- Determine in degrees and kilometres the distance of the north
and south extremities of Africa from the Equator.
- Locate the Tropics of Cancer and Capricorn on the map of
Africa, When is the sun at its zenith at the Equator, when at each
of the Tropics?
Fig. 130. A scheme showing when the midday sun is at its zenith between the Tropic of Cancer and the Tropic of Capricorn. The arrows show the displacement of the sun during the various months of the year.
Temperature. Africa is the hottest part of the world. It lies almost entirely within the torrid zone, and its average annual temperature is above 20°C. The extreme north and south are the coolest parts, having a subtropical climate. Themid day sun is high above the horizon throughout the year. Between the Tropics it is at its zenith twice a year and is very hot (Fig. 130). Since Africa is divided into two by the Equator and lies in both the Northern and Southern Hemispheres, the seasons are reversed on either side. When the north is having its winter, the south is
having its summer, and vice versa — when the north is having its summer, the south is having its winter.
Notice that the average January temperature is highest (over 25° C above zero) south of the Equator (examine the January isotherm on the climate map in your Atlas). In the north the temperature is much lower at this time of the year, being a little over 10°C at the north extremity. In July, on the contrary, it is very hot in the north. Notice that the isotherm of 30°C on the climate map is a closed curve and that in some places the average July temperature is 35° C above zero. The south has now a July temperature of only 10—15° C.
Off the north-western and south-western coasts' of Africa pass a number of cold currents through the Atlantic, which greatly lower the temperatures of these regions. For this reason the isotherms here bend towards the Equator.
Trade Winds and Rainfall. The climate of Africa is greatly influenced by the trade winds.
About the Equator, where it is hot throughout the year, lies a belt of low air pressure. On the other hand, there are two high-pressure belts that stretch around the globe near latitudes 30°N and 30°S. This difference in the pressure of the air causes winds to blow. Winds blowing from the high-pressure belts towards the Equator throughout the year are called trade winds. Trade winds are deflected to the west by the Earth's rotation on its axis: that is, to the right in the Northern Hemisphere and to the left in the Southern. Thus they move towards the Equator not from north to south, but
A scheme of the high pressure belts and trade winds. The arrows show the direction of the trades.
from north-east to south-west in the Northern Hemisphere and from south-east to north-west in the Southern (Fig. 131). In North Africa the trade winds blow across a great land mass and are dry. As they move southward, they are heated and the land they pass over receives almost no rain. In South Africa the trade winds blowing from the Indian Ocean bring rain to the Drakensbergs and other mountains, but then soon go dry. Thus the trade wind regions of Africa receive very little rain.
Closer to the Equator in summer, when the pressure of the air over the land is lower than
over the water, the trade winds change to monsoons which bring i rain to the continent.
In the equatorial belt the heated currents of moisture-laden air rise, and upon being cooled, cause heavy rains in this region. Rain is heaviest when the sun is at its zenith or close to it (from March to April and from September to October). At these periods the sun is hottest. Its vertical rays heat the land, causing much evaporation followed by cloudbursts. Rain is heaviest on the slopes of the mountains facing the wet ocean winds. The coast of the Gulf of Guinea, for example, receives up to 10,000 mm yearly.
Climatic Regions. Several climatic regions may be distinguished in Africa, that vary with the amount of rain and the temperature.
The equatorial belt is a region of low air pressure and the temperatures are uniformly high throughout the year (about 25—28°C). There is much rain at all seasons. The sky is usually clear in the morning. By midday it is covered with clouds, however, and then comes a heavy downpour often followed by a thunder shower. By evening the sky is clear again. It gets dark quickly and night falls almost immediately. And thus almost all the year round.
The stifling heat and moist air is hard to bear for Europeans unaccustomed to the climate and call forth various diseases. Everything iron rusts in the damp air. Many things become covered with mould. In this climate vegetation is lush and varied.
North and south of the Equator extend two tropical zones with periodic rainfall. The temperatures here are uniformly high throughout the year. Nearly all the rain falls in the summer, however, when the sun is close to its zenith and the summer monsoons blow. These regions on either side of the Equator, both in the Northern
Africa: Climatic regions.
Point out the equatorial climate belt and the "two tropical zones with periodic
and Southern Hemispheres, have two distinct climatic periods: a wet period in summer and a dry period in winter.
In the high-pressure and trade wind belts there is very little rain. Here extend two desert zones on either side of the Equator. The cold currents off the north-western and south-western coasts of the Atlantic Ocean do not favour
rain. The trade winds, besides, blow offshore. The deserts therefore extend to the very coasts of the ocean. In the part of Africa lying in the Northern Hemisphere stretches the vast Sahara Desert. The ground becomes very hot in the daytime under the clear, cloudless sky; the temperature in summer sometimes reaches 50—60°C in the shade. At night, on the other hand, it is cold, the temperature after a hot day sometimes falling to freezing-point and lower. Such winter frosts as in the Asiatic deserts do not occur in the Sahara, however. The average January temperature is high above freezing-point. Deserts occupy a smaller area in the south, as the continent is much narrower there.
Along the shores of the Mediterranean to the north of Africa and in the extreme south the climate is subtropical. The average annual temperature here is below 20°C, and there is a noticeable difference between the summer and winter temperatures. As in Southern Europe, summers along the shores of the Mediterranean are hot and dry, winters mild and wet. In the extreme south-west all the rain falls in winter, in the south-east it falls mostly in summer.
Questions and Assignments.
- What season is it in South Africa now?
- Study the January and July isotherms on the map and deter
mine the winter and summer temperatures:
- along the Red Sea coast;
- at the northern and southern extremities of Africa.
Why do the isotherms on the climate map sometimes form
closed curves and show a fall in the temperature?
- Study the map and notice the variations in the rainfall along
longitude 20°E. In which latitudes do the wettest and driest