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Climate in Primeval Times
Researchers assume that dry and humid phases alternated several times during the course of the last several millions of years. Fossils in soil samples from the Chad Basin show that the Sahara had at least occasionally been a desert already seven million years ago. During the Pleistocene (1.8 million until about 11,500 years ago), the Sahara was more water-rich and green. Numerous rock paintings and 1,000 km long wadis coming from the mountain prove this fact.
8,500 B.C., the tropics moved 800 km northward. The Sahara that had been a desert like it is today, then transformed into a fruitful savannah landscape. In the following millenniums, the climate changed again and large parts of the Sahara became desert again.
In the 5th century B.C., a successful oasis economy started in the centre of the Sahara. As time went by, the ground water deposits were depleted, thus limiting the oasis economy. The conquest by the Moslem Arabs marked the end of this fruitful era.
Climate Today
The Sahara is located in the subtropical high pressure belt. It belongs to the tropical deserts, and the descending airflow causes dissolution of clouds, thus allowing unhindered light and thermal radiation. The sky is usually clear with an annual duration of sunshine of 4,000 hours. The continuously high position of the sun, low relative humidity and missing vegetation make the Sahara the hottest large area worldwide. In the summer, temperatures reach average maximum values of over 45° C and locally up to 55° C in the shade, with massive differences between day and night temperatures. Since clouds and vegetation are missing, the temperatures fall significantly at night, with temperatures differences of up to 50° C in one day. Moreover, the seasonal temperature variations are very high. During winter, temperatures in the central Sahara often do not rise over 15° C, and particularly in the mountain regions they can fall down to -20° C at night.
The tradewinds (Passat winds) blow almost throughout the whole year and they bring only sparse precipitation. The relative humidity reaches maximum values from three to five percent; the annual potential evaporation in some parts of the eastern central Sahara is 6,000 mm. Therefore, sparse precipitation in the Sahara often does not even reach the soil, but it already evaporates several 100 m above the ground. At least 5 mm of precipitation are necessary to enable plant growth. The average annual amount of precipitation in the Sahara is approx. 45.5 mm. In some years, it does not rain at all. The average annual amount of precipitation in the Sahara amounts to approx. 45.5 mm. In some years it does not rain at all.
Since the 1990s, an increase of precipitation has been observed. These changes are apparently not necessarily attributed to the worldwide warming, but they are rather due to natural fluctuations. These natural fluctuations caused devastating droughts in the Sahel in the mid 1970s and 1980s.