to making the area much colder.The surface covered by equal suns rays reaching the surface of the earth increases away from the equator.The smaller area at the equator therefore receives more heat than further away.The atmosphere through which the rays pass is thinner at the equator and wider further away. A lot of heat is lost through the wider atmosphere. Areas around the equator are hotter than areas on latitudes that are away from the equator.

      The length of Day

      The longer the day the more hours of insolation. (Ammount of heat recieved from the sun).

      This raises the air temperatures.

      The shorter the days, the lower the insolation.

      This lowers the air temperature. For instance, when the sun is over head in June at the Tropic of Cancer, the days are longest in the Northern hemisphere.


      Altitude refers to the height of the land above sea level. The height of the land varies from the high mountains to the low coastal plains. Temperature drop or decrease as altitude increase.

      The rate of temperature decrease with increasing altitude is known as lapse rate which is 0.65 degrees Celcius/100 metres of ascent or 6.5 degrees c /1000 metres of ascent.


      Aspect means the direction that a slope faces to or away from the sun.

      Aspect influences Temperature where certain slopes in hilly areas receive direct rays of the sun and are warmer than the slopes that are shielded by hills and receive indirect rays. Aspect is felt mainly in high and mid latitudes (away from the equator)


      Wind is moving air. The winds are cold or warm depending on their origin.Winds from cold places lowers temperature of the areas they blow over while those from hot areas the winds raises the temperature.

      Distance from large water bodies.

      Distance from the sea implies how far an area is from a large water body. The land near the sea is said to be in a maritime location while those areas that are far inland are said to be continental.Coastal areas near the sea are maritimes hence the temperature are moderated.In these areas the summers are cooler and the winters are milder than places in the interior that experience extremes (very cold or very hot)

      Ocean currents

      An Ocean current is a mass of water in the Ocean with similar characteristics especially temperature and direction of movement. There are cold currents and warm currents.

      Warm currents cause a rise in temperature while cold current lowers temperature in the adjacent coastal areas where they flow.

      Cloud Cover

      The cloud type, thickness and amount of sky covered will influence temperature. When the sky is clear, there is continuous insolation and hence high temperature.

      Minimum Thermometer

      This thermometer records the lowest temperature reached in a day (24 hrs). It uses alcohol which responds to slight changes in temperature.

      When temperature falls, the alcohol contracts, its meniscus pulls the index towards the bulb until the lowest temperature is reached at night

      If the temperature rises, the alcohol will expand but the metal index remains in position.
      The reading is taken at the end of the index nearest to the alcohol meniscus. After reading, the bulb is raised and the index set back to the meniscus.
      The temperature readings of the day are usually read the following morning between 7.a.m. - 8.a.m

      Maximum Thermometer

      This thermometer records the highest temperature reached in a given day (24hrs).

      When temperature rises mercury inside the capillary tube expands Pushing the metal index in the free space up.this goes on until the highest temperature is reached and then the temperature starts falling. When this happens the mercury contracts but the index remains where it was pushed to. The maximum temperature is obtained by reading the scale at the end of the part of the index which was in contact with the mercury. After the reading , the index is reset using a magnet

      The Six's Thermometer

      A six's thermometer is used to measure maximum and minimum temperature. It is a U shaped glass tube that has combined the minimum and maximum thermometers. It contains both Mercury and alcohol.

      When temperature rises, alcohol in left tube expands and pushes mercury downwards, hence moving up on the right side column.

      The mercury then pushes the metal index up to record the highest or maximum temperature.

      The reading is then taken against the scale on the right hand column where the mercury meniscus touched the index.

      When temperature falls, alcohol contracts. The alcohol in right tube condenses and liquifies.

      Its weight exerts pressure on mercury on the right hand tube, pushing it downwards towards the left hand tube. The mercury on the left tube contracts and allows the mercury to flow upwards pushing the metal index towards a falling/reducing scale.

      The reading is taken from the part touching mercury meniscus on the left

      The mercury column should be continuous in the tube all the time.


      A thermometer is a self-recording thermometer which contains a bi-metallic strip in the shape of a coil.One end is fixed, the other moves a pen which traces a continuous record on to a chart fixed to a rotating drum.

      Mean temperatures

      If Maximum temperature in a day is 350 c and Minimum temperature in a day is 200 c, the mean daily temperature can be derived by calculating the average using the following formular.

      Interpretation of Temperature records

      In this section, we are going to look at how to calculate

      the diurnal range of temperature,

      mean monthly temperature,

      annual range of temperature,

      annual mean temperature

      and how to plot isotherms.

      The Diurnal range of temperature (daily range)

      Maximum temperature of the day minus the minimum of the same day e.g.
      Max 35 degrees c minim 20 degrees c =15 degrees c.

      The Diurnal range (daily range of temperature)

      Maximum temperature of the day minus the minimum of the same day e.g.
      Max 350 C - minimum 200 C

      The diurnal range of temperature is 150C

      Annual range of temperature

      This is the range between the highest mean monthly and the lowest mean monthly temperature in a given year. For example if the highest mean monthly temperature is 30 0 c and the lowest mean monthly temperature is 18 0 celcius. The annual range of temperature is 12 0 celcius

      The mean monthly Temperature

      The mean monthly temperature can be derived by calculating the sum of mean daily temperatures for one month divided by the number of days in that month. For instance the table below shows the daily mean temperatures recorded for a given station in June. You can derive mean monthly temperatures. The daily means are recorded below.

      The mean monthly temperature is 20.100c

      Annual mean temperature
      To obtain the annual mean temperature, add the mean monthly temperature for the year and divide it by 12 months

      The mean monthly is 23.66 0 c.


      Average temperatures can be plotted on a map and the lines joining places of equal temperatures are known as isotherms.

      By the end of the session you should be able to measure sunshine duration and intensity.


      Campbell stokes sunshine recorder

      Both duration and intensity are measured using an instrument called the Campbell stokes sunshine recorder.

      This instrument consists of a glass ball resting on a metal frame. The frame has a calibrated (scaled) sensitized paper mounted on it.

      The glass ball helps to focus the rays of the sun on this paper. As the sun moves across the sky, the rays also shift, burning a line on sensitized paper from sunrise to sunset.

      If the rays are strong (high intensity) the line is thick and dark, if the rays are weak, the marked line may be faint. If there was no sunshine at all the line trace is broken.

      The sensitized paper is removed at the end of the day and replaced with another.Several sheets removed are analyzed to show the sunshine duration and intensity per month.

      The main factor influencing sunshine is the type, thickness and area of the sky coverage.

      Cloud cover

      A cloud cover is a visible mass of tiny droplets of water and ice suspended in the atmosphere.The clouds form as a result of condensation.Condensation is the process by which water vapour changes into liquid ad a result of cooling below the dew point. Dew point refers to the temperature at which the atmosphere being called becomes saturated with water vapour.

      Types of clouds

      There are different types of clouds which are classified on the basis of altitude, appearance, structure and formation.

      High level clouds - cirrus, cirrostratus and cirrocumulus.

      Middle level clouds - altostratus, altocumulus, stratocumulus.

      Lower level clouds - Cumulonimbus, cumulus, nimbostratus, stratus

      Clouds are important in weather since they influence rain formation.

      Measurement of clouds

      This is a weather element which is recorded by observation of the sky at specific times and recorded in Oktas. One okta represents approximately one-eighth of the sky covered with cloud.

      Lines drawn on maps connecting places with equal cloud cover are known as Isonephs.

      Specific humidity

      This is the weight of the water vapour in a given mass of air expressed in grams of vapour per kilogram of air.

      Relative humidity

      This is ratio between the actual amount of water vapour in the air to he maximum amount of water that the same air can hold at the same temperature.

      It's expressed as a percentage. Relative humidity can be calculated using this formula.

      Measurement of humidity.

      Humidity is measured using an instrument known as a hygrometer.
      A simple hygrometer consists of two ordinary Thermometers. A dry and wet bulb thermometer.

      A wet bulb thermometer.
      It is described as wet because its bulb is wrapped in a muslin cloth and dipped in a small dish of water. When air is dry water evaporates from the wet muslin cloth cooling the bulb and lowering temperature as opposed to dry bulb thermometer.

      The readings are taken from the dry thermometer. The next reading is taken from the wet bulb thermometer. The difference in the readings is transferred to a humid table which gives the relative humidity as a percentage. If the difference in temperature is large, the relative humidity is low.

      The main factors that influence humidity are

      Air temperature

      Air pressure

      Supply of moisture


      Learning objectives

      By the end of the session you should be able to:

      identify differently types of wind

      name diferrent types of wind

      determine the wind direction and speed using weather instruments.


      Wind is air in motion. Its characteristics are expressed in terms of speed and direction.The direction is always given as that form which it is blowing and expressed form the points of the compass or in degrees from the true mouth.

      Movement of air is caused by differenced in pressure, that is, air moves from areas of high pressure to those of low pressure.It is an important weather element because it is a medium of transfer of heat, moisture and movement of clouds.

      Factors influencing wind direction and speed

      The wind blows in different direction and in varying speed. This is caused by;

      Pressure gradient

      This is the distance between the high pressure cell and low pressure cell or the difference in pressure over sea or land. The wind tends to blow from regions of high pressure to regions of low pressure. Steep gradient winds move faster while gentle gradient winds are slow.

      Coriolis Force

      The force resulting from the rotation of the earth that tends to deflect object/Air to the right in the Northern hemisphere and to the left of Southern hemisphere.

      Centrifugal force

      This is the clear tendency for air to move clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.


      Presence of relief barriers

      The rough natureof the earth's surface creates friction to wind, as obstacles or shelter often changing wind direction.

      Type of winds

      Planetary winds

      These are the worlds major prevailing (blow frequently) winds. There are found in the Northern and southern hemisphere.

      Winds shift according to change in air pressure systems. Air moves from an area of high pressure to one of low pressure.

      Monsoon winds

      These are seasonal winds that reverse the direction as the season changes

      Experienced in S.E Asia, Japan, India and N.Australia.

      Other examples of lanetary wiinds are trade winds, westerlies and polar winds

      Local winds

      These are winds affecting a small area and for a short period of time. They are influenced by the presence of a relief feature such as a mountain or a large water body such as lakes and seas. The example of local winds are the breezes mountain and desert winds.

      Land breezes

      Land breeze lowers night temperatures

      A breeze is a gentle wind affecting places near water bodies.

      The sea breeze

      The sea breeze during the day lowers daytime temperatures. Breezes change within 24 hours and they are also called diurnal winds.

      Mountain and valley winds

      In the mountain areas the local winds that blow up a mountai are called anabatic winds.

      These are local winds which blow from valley bottoms up the valleys to the hills tops. They are cold during the day.

      Katabatic winds (at night).

      They are cold local winds which blow down the hill to the valley. Lower temperature in the valley bottom.

      Measurement of Wind

      Wind direction

      The wind direction is determined using a windvane.The arrow points in the direction from which the wind blows which is the point of origin and the name of the wind direction.

      The windvane should be placed on a tall building at least 10 metre form the ground. Obstacles like tall structures and trees should be avoided because they cause friction reducing speed and changing direction to the wind.

      A windsock

      A windsock could be used to give general direction for the wind.

      The mouth of the sock faces the direction of origin fo the wind. A Windsock is mostly found in airstrips.

      Wind speed

      Wind speed is measured using an anemometer. It consists of metal cups at the end of the arm/rods that rotate freely when wind blows.

      The faster the wind, the more the rotation which are transmitted and recorded on a meter in units known as knots or km/hr.The wind speed can also be transmitted using physical observation of objects as the wind blows. It is known as the beaufort wind scale. It runs from 1 - 12.
      Selected examples on wind speed

      Foctors influencing atmospheric pressure

      The following are factors that influence atmospheric pressure. Altitude, Temperature, and rotation of the Earth.


      Pressure decreases as altitude increases. This is because rising air expands, spreads over a larger area giving less pressure.


      when temperature rises the air becomes warm and light and the pressure drops. Conversely, when temperature drops the air is heavier which causes the pressure to rise. The earth's surface therefore is divided into low pressure cells and high pressure cells according to seasons.The pressure cells in turn leads to the development of wind systems over the earth since air moves from high pressure to low pressure areas.

      Rotation of the earth.

      The Earth makes a rotation in 24 hrs and yet the circumference reduces towards the poles.

      Air tends to be pushed towards the equator where there is more space, spreads and exert low pressure at the equator throughout the year.

      Atmospheric pressure Zones

      Pressure distribution over the earth can be summarized as; Sub tropical high pressure zone, temperate low pressure and Polar high pressure.

      The equatorial low pressure zone

      Experiences high winds called doldrums. Main features of this zone are,

      high temperatures throughout the year,

      high humidity,

      low pressure

      convergence zone of trade winds hence the inter tropical convergence zone (ITCZ)

      The ITCZ moves north and south of the equator following the apparent movement of the sun within the tropics (23 degrees N - 23 degrees S).
      Due to convergence, low pressure and high temperatures, convectional rain the thunderstorms are common weather elements within the equatorial belts.

      Sub tropical high pressure zone

      Sub tropical high pressure zone found around 300 N and 300 S

      Source of trade winds and westelies.
      Air moves out of this zone to low pressure areas in the northern and southern

      The temperate low pressure(sub-polar low pressure)

      This zone is found around latitude 60 degrees N and 60 degreesS. Winds which blow into this areas are westerlies and polar easterlies

      Polar high pressure

      The poles (90 degrees N and 90 degrees S) experience polar high pressure

      Pressure belts shift but are discontinuous on the earth's surface due physical variations. Pressure cells move/shift with air masses.

      Air masses

      An air mass is large volume of air with uniform characteristics in temperature and humidity depending on the point of origin of the air mass.

      There are four main types of air masses are, Equatorial air masses, Tropical air masses, Polar air masses and Arctic and Antarctic air masses

      A boundary separating masses of different characteristics is known as a front when the different air masses meet, warm air tends to rise as cold air undercuts it then descends forming a depression.

      The two main pressure systems in the world are classified as

      • Cyclones
      • Anticyclones


      They develop low pressure at the centre and increases outwards. The cyclones are called depressions in the mid-latitudes and tropical cyclones in equatorial areas.The center with the lowest pressure is the 'eye' very strong winds from the outside high pressure area rush in towards the eye causing very turbulent weather.Cyclones can develop over oceans and move towards the ITCZ. Many regional names exist for this turbulent weather

      • Typhoons
      • Willy willies
      • Hurricane
      • Tornadoes

      Anti cyclones (high pressure system)
      The high pressure is at the centre and decreases outwards. Air descends from the atmosphere, then spread outwards on the ground while being deflected clockwise in the Northern hemisphere and anticlockwise in the southern hemisphere.

      The weather accompanying anticyclones includes light winds at the center and stronger outwards
      Cyclones         Anti cyclones
      - Very strong widns at the centre    - Light winds at the centre
      - Cover smaller areas within       - Covers a wider are with stable weather
         violent unstable weather          
      - Known as hurricanes in tropical    - Found the high latitudes (Horse latitudes)
         areas or depressions in mid-latitudes

      Measurement of atmospheric pressure

      Simple mercury barometer Air has weight . It therefore exerts pressure on the earth's surface.The atmospheric pressure is measured using a simple mercurybarometer or an aneroid barometer.A simple mercury barometer consists of a glass tube 1 metre long and a bowl containing mercury.

      The aneroid barometer

      This is a more refined automatic instrument. An aneroid barometer is made up of a collapsible metal box from which air has been expelled.

      The sides of the box are flexible; they expand or contract according to the air pressure. The box expands when the pressure is low and contracts when the pressure is high.

      These movements are then conveyed to a pointer by a system of levers with a dial on a scale where the pressure can be read and recorded in millibars.The movements may also be conveyed to a rotating drum covered with a chart where a pencil automatically draws or traces the pressure changes giving pressure readings in gm/cm2.

        By the end of the session you to be able to;

        • identify different forms of precipitation.
        • describe different ways of rainfall formation.
        • measure rainfall.


        Types of fog exist depending on formation thus; Radiation frog, Advection fog, Hill fog, Frontal fog, Ice fog, arctic smoke and Urban fog (smog).

        Measurement of fog/mist is based on visibility. Example

        Relief rain
        Relief rain occurs in areas where onshore winds rise over hilly or mountainous regions lying parallel to the coast. It also occurs in areas where moist air forced to ascend relief features far from water bodies. The rain bearing winds loose their moisture on the windward slopes winds.

        Convectional Rain

        Convectional rain is commonly experienced in tropical areas or mid-latitudes during the summer season. Areas around the equator get intense heating form the sun. This results into warm air rising in the form of conventional currents.

        When this air reached higher in the atmosphere, the moisture in it condenses to form cloud, which later fall as rain.
        Convectional rain falls mainly in the afternoon, and is accompanied by thunderstorm

        Frontal Rain

        This type of rain is common in temperate areas. When warm moist tropical winds meet cold dry polar winds, frontal rain may result.

        When the warm air mass meets with the cold air mass it is forced to rise up. This is because of the undercutting by the cold air and the warm air is lifted up. It gradually cools, condenses and falls as rainfall. Frontal rainfall is usually very heavy and is accompanied by thruder and lightening.

        Measurement of rain

        The basic instrument used in measuring the amount of rainfall is a rain gauge. A standard rain gauge consists of a metal cylinder.

        Inside the cylinder is a jar in which rain water collects.


        A funnel is used to direct the rainwater into a jar.

        In measuring rainfall the rain gauge should be in an open place away from buildings and tall trees.The space should not be prone to floods.

        Should be firmly fixed on the ground.

        Rain water that is collected is measured in 24 hours using a special measuring cylinder marked in millimetres.

        The water collected in the jar is measured the following day about 8 a.m before evaporation takes place. A measure of 20mm means that on a flat ground water will accumulate to a depth of 20mm without sinking or evaporating.

        Rainfall record

        Rainfall records include;

        • Monthly rainfall total
        • Annual rainfall total

        Mean monthly rainfall

        In calculating the mean monthly rainfall, get the figure by getting the sum of the monthly rainfall totals for a particular month, observed over several years (e.g. 10) and dividing this sum by the number of years of observation as follows

        Get the records for October for ten years and divide by ten 200, 220, 180, 200, 170, 180, 200, 150, 250, 200.
        = mean monthly rainfall for October is 195mm.

        Mean annual rainfall

        Effective rainfall = Rainfal - evaporation = water gain

        By the end of the session you should be able to;

        describe the composition of the atmosphere.

        describe the structure of the atmosphere.

        The structure of the atmosphere

        The atmosphere is about 330km in thickness. There are four main layers of the atmosphere.

        The troposphere

        The stratosphere


        This layer extends up to a height of about 80 kilometres upwards. Temperatures fall with increase in height from 0 degrees at the stratopause to about - 100 degrees at the upper limit. This is the layer from which the strongest wind in the atmosphere blow.
        The uppermost layer is marked by a zone of constant temperatures known as mesopause.


        Extends from 80km upwards to about 600 kilometers. Temperatures rise with increase in height. this is due to the absorption of the solar energy.
        At this zone the gases separate into different layers.

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