Previous Lecture Complete

S.3 Geography Notes

Factors that influence the Climate of Africa.

The main factors that influence the climate of Africa are:

Altitude: The climatic differences in Africa are partly attributed the Altitude. The temperature drops or becomes cooler by 10C for every 150 metres of ascent.  This is known as temperature Lapse rate.  Altitude has a remarkable effect on temperature distribution in Africa.  

It influences climate as follows: 

  • The highland areas of Ethiopian massif, Atlas have lower temperatures because the temperatures tend to decrease with altitude at the rate of 10c after every 150 metres. This partly explains why mountain Rwenzori experience snow on its top despite its location near the equator.
  • High altitude areas of Africa receive heavy orographic/relief rainfall because the warm dry ascending air cool, condense, and form cumulus and cumulonimbus clouds as it ascends a mountain leading to heavy rainfall on the wind ward sides.
  • It also affects pressure in that atmospheric pressure decreases with an increase in altitude that is why high-altitude experience low atmospheric pressure and higher pressure on the foothills of the mountains.
  • There is also low humidity in the highland areas because of the low temperatures experienced in mountainous areas.
  • Low lying areas have high temperatures rising above 240C for example in the rift valley areas, while the mountain peaks have cool temperatures.

The influence of relief

Relief is the general appearance of the land of the land surface. Africa has a varied relief ranging from plains to high mountains all these have influence on the climate of Africa as follows:

  • Flat areas experience dry weather conditions because there are hardly any hills or mountains which could hold back winds.As a result, the winds gather speed over such areas and drive away clouds to other areas.
  • Highland areas like Ethiopian highlands, Cameroon highland and Atlas receive heavy rainfall due to the fact that the incoming winds are forced to rise when the blow towards the highlands and in the process cool, and the moisture condenses into orographic rainfall on the wind ward side.

The leeward side is however in the rain shadow and they have arid/dry conditions as illustrated below; illustration of the relief rainfall

Latitude:

The latitudinal position of Africa explains the tropical nature of climate generally characterised by hot temperatures since the sun is normally over head the tropics that is the sun migrates within the tropics. This is because the concentration of the sun rays is greater within the tropical region.Places near the equator experience maximum heat from the sun and therefore, experiencing warm to hot temperatures all the year except in hilly areas.The warm to hot temperatures lead to high moisture content in the atmosphere which results into high precipitation with two peaks of rainfall or bimodal rainfall.

Areas far away from the equator have one rainy season followed by a long dry season.

Wind systems:

Africa is placed in the global context of winds.

The most influencing ones on her climates are the:

North east, south east and south west prevailing/trade winds.

Generally, the low-pressure trough that develops over Africa in the tropics attracts winds from centres of high pressure usually over the cooler water bodies. The winds are drawn in into the trough and converge over the tropical lands and causes convectional rainfall over many places in Africa.

Activity: Draw the sketch map of Africa showing the trade winds that affect climate.

At the front of the winds heavy rainfall develops and falls as convectional rain. The winds blowing from ocean surfaces pick moisture from the water and deposit it over the surface of the continents.

This explains why much of West African coast is wet.

However, some of these trade winds are dry especially when they passed over the continents or highlands which rob them of the winds. Examples are the Harmattan trade winds in West Africa and the northeast trades over Ethiopian highlands.

Activity: Draw the diagram in Africa by Minns showing the wind patterns over Africa

Ocean currents:

There are two types of ocean currents namely the warm ocean currents and the cold ocean currents. Both affect the climate of Africa and for effects refer to ocean current notes.

Vegetation cover:

Areas that are covered with extensive, dense and thick forests like in the Democratic Republic of Congo receive heavy rainfall throughout the year because of high rates of evapo- transpiration from the thick leaves of plants. Areas without vegetation or with scatty vegetation cover are dry.

Water bodies - lakes:

Some places close to large masses of water bodies experience local climate. For example, through lake and breeze processes the temperatures and rainfall of adjacent lands bring about special climate conditions.

A lake breeze is a blow of wind from the water surface to the land to replace a rising warmer air. Such winds blow during the day when the temperatures over the land become higher than those over the water surface. The lake breezes impregnate the air with moisture for usually heavy afternoon rains.

During the night when temperatures fall, the land cools faster than the water and so the pressure over land is high compared to that over the water. A wind from the land blows to the water as a land breeze leading to heavy stormy rain over the water surface and adjacent lands.

Activity: Draw diagrams of lake and land breezes

Apparent movement of the overhead sun

The position of the overhead sun influences the moment and subsequent position of the inter Tropical convergence zone (ITCZ is the low-pressure zone of unstable air masses which keep on shifting depending on the position of the overhead sun).

The ITCZ have a great influence on the prevailing winds - North east and south east Trade winds because they are forced to blow into low pressure belt from regions of high pressure. The inter-tropical convergence is responsible for the seasonal pattern of rainfall distribution in many areas of Africa. It influences climate in the following ways;

  • At the time when the sun is overhead around the equator in March and September each year, this belt receives intense heat up air and air masses mainly North east and South east trade winds come to replace the rising air.The convergence of winds along the Equator in March and September causes heavy rainfall which is well distributed throughout the year with two peaks (Bimodal).
  • As the sun moves northwards to the Tropic of cancer the rainfall belt also swifts because the air masses are now converging further north of the Equator. The sun is at the tropic of Cancer around June and the northern part of the equator in Africa experiences a rainy season from April up to August.
  • The other months of the year experience little or no rainfall. The highest temperatures occur just before the onset of rainy season in the Northern hemisphere.
  • As the sun continues its journey Southwards beyond the Equator, the rain belt also shifts because the convergence of the air masses is somewhere south of the Equator. The sun is apparently overhead at tropic of Capricorn in December. Therefore, areas south of the Equator like Harare in Zimbabwe experience a rainy season from around October to March.

Human/man’s activities:

Environmental unfriendly human activities like deforestation, bush burning, wetland reclamation, sinking of boreholes, overstocking of cattle, urbanization, industrialization lead to desertification.

However, environmental conservation practices like afforestation, re-afforestation, and conservation of forested areas increase the amount of rainfall and also lower the temperatures.

Discussion

Get answers directory from your instructor
if you have any question about this topic.