CLIMATE. 



27 



Among- the localities in the extreme .southwestern United States 

 selected for comparison, Tucson, Ariz., with an elevation of 2,387 

 feet, resembles in situation Bou Saada. Phoenix (altitude, 1,100 feet) 

 may be compared with Biskra, At Yuma (altitude, 137 feet), and 

 still more at Volcano Springs (228 feet above sea level), conditions 

 would be expected to resemble in many respects those prevailing at 

 Tougourt and at Ouargla. A comparison of the figures in these ta))les 

 shows that the Colorado Desert in southern California is warmer than 

 the Sahara in Algeria. Volcano Springs has an annual mean tempera- 

 ture 10.7'^ F. higher than Tougourt, and in summer the maxima are 

 higher. The extreme minimum temperatures in Arizona and Cali- 

 fornia are lower than those in the Sahara. For example, the lowest 

 recorded temperature at Biskra is 20.7- F., while at Phoenix, Ariz., 

 the minimum frequently falls to 25^ F., and has been as low as 12"" F.« 



HUMIDITY. 



While the actual amount of water vapor in the air is sometimes 

 quite appreciable in the Sahara, the relative humidity is always low, 

 because of the high temperatures. In summer the average relative 

 humidity is only 28 per cent, and for this reason the excessive heat is 

 less uncomfortable than would otherwise be the case. So extreme is 

 the dryness of the atmosphere that one's skin is seldom wet with per- 

 spiration, even on the hottest days. Dew is rarely precipitated, and 

 although freezing temperatures are by no means unknown in winter, 

 white frost is not common. The sky over the Sahara is generally 

 cloudless and very clear, particularly in the night time. 



Table 4. — Mean relative luunid'dy {in percent((</es) af Io(y:tlities in the desert region of 



Algeria, as coitipaved ivitlt Yuma, Ariz. 



Month. 



Algeria. 



Biskra. 



January fil.6 



February 55.3 



March.." 52.0 



April 48.2 



May 42.9 



June 36.4 



July 32.6 



Ouar- 

 gla. 



60.1 

 67.5 

 .55.0 

 47.0 

 37.3 

 35.6 

 29.4 



Bou 

 Saada. 



65.5 

 60.7 

 50.7 

 46.9 

 42.0 

 34.1 

 25.4 



United 

 States. 



Yuma. 



45.4 

 43.8 

 43.0 

 35.1 

 36.7 

 34.7 

 42.8 



Month. 



August 



September 

 October . . . 

 November 

 December. 



Year . 



Biskra. 



Algeria. 



Ouar- 

 gla. 



35.6 

 44.1 

 .51. 2 

 58.5 

 62. 5 



48.4 



25.3 

 28.6 

 52.0 

 62. 1 

 66.4 



47.2 



Bou 



Saada. 



26.4 

 39.1 

 47.6 



57.8 

 64.8 



United 

 States. 



Y'uma. 



47.7 

 44.7 

 46.2 

 43.3 

 51.4 



46.8 



42.9 



The three stations in the Algerian Sahara where records of relative 

 atmospheric humidity have been kept all show an annual mean higher 



"For a detailed comparison of the climate of the Aljj;erian Sahara with that of the 

 extreme Southwestern States, see Bulletin No. 5.'^ of the Bureau of IMaut Industry, 

 U. S. Department of Agriculture, The Date Palm and Its Utilization in the South- 

 western States, by Walter T. Swingle, 1904, pp. 52-70. 



