Due to the spherical shape of the earth, the different latitude zones of the earth experience different levels of energy input from the sun. With the decreasing solar angle from the equator to the poles, the energy supply decreases more and more. The intense solar radiation in the tropics and subtropics leads to an energy surplus there compared to the higher latitudes. The atmosphere tries to balance these energy contrasts between equatorial and polar latitudes.
Warm and thus lighter air masses rise near the equator. On the ground, the equatorial low-pressure trough creates an area of low air pressure, while an altitude high forms in the upper layers of the atmosphere. The cold and therefore heavy air masses sink in the area of the poles. In this way, a high altitude low arises above the poles, while the air masses collecting in the lower layers of the atmosphere create a cold high near the ground. This creates a strong contrast in air pressure between the equatorial and polar latitudes, especially at altitude. These differences in air pressure cause an equalization flow (altitude flow )that runs from the equatorial regions towards the polar latitudes. Due to the deflecting force of the earth’s rotation (Coriolis force), these air currents are deflected more and more with increasing distance from the equator – to the right in the northern hemisphere and to the left in the southern hemisphere. By sinking air masses in the Sudtropen arise especially over the ocean surface stable ground-level high-pressure cells. A balancing flow near the ground (ground flow) runs from them in the direction of the low air pressure along the intertropical convergence zone at the equator, where they converge as northeast and southeast trade winds as a result of the deflection by the Coriolis force.
The equatorial low pressure channel is more developed over the continents and shifts more strongly to the north and south with the seasons than over the oceans (148.1–2). Over land areas, the trade winds resulting from the descending air movements of the subtropical high pressure areas are very dry, such as in North Africa (148.3). If, on the other hand, they initially stroke over larger areas of the sea, the moisture absorbed in the process can lead to high levels of precipitation when it crosses over to the land, as can be observed on the east coast of Madagascar (148.3).
Between June and September the Innertropical Convergence Zone (ITCZ) lies over the mainland far north of the equator (148.2). As a result, the southeast trade winds in the Gulf of Guinea and in East Africa blow across the equator to the northern hemisphere, where they are diverted to northwest winds. If they stroke the surface of the sea, as in the Gulf of Guinea, they lead to real monsoon rains.
The natural diversity, the historical development, the state of development of the countries and the current framework conditions on the world agricultural markets are essential factors that cause the coexistence of very different forms of agricultural use in Africa. Check COUNTRYAAH.COM to see countries in southern part of Africa.
The agricultural map provides an overview of land use, the extent of the cultivated and pasture land and the crops grown. In many ways, relationships can be established with the climate maps and the physical maps. On the other hand, it does not provide any information about other important factors in agriculture: farm structures, economic methods and ownership. Statements on the export orientation can be derived from the hatching (personal use) or indirectly from the cultivated crops.
PROBABILITY OF DROUGHT
Due to large-scale displacements of the Hadley cell (249.2), large parts of Africa develop rainy and dry seasons of different lengths, which determine the rhythm of life on the continent. The seasonal fluctuations in precipitation are mainly reflected in the number of humid months. Rooms in which the total amount of precipitation is higher than the potential evaporation from the landscape for less than six months of the year are considered dry rooms. The limit here is around 500 millimeters annual precipitation (148.3).
In addition to seasonal fluctuations, broad strips of Africa are also affected by high inter-annual rainfall variability. In years with little precipitation, long periods of drought with negative consequences for all areas of life can occur.
Due to the sharp decline in tropical rainforests and monsoon forests and the associated higher evaporation over land, the natural exposure to drought events is also increasing in the semi-humid and fully humid inner tropics.
The precipitation in the Sahel zone does not result from the ITCZ and its relocation, but from low pressure disturbances, “Easterly Waves”, which arise under the eastern high-altitude currents of the “African Easterly Jet”. The monsoons feed moisture into this wave current from the south, which can rain down over the Sahel zone. A large part of the water vapor comes from the equatorial rainforests with their high transpiration rates. However, due to the progressive deforestation of these forests in the past decades, the moisture content of the air and thus also the willingness to rain fall.
In addition to these anthropogenic influences on precipitation in the Sahel zone, natural processes such as changes in the surface temperatures of the seas, changed atmospheric and oceanic circulation conditions as well as variabilities in solar activity have an influence on the temperature contrast between Guinea and the Sahara and thus on the “African Easterly Jet” can lead to periods of drought.
In contrast to the arid regions of the tropical Sahel zone, there is currently no trend towards increased droughts in the subtropical arid regions.
However, model calculations in connection with the anthropogenic greenhouse effect show that with a sustained warming trend due to changes in circulation conditions within the precipitation, a decrease in precipitation can also be expected in North Africa (250.3).