New Facts and Some Reflections on the Sahara

Abstract

The fundamental fact and problem is the aridity that makes the desert and gives it its unity. We now know that this aridity is not total. Up to I939 the only regular meteorological station in the was at Tamanraset.' Throughout that year, however, daily observations were carried out by French officials at 3 I stations scattered through the northern and central under a plan sponsored by the Meteorological Institute of the University of Algiers and the Direction des Territoires du Sud. The results were published in I940 in a series of monthly maps and a map for the year under the paradoxical title of Rainfall and Floods of the Sahara (scale I 5,ooo,ooo). The picture they present changes considerably our ideas of the Saharan climate, as R. Capot-Rey, Gautier's pupil and successor at the University of Algiers, points out in his commentary.2 The year map discloses a rainfall larger and less erratically distributed than has generally been believed. It decreases toward the south, increases with altitude, and shows the usual contrast between windward and leeward slopes. The fact that one can draw isohyets for the heart of the calls for a revision of the old definition of the desert climate-a climate without an average yearly rainfall.3 On the monthly maps one can follow the progress of regional rains crossing the desert. Capot-Rey classifies the Saharan rains thus: I. Mediterranean cyclonic rains, caused by low-pressure centers crossing the Mediterranean from west to southeast in autumn or winter. Ordinarily, they bring water to the Atlas piedmont; exceptionally, to El Golea and Wargla; to the south they bring sandstorms without rain. 2. Saharan cyclonic rains, caused by depressions crossing the desert from southwest to northeast, typhoonlike in their progression. They generally occur in winter and follow the foot of the Anti-Atlas and the Saharan Atlas or cross the itself, giving rains to the Ahaggar and Tasili. In March, I939, such a storm gave Wallen, on the northern border of the Tanezruft, 8I millimeters of water in three days! The origin of these depressions is not yet clear. 3. Monsoon rains, from Atlantic sources penetrating to the heart of the in the region of Air and the Ahaggar. Humid air masses from Lake Chad were the cause of the rains that gave Tamanraset 20 millimeters (out of a yearly total of 26 millimeters) in