Africa

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Semiarid and Arid

Surrounding the tropical summer rainfall zone are areas of semiarid and then arid climates in north central Africa, east central Africa, and southern Africa. The semiarid, or hot steppe, zone has a short rainy season of up to three months with about 250 to 500 mm (10 to 20 in) of rain per year. Precipitation is unreliable and scarce, creating difficult conditions for plant growth. Temperatures vary in the semiarid zone, with average daily highs ranging from 25° to 36°C (77° to 97°F). Africa’s arid desert regions receive little rainfall. Although classified as hot deserts, these regions have significant annual variations in temperature, and extreme fluctuations in temperature over the course of a day. In the Sahara, daytime summer temperatures can exceed 50°C (120°F), and winter nighttime temperatures can drop below freezing.

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Highland and Mediterranean

Tropical highland climates are common in much of East Africa. Temperatures in the highlands of Ethiopia and Kenya average 16° to 21°C (60° to 70°F), on average about 5 Celsius degrees (9 Fahrenheit degrees) cooler than the lower plateau areas of Kenya, Uganda, and Tanzania. In most parts of the world, higher elevations receive higher levels of precipitation, but the highlands of East Africa are an exception, experiencing rather low levels of rainfall. However, the highest mountains and the southeastern flank of the Ethiopian plateau receive greater precipitation on their windward slopes.

The coastlands of the Cape region of South Africa and the North African coast from Morocco to Tunisia have Mediterranean climates. These areas have mild, rainy winters followed by a prolonged summer when conditions are warm and dry. They receive between 250 to 1,000 mm (10 to 40 in) of rainfall per year.

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Major Climate Controls

Several factors influence the climate of Africa, determining the continent’s climatic regions, creating seasonal variations, and altering day-to-day weather. The most important climate control is related to atmospheric conditions and wind patterns; other major controls include topography, ocean currents, and clouds and other airborne material.

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Seasonal Shifts of Winds, Atmospheric Pressure, and Air Masses

Air masses moving over Africa have different characteristics depending on their areas of origin. Air originating over the tropical oceans, known as maritime tropical air, is warm, moist, and unstable. When this moisture-laden air is forced to rise and become cooler, condensation and precipitation often occur. Conversely, the hot, dry air that originates over the African continent (continental tropical air) is incapable of producing rain, even if it is forced to rise.

Over most of Africa, air moves toward the equator. Northern Hemisphere winds from the northeast and Southern Hemisphere winds from the southeast converge at the intertropical convergence zone (ITCZ), a low-pressure zone centered on the equator. Air rises at the ITCZ, spreads out, and descends in the north and south, in subtropical high-pressure zones centered on the Tropic of Cancer and Tropic of Capricorn. After descending, the air flows outward, either poleward into temperate regions or back toward the equator.

The ITCZ constantly shifts its position, moving north of the equator during the Northern Hemisphere’s summer, and south of the equator during the Northern Hemisphere’s winter. This shift brings about seasonal changes, notably in the tropical summer rainfall zone. During the summer season (July to September in the Northern Hemisphere and December to February in the Southern Hemisphere), moist maritime air is drawn in toward the ITCZ and produces rainfall when it is forced to rise. During the winter season (December to February in the Northern Hemisphere and July to September in the Southern Hemisphere), when the ITCZ is centered in the opposite hemisphere, dry conditions prevail due to the dominance of hot, dry winds coming from the subtropical high-pressure zone. These global, seasonal shifts in wind and pressure zones also bring about the seasonal changes that characterize the Mediterranean climates of Africa’s northern and southern margins.

These seasonal shifts have a comparatively minor impact on rainfall patterns in equatorial and desert regions. The mid-Sahara receives little rain, even in summer, because the ITCZ seldom advances that far north. On the other hand, equatorial regions lack a well-defined dry season because they continue to benefit from moist, maritime air throughout the year. Their position near the equator also ensures that temperatures remain uniformly high year-round. In contrast, Africa’s desert regions experience significant temperature shifts seasonally, a reflection of their higher latitudes.

Most of the continent receives moisture from air originating over the Atlantic Ocean. In eastern Africa, however, rainfall south of the equator comes from large tropical cyclones originating over the Indian Ocean during the Southern Hemisphere summer. Rainfall from these cyclones is particularly high in eastern Madagascar and the coastal mainland between South Africa and southern Tanzania. North of the ITCZ—centered during January over Tanzania—East Africa receives little rain because its air originates in dry, mainland Asia.

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Altitude and Relief

Africa, unlike other continents, has no great mountain ranges to impede or channel atmospheric circulation. However, altitude is an important influence on temperatures, notably in the high plateaus and highlands of southern and East Africa. Highland regions have lower temperatures throughout the year, compared to lower-elevation areas at the same latitude. Highlands also influence rainfall patterns: Moisture-laden air forced to rise over a mountain barrier produces more rainfall on the windward side than on the leeward side.

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