28 AGRICULTURAL EXPLORATIONS IN ALGERIA. 



than that of Yuuia, the only locality in the desert region of the south- 

 western United States where accurate records have l)een kept. But, 

 while in winter the humidit}' is greater in the Algerian Sahara than 

 in southwestern Arizona, in summer the reverse is true. 



PRECIPITATION. 



A widel}' received explanation of the peculiar conditions of the 

 Sahara, as regards atmospheric water, is as follows: The central por- 

 tion of the desert is sufficienth' elevated to he consideral>ly colder in 

 winter than the Atlantic Ocean to the west and the Mediterranean Sea 

 northward. Consequently, the general direction of winds in winter is 

 from the center toward the edge of the desert, which precludes the 

 possibilit}^ of much rainfall at that season. In summer, on the other 

 hand, the normal winds blow toward the highly heated center of the 

 desert, although there are occasional siroccos in the contrary direction. 

 These normal summer winds from the Atlantic and Mediterranean 

 would cause rainfall in summer were it not that physiographical con- 

 ditions intervene to prevent this. Winds from the west encounter a 

 cold current that follows the Atlantic coast of northern Africa, and 

 the greater part of the moisture they carry is condensed before they 

 reach the mainland. The high summits of the coastal mountain sys- 

 tem of Algeria intercept and condense most of the water vapor that is 

 brought in by winds from the Mediterranean, What little moisture 

 escapes this barrier and crosses the high plateau is mostly given up 

 when the mountains along the northern border of the Sahara are 

 encountered. Furthermore, in the desert itself there are few moun- 

 tains of sufficient elevation to condense what water vapor passes the 

 second barrier. 



Notwithstanding these conditions, rain is b}" no means unknown in 

 the Sahara. Heavy precipitation sometimes occurs, ))ut its distribution 

 is ver}' irregular, both in point of time and of place. Localities in the 

 desert are known which have received no appreciable amount of rain 

 for ten years or more. At other times a cyclone may cause a sudden 

 heavy downpour. Violent torrents are formed and a great amount of 

 erosion is accomplished in a few hours. The higher elevations of the 

 isolated mountain masses of the Sahara have a somewhat more regular 

 rainfall, but it is believed that, on the whole, evaporation exceeds pre- 

 cipitation in the Sahara, and that its aridity is steadily, although 

 imperceptibly, increasing. 



