Understanding Weather and Climate
WEATHER AND CLIMATE
Weather. Weather refers to the state of the atmosphere (atmospheric conditions) of a certain place over a short period of time. This time may e an hour, a day or a week. In description of weather you hear people saying that the weather of the day is warm or cold, it is a cold morning, a sunny afternoon, a rainy day or a chilly night and so on. It is never static, and therefore cannot be generalized. The weather of a certain area can vary tremendously. It may be raining heavily in one part of the district but sunny a few kilometers away. It is also important to note that any place can be subjected to hazard changes in weather at any time.
Climate. Climate is the average atmospheric conditions of an area over a considerable time. For climatic averages, a minimum period of 35 years is desirable. This involves the systematic observation, recording and processing of the various elements of climate like temperature, rainfall, pressure, humidity, air pressure, winds, sunshine and clouds before any standardization of the climatic averages or means can be arrived at.
The structure of the atmosphere. The atmosphere is a layer of air surrounding the earth. It is about 330 km thick and consists of a mixture of various gases namely: Nitrogen (78%), Oxygen (21%), Carbon dioxide (0.03%), rare gases (10%) and dust particles salt particles and so on of negligible percentage. The atmosphere is subdivided into four layers basing on temperature changes. These layers include: the troposphere, the stratosphere, mesosphere and thermosphere.
The troposphere. This is the lowest layer of the atmosphere. This zone extends from the earth's surface to a height of between 16 km at the equator to about 11 km at the poles. In the troposphere temperatures decrease by 6.4oc for every 100 metres of ascent. This decrease in temperature with increase in altitude is called the environmental lapse rate or normal laps rate.
The stratosphere extends from the upper surface of the troposphere, called the tropopause, to a height of about 100 km. This zone is characterized by a steady increase in temperature and this phenomenon is called temperature inversion.
The increase in temperature in this zone is caused by a concentration of Ozone (03) which absorbs incoming ultra-violet (uv) radiation. Because of increase in temperature with altitude in this zone, there are no aid currents. Besides, the stratosphere contains very little or no water vapour, and thus there are no clouds.
The mesosphere is above the stratosphere. In this zone temperatures fall rapidly as there is no water vapour, cloud, dust or Ozone absorb incoming radiation. This layer has the lowest temperatures (-90oc).
The thermosphere extends from the upper surface of the mesosphere, called the mesopause. Temperatures rise rapidly with height, perhaps reaching 1500oc. This is due to an increasing proportion of atomic oxygen in the atmosphere, which, like Ozone, absorbs incoming ultra-violet radiation.
Diagram.
Elements of weather and their measurements. To collect various climatic data to prepare maps and charts of them, the following elements of weather are daily observed and measured by weather instruments kept at a weather station. The elements are: rainfall, humidity, temperature, pressure, wind, sunshine and clouds.
Weather station. A weather station is a place where all elements of weather are measured and recorded. Each weather station has a standard meteorological shelter known as the Stevenson screen. It consists of a white wooden box raised 1.2 metres above the ground. The roof is double layered with an intervening air space to prevent the sun's heat from reaching the inside of the screen. The four sides of the box are covered to allow free circulation of the air. One side of the screen is hunged to serve as a door, which can be opened and closed to give access to the instruments kept inside. The floor of the screen is also covered. The screen contains four thermometers, all hung from a frame in the center. These are the maximum and minimum thermometers, dry and wet bulb thermometers. See diagram below:
Importance of a school weather station:
-It helps the students to study the weather conditions around their school.
-It makes geography real. Students are able to relate the classroom principles
with what actually happens in the field.
-Students also acquire skills of measuring weather elements
-Students also acquire skills of recording and compiling statistics and so on.
Rainfall. Rains are tiny water droplets falling from the cloud. Rainfall including other forms of precipitation (snow, sleet and hail) is measured by a metal instrument called a rain-gauge. It consists of a cylindrical copper container in which there is a copper collecting can containing a glass bottle, and a metal funnel that fits on to the top of the container as shown in the diagram below. The hole in the funnel that leads down to the container is very small to minimize evaporation of collected rain. The gauge should be sunk into the ground so that the top of the funnel is at least a third of a metre (one foot) above the ground and firmly fastened, to avoid splashing. The instrument is placed well away from tall buildings, high trees and other objects to avoid run off from them getting into the funnel. Besides, the outer case is sunk in the ground to prevent the sun's heat from evaporating any of the rain collecting in the glass far.
Rain Gauge
The measurement of the rainfall is done by removing the funnel, Emptying the water in the glass bottle into a graduated cylinder, usually every 24 hours. The reading should be done at eye-level from the scale, which is in millimeters. For greater accuracy, a special kind of taper measure, which tapers at the bottom, may be used.
The daily records of rainfall are added at the end of the month to obtain the total monthly rainfall. The total of each month is again added at the end of the year to obtain the annual rainfall. The mean annual rainfall is got from the averages of annual rainfall taken over a long period of say 35 years. For plotting in rainfall maps, places having the same mean annual rainfall are joined by a line called an iso...........?
Description of rainfall statistics
Annual rainfall in (mm) describing words
Over 1500 Very wet
1000 to 1500 Wet
500 to 1000 Moderate
250 to 500 Dry
Below 250 Very dry
Formation of rain. Rainfall is the tiny droplets falling from clouds that formed from a rising air when the temperature is above 0oC. The air around us contains water in the form of gas (vapour). As this air rises and is sufficiently cooled, condensation (changing from gas to water droplets) may take place leading to large masses of tiny water droplets which form clouds.
The atmosphere contains minute particles called ‘nuclei' comprising of dust, smoke, soot, salt, etc. Water droplets join together around the nuclei to form rain droplets. The raindrops are large enough and heavy, they fall down on the surface of the earth as rain under the force of gravity.
Condensation takes place in three main ways:-
- Intense heating causing moist air to rise, cool and condense
- When warm, moist air is forced to rise over a relief barrier, it cools and condenses.
- When warm, moist air is forced to rise over a heavier, cooler and drier mass of air, it cools and condenses.
Types of rainfall in East Africa.
The type of rainfall in any place on the earth depends on the way condensation takes place. Since condensation takes place in three main ways, therefore, three main types of rainfall can occur in East Africa.
(a) Convectional rainfall
(b) Orographic (relief) rainfall
(c) Cydronic (frontal) rainfall.
Convectional rainfall. Convectional rainfall results from vertical movement of warm moist air. When the air over a warm ground surface is heated, it rises up above cooler, denser air. This process is known as Convection. As the warm moist air rises, it is cooled adiabatically. When dew point temperature is reached, Condensation takes place resulting in clouds. The clouds continue to grow bigger and bigger forming the massive and huge cumuli nimbus clouds, which may reach a height of about 12,00m. These clouds give rise to heavy rainfall accompanied by thunder and lightening.
Convectional rainfall is common in the Lake Victoria basing of East Africa. See diagram below:
Earth's hot surface heats the air above it. The heated air expands and becomes lighter than the surrounding air. It therefore rises.
Orographic (Relief) rainfall. Orographic rain occurs in all latitudes. It occurs when warm moist air is forced to rise over a hilly or mountains region lying at right angles to the direction of the winds. As the air rises, it cools and condenses forming clouds leading to heavy rain falling on the windward side. The descending air on the leeward side of the mountain is warmed and its capacity to hold moisture increases. Instead of bringing rain, it brings dry conditions and this becomes a rain shadow area. Examples of rain shadow areas in East Africa include North Easter Uganda, Kasese, Ankole, Masaka corridor (Uganda). Naivasha, Nanyuki, Nyeri, Naromoru, North Eastern Kenya are in the rain shadow of Mt. Kenya.
In East Africa relief rainfall is common in the highland areas such as Kigezi highlands, Mt. Ruwenzori region, Mbale, Sironko, Kapchorwa (Mt. Elgon) in Uganda Kisii, Kericho, Eldoret, Nandi hills, Aberdares, Mt. Elgon, Mt. Kilimanjaro, Mt. Usambara, Mt. Pare, the Southern highlands in Tanzania.
Cyclonic (frontal) rainfall. This results from the meeting of two air streams of different characteristics in an area of low pressure. For instance, one warm and moist and another cold and dry meet. Within the tropics, this occurs at the Inter Tropical convergences zone (ITCZ or Equator Trough). The ITCZ is characterized by rapidly rising warm and moist air. As the air rises, it cools and condenses to form clouds and heavy rain often falls in the afternoon. The ITCZ is responsible for the distribution of rainfall in East Africa. The two air streams that are responsible for the pattern of rainfall in East Africa are the North East and South East Trade Winds.
Temperature. Temperature is the degree of sensible heat or cold in the atmosphere. Temperature is a very important element of climate and weather. The instrument for measuring temperature is the thermometer which is a narrow glass tube filled with Mercury or alcohol. On thermometers temperatures are marked in two ways: Centigrade oc and Fahrenheit oF. But for most scientific purposes, the centigrade oc is preferred. Its freezing point is 0oc and boiling point is 100oc. In Fahrenheit oF, the freezing point is 32oF and the boiling point is 212oF.
Conversion of one scale into another. To convert one scale into another, the following formulae can be used:
To be obtain centigrade = (oF - 32 : 9 (1.8)
5
e.g. to convert 69oF into centigrade:
(69 - 32o) : 9 = 37 : 9 = 37 x 5 = 20.5oC
5
To be obtain Fahrenheit = (9 x oc) + 32oF
5
e.g. to convert 30oc into Fahrenheit
(9 x 30oc) + 32oF = 54o + 32o = 86oF.
Shade Temperatures. Shade temperature is the temperature of the air. These temperatures are obtained by placing the thermometers in the Stevenson screen to exclude the intensity of the sun's radiant heat.
Maximum and minimum temperatures are measured by the maximum and minimum thermometers. They are either separate thermometers or joined in a u-shaped glass tube known as the six's thermometer. The maximum thermometer records the highest temperature reached during the day. When the temperature rises, the mercury expands and pushes the index along the tube. When the temperature falls, the mercury contracts but the index remains at the maximum level. Then the end of the index nearest the mercury gives the reading of the maximum temperature as indicated in that figure below which is 25oc (77oF). The index is then drawn back to the mercury by swinging the thermometer hard or by a magnet. The maximum temperature reached during 24 hours is registered.
The minimum thermometer records the lowest temperature reached the day. It occurs early in the morning. The glass tube is filled with alcohol, which allows the index to slide freely along the tube. When the temperature drops the alcohol contracts and its meniscus drags the index towards the bulbs. When the temperature rises, the alcohol expands and flows past the index leaving it where it was. The minimum temperature is obtained by reading the scale at the end of the index, which is nearer to the meniscus. In the figure below the minimum temperature is 15oc (59oF). The thermometer is then reset by a magnet for the next 24 hours reading.
Six's thermometer. The Six's thermometer can also be used to measure maximum and minimum temperatures. When the temperature rises, the alcohol in the left-hand limb expands and pushes the mercury down in the left-hand limb and up the right-hand limb. The alcohol in this limb heats up and part of it is vaporized and occupies the space in the bulb. The end of the index nearest the mercury on the right-hand limb gives the reading of the maximum temperature. When the temperature falls, the alcohol in the left-hand limb contracts and some of the alcohol vapouir in the comical bulb liquefies.
The daily readings of the maximum and minimum thermometers are used to work out the averages or mean temperatures for one day and the temperature range for one day. The mean daily temperature is the average of maximum and minimum, e.g.
Maximum temperature and Minimum temperature
The sum of the daily average (mean) temperatures for one month divided by the number of days for that month, gives the mean monthly temperature.
The mean annual temperature is the sum of the mean (average) monthly temperature divided by 12 in a year.
The difference between the maximum and minimum temperatures of a day gives the diurnal (daily) range of temperature. The difference between the hottest month and the coldest month gives the annual range of temperature. In diagrammatic representations, monthly mean temperatures are shown in simple temperature graphs or in temperature distribution maps. For plotting in temperature maps, places having the same mean monthly temperature are joined by a line called isotherms.
Humidity. Humidity is a measure of the dampness of the atmosphere (amount of water vapour in the air) which varies greatly from place to place at different times of day. This may be expressed either absolutely or relatively.
Absolute humidity is the actual amount of water vapour present in a given volume of air, which is expressed in gram per cubic metre. Absolute humidity increases with increase in temperature. However, relative humidity is more important from the point of view of weather studies. This is the ratio between the actual amount of water- vapour and the total amount the air can hold at a given temperature, Expressed as a percentage. Relative humidity increases with decrease in temperature or falls with increase in temperature.
Warm air can hold more water vapor than cold air. If the air contains only half the amount it could carry, then the relative humidity is 50 percent. When the relative humidity reaches 100 percent, the air is said to be saturated and the air temperature is at dew point. Further cooling will condense the water vapour into clouds or rain. Thus, when relative humidity is high like between 80 and 90 percent, the air is moist and when it is low, like below 50 percent, the air is dry as the deserts.
The instrument for measuring relative humidity is the hygrometer. It compasses wet-and dry bulb thermometers placed side by side in the Stevenson screen. The dry bulb thermometer ordinary thermometer measures the shade temperature. The wet-bulb is kept wet y a thin muslin cloth that dips into a reservoir of distilled water as seen in the diagram below:
The hygrometer consisting of wet and dry bulb thermometers.
When the air is not saturated, evaporation which produces a cooling effect, takes place from the muslin cloth. This causes mercury in the wet-bulb thermometer to contract and the wet-bulb shows lower reading than the dry-bulb. With reference to prepared tables, the difference between the readings is used to calculate the relative humidity. See table below:
|
Dry bulb |
Wet bulb |
|
20oc 25oc 30oc 35oc |
20oc 22oc 24oc 26oc 28oc 30oc 32oc 34oc 100% - - - - - - - 65% 80% 95% - - - - - 40% 50% 60% 80% 90% 100% - - 24% 30% 35% 45% 67% 70% 83% 95% |
A part of a set of humidity tables.
Dry-bulb temperature = 25oC
Wet-bulb temperature = 22oC
Relative humidity = 80oC
With reference to the above table, under the difference column of dry and wet-bulb reading, the relative humidity can be obtained as a percentage. A large difference indicates a low R.H and a small difference a high R.H. If both thermometers have the same reading, R.H. is 100 percent; the air is saturated.
Factors that influence the humidity of a place:
The amount of water vapour in the atmosphere is determined by evaporation. The following factors influence the humidity of a place.
Temperature. High temperatures lead to greater capacity of air to hold water vapour while low temperatures lead to loss of ability of air to hold water vapour.
Water bodies. Water bodies act as sources of water vapour. The amount of water vapour will also depend on the constant supply of water vapour from water bodies
Like lakes and swamps.
Availability of vegetation. Forests are sources of water vapour and therefore help to recharge the atmosphere through a process known as evapo-transpiration.
Air movement. Air movements cause more evaporation and hence humidity than in places where air is calm.
Availability of swamp soil. Sandy or base rock surface contributes little or no water to the atmosphere through evaporation while damp soil contributes much water vapour in the atmosphere.
Season. In the tropics high temperature and high rainfall lead to high rates of evaporation hence much humidity in the atmosphere. In dry months the amount of water released to the atmosphere is considerably reduced.
Relief. On leeward sides of mountains humidity is low because the winds, which blow down these slopes, will have lost most of their moisture through condensation on the windward sides.
Time. Humidity is usually greater during the day than at night.
Latitude. Humidity is usually highest in low latitudes and decrease towards the poles.
Pressure. Pressure is the force or weight exerted by air per unit area over the earth's surface. Pressure varies from place to place and from time to time. The instrument for measuring pressure is a barometer. Atmospheric pressure is measured in Millibar (mb). A normal atmospheric pressure equivalent to 1003 kg per cm2 (14.7lb. Per square inch) in weight or a reading of 760 mm of mercury in the column at sea level is 1013 millibars. On maps, lines called isobars join places of equal pressure.
Mercury barometer. This is an ordinary mercury barometer which consists of a long glass tube, sealed at the upper and open at the lower end. The lower end is inserted in a bowl of mercury, whose surface is exposed to the air as seen in the diagram below.
Variations in the atmospheric pressure on the mercury surface are balanced by the column of mercury in the glass tube. This gives the pressure of the air and can be read off quickly from the scale on the glass tube. At sea level, the mercury column is 760mm. If the pressure increases, the air pressing on the surface will force up the mercury column to about 790mm (high pressure). When the pressure decreases, as lost air pressure on the surface, the mercury column will drop about 700mm (low pressure).
Aneroid barometer. This instrument comprises a small metal container, which contains very little air. The top of the box bends in registering high pressure by an indicator on the revolving dial. When there is a decrease in pressure, the spring pushes the box outwards, registering low pressure by the indicator. See diagram below:
In aeroplanes, a modified type of aneroid barometer called an altimeter is used.
Pressure varies with a number of factors, e.g. temperature, altitude and so on. Areas of high temperatures have low pressure and areas of low temperatures have high pressure. Also areas of high altitudes have low pressure because as one ascends there is less air above and so the weight, or pressure is less.
Winds. Wind is air in motion. It has direction, speed and strength. Winds are named from the direction they blow. Wind can only be felt but not seen. When leaves fall, trees sway and dust particles more, we realize that the wind in blowing. The movement of the wind is induced by pressure gradient between two points. Winds move from high to low pressure areas. The speed of wind is also dictated by the pressure gradient, i.e. the steeper it is, the faster the wind flow is and the less steep it is, the slow the movement is.
The instrument used for measuring wind direction is a wind vane (weather cock). It comprises of an arrow or vane on the top that moves freely with the prevailing wind. The other part with four compass points (North, East, South and West) is stationary and shows the direction which the wind is moving. See diagram below:
A wind rose is used to record the direction of prevailing winds of a place over a period of a month. It is an Octagon with the eight compass points. Each of the small, rectangles represents the date in which the wind comes from that e.g. on the second ot the month, the wind is north. The days which are without any wind are indicated in the center of the Octagon e.g. 3 days in that month. See diagram below:
Anemometer. An anemometer is used to measure the speed of wind. It is made up of three or four semi-circular cups fixed to the ends of horizontal arms that rotate freely on a vertical spindle. The cups rotate when there is wind. The cups rotate very fast when the wind is strong. The number of rotations, is recorded on a meter to give the speed of the wind in kilometers per hour. See diagram below
Sunshine. The amount of sunshine received in a place depends on the seasons. The duration of sunshine is a place each day is measured by a sunshine recorder. This is a glass sphere with a diameter of 102mm partially surrounded by a metal frame on the inside of which is a sensitized card, graduated in hours. See diagram below:
The glass sphere focuses the sun's rays upon the card. A trace is made on the card when it is sufficiently heated. At the end of the day, the card is taken out and the length of trace is turned into hours, which represents the total amount of sunshine for the day. On maps, places with equal sunshine duration are joined by isohels.
Clouds. Clouds are tiny droplets of water vapour, which are too small to fall as rain or snow floating in the air. Cloud formation is due to cooling and condensation of water vapour in the atmosphere.
When air rises, it is cooled by expansion. After dew-point temperature has been reached, some of its water vapour condenses into tiny droplets of water, which continue floating in the air known as clouds. Mists and fogs are also made of water droplets and therefore they are clouds which form hear the surface. Their form, shape, height and movement indicate the conditions in the sky and the likely weather to be experienced. The amount of cloud-cover in the sky is expressed in eights or oktas e.g. 2 O is three-quarters covered; and 8 O is total overcast. 8 8
On maps places with an equal degree of cloudiness are joined by lines known as isonephs.
The classification of clouds is based on their appearance, form and height. There are four major cloud types. These include the following:
(a) High clouds. These are mainly cirrus (ci) of feathery form at 6,100 - 12,200 metres above the ground and include cirrus, cirrocumulus (cc) and cirrostratus (cs).
(b) Medium clouds. There are mainly alto (Alt) at 2100 - 6000 metres. They include Alto-cumulus (alt-cn) which are wooly and bumpy clouds arranged in layers and altostratus (Alt-st) which are denser, grayish clouds with a ‘watery' look.
(c.) Low clouds. These are mainly stratus or sheet clouds below 2100 metres. They include stratocumulus (st-cn) which is a rough, bumpy dull weather with high drizzle and reduces visibility of aircraft and Nimbostratus (Ni-st) which is a dark, dull cloud, clearly layered and also known as a ‘rain cloud'.
(d) Clouds with great vertical extent. Mainly cumulus or heap clouds with no definite height (6,100 - 9,000) metres. These include: Cumulus (cn). This is a vertical cloud with a rounded top and horizontal base typical of humid tropical regions and cumulonimbus (cu-Ni) which is an overgrown cumulus cloud, extending to vertical height from a base of 600 metres to over 9,000 metres. It is frequently seen in tropical afternoon. It is referred to as a thundercloud and rings convectional rain.
Other elements affecting visibility include haze, mist and fog.
(a) Haze. It is caused by smoke and dust particles in industrial areas or may be due to unequal refraction of light in air of different densities in the lower atmosphere.
(b) Mist. This is formed by the condensation of water vapour in the air, which causes small droplets of water to float about forming clouds at ground level. Mist reduces visibility to about 1000 metres.
(e) Fog. Ordinary fog is due to weather condensing on dust and other particles like smoke from houses and factories. It occurs in the lower strata of the atmosphere as dense ground cloud. In industrial areas very thick smoky fog called smog is formed. The visibility may be reduced to 200 metres.
Climate is the average weather conditions of a place for along period of time roughly between 30-40 years. Climate is very important because it influences man's behaviors like settlement, dressing, feeding etc. It also influences the physical feature s of the environment especially vegetation. Climate is therefore important in determining various social and economic activities.
(A) Major climatic belts (zones) of East Africa.
The climate conditions experienced in East Africa are complex. Under a simple classification, East Africa can be divided into four major climatic belts.
1. modified equatorial climate
2. Tropical Savannah/continental climate
3. Semi-arid/arid climate
4. Montane climate(highland sub-tropical climate)
Modified Equatorial climate
This type is climate is limited to a narrow coastal strip extending from Lamu in the north (Kenya) to Tanga area south of River Pangani in Tanzania. It is also found around the lake Victoria basin with regions like Buganda, and south west Uganda (Maramagambo area) and the Nyanza province in Kenya (Kisumu, Kissi etc)
This zone receives heavy rainfall through out the year with a double maxima. The first rains march-may are heavier than the second rains that is September-November. Annual rainfall is between 1500-2000 mm per annum particularly in the lake region. The rainfall is conventional and therefore the region is wet.
The zone also experiences generally high temperatures for most of the year ranging between 21°c- 30°c with a small annual temperature change between
2°c-7°c. Humidity is high through out the year(extensive cloud cover). The area is dominated by equatorial type of vegetation (Tropical rain forest).
Tropical Savannah (continental) zone.
This is an extensive zone in East Africa. It covers regions like northern Uganda, most of Tanzania especially southern and south-west Kenya. This climatic group experiences alternating wet and dry seasons. There is heavy rainfall in the wet season. It has double maxima that is march-April and September-October, with an annual rainfall ranging between 850mm to 1,250mm. Temperatures range between 22°c-36°c. Diurnal and monthly temperatures ranges to up to 10°c. Savannah vegetation covers much of the region. Hence this zone is popular for grazing and tourism- National parks and Game reserves.
Semi-arid and arid climate
This is found on North Eastern Uganda, Karamoja, Northern and North-Eastern Kenya-Turkana areas, central Tanzania and Ankole- Masaka corridor in Uganda. These areas are very hot and dry. The region has prolonged drought with very little and unreliable rainfall. Rainfall received varies between 325 mm and 620 mm per annum the semi arid region. But in the arid regions rainfall received is less than 250 mm per annum, e.g. Wajor receives an average of about 221 mm p.a.
Temperatures are usually very high i.e. above 35°c with a large diurnal clear and the winds are dry. The common type of vegetation is mainly shrub, thicket and other related thorny bushes.
Montane climate (highland sub-tropical climate).
This zone is limited to the highland areas of East Africa such as, Kenyan highlands, Rwenzori, kigezi highlands and Mt. Elgon region in Uganda,
Kilimanjaro etc. These regions have varied climate types ranging from tropical to temperature. The regions receive ver high rainfall which is mostly orographic and cool temperatures, for example the Kenyan highlands receive beyond 200 mm of rain per annum with temperatures at times going as low as below 0°c. This explains why there is glaciation on some of the mountains of East Africa.
SKETCH MAP SHOWING CLIMATE ZONES OF EAST AFRICA.
B). Factors influencing climate in East Africa.
East African region experiences a great deal of variations in the types of local climate from place to place. The climate of the lake shore region to the semi-desert and desert climate of northern and North Eastern Kenya. This variation in climate is brought about by a number of factors. These factors include:
1. Position of the sun over head
2. The nature of relief
3. Influence of latitude
4. Influence of Altitude
5. Presence of water surfaces
6. Wind systems
7. Ocean currents
8. Vegetation
9. Aspect
10. Human activities
11. Continentally/distance from the sea.
1. Position of the sun overhead.
The position of the sun overhead influences the movement and subsequent position of the Inter-Tropical Convergence Zone (I.T.C.Z), which is a low pressure belt and this greatly influences the movement of winds.
SKETCH MAPS FROM YOUNG TO LOWRY.
The apparent movement of the over head sun causes the I.T.C.Z to oscillate to the north and the south of the equator bringing heavy rainfall in areas around Gulu in
Northern Uganda and Lowdar in Kenya from may-September, Entebbe, Nairobi, Kabale in March and September and much of south and northern Tanzania from October -march.
The nature of relief
East Africa has different relief features. They include high mountains, plateaus, plains ad the rift valley floors. The high mountains act as barriers to the blowing winds which cause them to use. In the process, the winds cool and condense resulting onto heavy orographic rains on the wind ward side, while the leeward side may receive little or no rainfall at all, for example Kasese is dry because of the rain shadow effect cast by Mt Rwenzori.
Influence of latitudes
Latitude determines the temperature at any point on the earth's surface. Temperatures vary with the angle at which the sun's rays strike the ground and the length of day. But the angle of the overhead sun and the length of day vary with latitudes within the tropics, the midday sun is always high in the sky and the sun's rays strike the surface at right angles thereby concentrating their power onto a relatively small area while at the opposite do occur.
Since East Africa lies astride the equator, the angle of the sun's rays is nearly vertical throughout most of the year. This means that the heating effect of the rays will be maximum since they are concentrated onto a small area. As a result, temperatures in most parts of East Africa are generally high except in areas of higher altitude.
This can be illustrated in the diagram below.
Influence of Altitude.
Altitude(height above sea level) is very important in influencing temperatures. At higher altitudes the air is thin, dry and dust- free. The sun's rays pass through it without doing much heating. The rays heat the ground surface first and this in turn heats the surrounding air. This means in effect that air at sea level is hotter than air at a higher altitude. Generally, temperatures drop at a rate of 1°c for every 156m rise in altitude. This rate of temperature drop is ELR. This is shown in the diagram below.
Presence of water surfaces.
These are provided by the large fresh water lakes found in East Africa like lake Victoria, L. Tanganyika, L. Kyoga etc. Rivers, swamps and the Indian ocean. These water surfaces affect the climate of East Africa in a number of ways:
(i) They act as sources of water vapour and through a high degree evaporation, they add to the humidity content of the atmosphere.
(ii) Land and sea (lake) breezes occur on the Indian ocean and the lake Victoria which modify the climate of the surrounding areas.
(iii) The water surfaces recharge the dry winds(NE and SE trade winds) as the blow over them causing high rainfall in the surrounding areas, eg the SE trade winds get recharged when they pass over lake Victoria after loosing much of their moisture in southern Tanzania.
Wind systems
The climate of East Africa is influenced by the following wind systems.
§ The North east trade winds
§ The south east trade winds
§ Westerlies (the existence of the Zaire basin)
§ Land and sea (lake) breezes.
(a) North East trade winds.
These are dry winds from Egypt, the Sudan, the Horn of Africa and Arabia. Their source is dry desert and they bring little rain to East Africa. They are partly responsible for the aridity of northern and north eastern Kenya and north eastern Uganda.
(b) South East trade winds.
These originate in the south Atlantic and the Indian ocean. They are moist winds and bring heavy rains to East Africa especially the coastal lands and southern Tanzania. They got to control Tanzania when they are discharged (dry) but get recharged when they pass over lake Victoria. And when they cross the equator, they become S.W. winds and they cause heavy rainfall in northern and north western shores is lake Victoria. Besides are major factors responsible for heavy rainfall along the North western slopes of Mt Elgon and western Kenya.
(c) Westerlies (The Zaire Basin influence)
These winds influence much of western Uganda from August to January in certain conditions, a stream of Zaire air may drift in from the north-west for several days and bring prolonged showery rain over the western Uganda, e.g. Mbarara, Bushenyi, parts of kigezi and Toro (kasese, Bundibugyo and Kabarole and kyenjogyo)
(d) Land and sea (lake) breezes.
These are local winds which are common along the coastal areas and within the interior regions especially around Lake Victoria. They are caused by the differences in specific heat capacities of land and water.
(i) Land breeze process (Night time)
At night land cools faster than sea hence temperatures are cooler overhead than the sea which retains much of its heat. Low pressure is created over the warm sea and high pressure over the cold land. Winds blow from the land to the low pressure belt over the sea as a Land breeze as seen in diagram below cause rainfall at night and cool air brought by land breeze moderate the temperatures.
(ii) Sea breeze process(Dry time)
There is intense heating of coast land during day. Land warms faster than sea hence temperatures are high over land and colder over sea. Warm air arises over land and creates low pressure at the surface. Cool and moist winds(sea breeze) blow towards land to replace the rising air is from high pressure to low pressure on land as seen in the diagram below.