Figure 3- 

 Combined Temperatures and Relative Humidities 



water course in the centre of the wady. Fig. 4 A shows the gradually changing tem- 

 perature at different levels in the sand during 36 hours in August. Surface temperature 

 reached a maximum of 56*^ (133°F) at about 1 p.m.; at 5 cms. the maximum was 44°C 

 (111°F) at about 3 p.m.; at 10 cms. just over 40°C (104°F) at about 5 p.m.; at 18 cms. 

 36*^ (97°F) at about 7 p.m.; while at 28 cms. the temperature only ranged from about 

 23° to 24°C (73-75°F) with a maximum about midnight. The minima ranged from about 

 20°C (68°F) at 5 a.m. at the surface to 23°C (73°F) at 28 cms. about mid -day. Fig 4B 

 shows the temperature contours at different depths during the day and Fig. 4C shows 

 the movements of heat in the sand at different times, the surface heating during the day 

 and cooling during the night. The lines where the heat movement is momentarily zero 

 have been called the 'thermostatic lines* (see McKenzie Taylor and Williams 1924). 



Fig. 5 shows how the observed changes in temperature at different depths in the 

 sand in August support the theory that the range of temperature at a depth x is given 

 by the formula R = Ra'^ where R is the range at the surface, and a is a constant for the 

 particular sand or soil. The figure shows above the observed maxima and minima, and 

 below the expected range calculated from the above formula with (3 = -01 3, and as 'cros- 

 ses* the observed ranges which fit extremely closely to the calculated values. 



22 



