14 



AGRICULTURE IN THE SAHARA DESERT. 



Almost every winter freezing temperatures are reached, although 

 ])robably the minima are higher and frosts are less frequent in the 

 Oued Souf than at Tougourt and at Tozer. In the winter of 1903-1 

 the absolute minimum was 32° F., and for several preceding winters 



'27° F. 



In respect to atmospheric humidity, the absence of surface water 

 probably tends to keep the air drier than is generally the case in the 

 oases of the Sahara. 



The rainfall is said to be even less than in the Oued Eirh, wh'jre 

 the average yearly precipitation (at Tougourt) is only 5.3 inches. 

 Most of the rain is divided between two periods — October to Novem- 

 ber, and February to. March. The rains are generally torrential in 

 character and fall during several successive days, with intervals of 

 sunshine. 



The sky is nearly always clear in summer. Toward the end of 

 August light clouds appear in the morning and evening, but no rain 

 falls until October. During the writer's four days' visit to the Souf 



the calculations for Mecca, experience having shown that the temperatures 

 recorded at Mecca and at Imperial are very nearly identical. 



Table I. — Sum of daily mean temperatures above 6J,..',° F. (18° C.) from May l to October 



SI, 190J,. 



Locality. 



El Oued, Algeria. 

 Ouargla, Algeria . 

 Biskra, Algeria . . 



Tozer, Tunis 



Mecca, Cal 



Degrees 

 Fahren- 

 heit. 



Degrees 

 centi- 

 giade. 



3,6T2.6 

 3,961.3 

 3, 140. 4 

 3,831.3 

 3,591.5 



2,040.3 

 2, 200. 7 

 1,744.6 

 2,128.5 

 1,995.3 



The sum of daily means at Biskra for 1904 is nearly 200° F. lower than the 

 normal, as based upt)n observations covering a period of 10 years (see W. T. 

 Swingle, Bui. No. 53. Bureau of Plant Industry, p. GO), and the sum at Mecca 

 for that year is about 400° F. lower than the normal for Indio, Cal.. a few miles 

 distant. 



Table II. — Sutn of daily maximum temperatures above 6k4° F- {1S° C.) from May 1 to 



October 31, 190!,. 



Locality. 



El Oued, Algeria. 

 Ouargla, Algeria 

 Biskra, Algeria . - 



Tozer, Tunis 



Mecca, Cal 



In calculating the sums of daily ma.ximum temperatures allowance was made 

 for mean monthly minima falling below 04.4° F., in accordance with the prac- 

 tice suggested by Swingle (ibid., p. 07). The sum of daily maximum tempera- 

 tures at Biskra during May to October. 1004, is alwut 2.50° F. lower than the 

 normal for 12 years (Swingle, ibid., p. 68). The sum for Mecca is about 700° 

 F. lower than that for Imperial, Cal.. in 1902. 



