RODENTS OF LIBYA 15 



of rain per year, and some parts of the Fezzan receive no rainfall at 

 all, sometimes extending over many years. 



Most precipitation results from violent convectional showers, 

 which usually are limited to small areas. During the winter, there 

 are occasional widespread rains of a cyclonic rather than convectional 

 origin in the northern portion of the Libyan Sahara. 



Skies are most frequently clear in the Libyan interior, particularly 

 during the winter months (over much of the Sahara, December and 

 January have a cloudiness of only 1/10). 



Relative humidity is almost always low in the Saharan interior 

 of Libya (12-30% for the midday hours) , and evaporation is extremely 

 high. Relative humidity as low as 2 percent, with temperatures of 

 100° F., has been recorded from the Egyptian Sahara, and doubtless 

 similar conditions prevail in the Libyan Sahara. Even though the 

 desert air is physiologically dry and does not approach saturation, 

 absolute humidity is quite high because the hot desert air contains 

 a considerable quantity of water vapor. Potential or actual evapora- 

 tion, owing to high temperatures and low relative humidity, is exces- 

 sive, often being 20 or more times the amount of precipitation. 



In the Libyan interior it is the excessively high temperature in 

 summer, rather than the cold winter temperature, which is responsible 

 for the marked differences between the seasons. Dry air, cloudless 

 skies, and bare, dry earth which produce relatively large temperature 

 differences between the extreme months are also responsible for pro- 

 ducing marked temperature variations within a 24-hour period. 



During the high-sun period (summer) intense diurnal heat prevails. 

 In the northern Sahara average daily maxima of 99° are followed by 

 minima approximating 71° F. The highest air temperature (136.4° F) 

 ever recorded anywhere in the shade under standard conditions was 

 from El Azizia on the Gefara Plain south of Tripoli. 



During the period of low-sun (winter), the days are still warm; 

 the daily maxima usually average 60° to 70° F, but occasionally reach 

 80° F. 



As is true with the rest of the Sahara, the sun largely controls 

 local weather, resulting in a great deal of similarity between successive 

 days. 



The Saharan portion of Libya tends to be windy owing to the 

 lack of obstruction of the moving air by the sparse vegetative cover. 

 Also the rapid daytime heating of the lower air tends to convectional 

 overturning. Nights are generally much less windy owing to the rapid, 

 nocturnal convectional cooling of the surface air. 



Semiarid or Steppe Type 



A region of low latitude steppe climate characteristically bounds 

 the true Saharan portion of Libya to the north and is a transitional 



