384 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1942 



climate between the arid Sahara and the Sudan, where rainy and dry 

 seasons are of about equal length. As in all other semiarid regions, 

 its rainfall is unreliable. The heaviest cloudbursts are brought by 

 the rather infrequent "tornadoes." These may occur at any time 

 during the rainy season but are likeliest at the beginning. Dense 

 clouds pile up swiftly from the east, sky and sea turn black, and a 

 violent thundersquall ensues. Within half an hour the wind turns 

 west and the storm is over, but rarely without damage to trees and 

 habitations.^ 



Table I. — Rainfall in Dakar, average for 1903-1933 ^ 



Maximum annual rainfall (1906), 947 mm.; minimum annual rainfall (1913), 308 

 mm. 



^ I Data from L. Weisse, Note sur la repartition mensuelle, journaliere et horaire de la pluie h Dakar, in 

 Etudes mfitfeorologiques siir I'Afrique Occidentale Frangaisc, Pubis. Com. Etudes Hist, et Sci. Afrique 

 Occidentale Frangaise, Ser. B, No. 3, pp. 71-75, 1937. 



The ordinary rains are brought by monsoon winds from the west 

 during the period of maximum insolation in the northern half of the 

 African continent. Throughout the season the air is steamy and the 

 horizon white with water vapor. For a month or so after the rains 

 the sky is cloudy during a considerable part of the daytime, and the 

 humidity remains high. 



The dry season is marked by light northerly winds that prevail 

 to an altitude of about a thousand meters. Since they pass over 

 the ocean to reach the peninsula, they bring rather high humidity. 

 Occasionally the dusty harmattan blows out of the east, reducing 

 relative humidity and raising the temperature. It seems to persist 

 above the northerly winds during most of the dry season and perhaps 

 accounts for the whitish blue of the sky. The uppermost air currents 

 move from south to north during the dry months, at about 40 kilo- 

 meters an hour.* The stability of the winds favors aviation. Except 

 for tornadoes, the movement of air at ground level is a breeze rather 

 than a wind. 



The annual range of temperature is rarely greater than 6° C, 

 the diurnal variation is commonly 5° (23° to 28° are typical readings). 

 It gets hot as soon as the sun is fairly up and stays hot until near 



3 On the mechanism of the storms see Henry Hubert, Sur les grains orageux dans 

 TAfrique de rOuest, Compt. Rend. Acad. Sci. [Paris], vol. 205, pp. 464-466, 1937 ; also Les 

 masses d'air de I'Ouest Africain, Ann. Phys. Globe France d'Outre-Mer, vol, 5, pp. 33-64, 

 1938 (noted in Geogr. Rev., vol. 29, pp. 154-155, 1939). 



* M. Buroleau, Note sur les courants a€rienes h Dakar, in fitudes m^t^orologiques sur 

 I'Afrique Occidentale Frangaise, Pubis. Com. fitudes Hist, et Sci. Afrique Occidentale 

 Francaise, Ser. B, No. 3, pp. 35-40, 1937. 



