SOME BIOCLIMATIC OBSERVATIONS IN THE EGYPTIAN DESERT 



Dr C.B.Williams. 

 {Rothamsted Experimental Station) 



I resided in Egypt from July 1921 to June 1927, and during this period I made three 

 short expeditions to the hilly desert to the south- east of Cairo to take observations on 

 bioclimatic — or what perhaps today would be called microclimatic conditions. The 

 object was to discover the range of temperature and other environment conditions avail- 

 able to animals with a power of choice, and of movement over short distances. 



The results have already been published in technical and scientific bulletins of 

 the Ministry of Agriculture of Egypt (see bibliography at end), but as these are not easy 

 to consult in libraries, it was thought that a new summary might be useful to ecologists. 



The locality chosen was in Wady Digla, a dried watercourse about twelve miles 

 south- east of Cairo and about seven miles from the nearest cultivation in the Nile Val- 

 ley. The wady (or valley) at the point chosen runs from east to west and is about 300 

 yards across at the top, about 80 yards across at the bottom and about 200 feet deep. 

 The rock is a pale brown limestone. Rain falls on an average not more than once a 

 year. 



Three visits of eight days each were made in August 1922, March 1923 and Decem- 

 ber 1923, and on each occasion meteorological readings were taken, for seven consecu- 

 tive days, every hour from 5 a.m. to 11 p.m. and again at 1 a.m. In addition to these 

 three longer visits, many one or two day visits were made at all times of the year, and 

 on some of these temperature and humidity conditions were recorded. 



Readings were taken in a variety of locations, including a Stevenson's screen in 

 the middle of the wady — in the shade of the rock on the south side of the wady — under 

 a large rock where it was just possible to crawl — on the plateau above the wady — at 

 different depths up to 30 cms. in a sand patch alongside the dried water course — at a 

 depth of about 75 cms. down a Jerboa burrow — in a bird's nest in a bush, and in an- 

 other hole in a rock- in two *ant- lion' pits one in the sun and one in the shade — and 

 at various depths in two caves. The records included black and white bulb tempera- 

 tures — wet and dry bulb temperatures with a sling-psychrometer — wind with two cup 

 anemometers — and evaporation with 'Piche' evaporometers. 



Table I shows a summary of most temperature and humidity records (except those 

 in the caves) in each of the three periods of observation, and Fig. 1 shows diagrama- 

 tically the means and extremes in many of the habitats. 



It will be seen that in August the black bulb thermometer reached 74°C (166°F), 

 the surface sand reached 58°C (136°F) and the temperature in a bush reached 44°C 

 (111°F), while the shade temperatures only reached 35°C (95°F) which was less than 

 the maximum of the sand at 20 cms. The August week was not exceptionally hot, and 

 shade temperatures 5° or even 10°C higher might well occur at this time of the year. 

 During this period a temperature of 44. 2°C (112°F) was recorded in the sand of an 'ant- 

 lion' pit, and at this temperature the ant-lion larva just below the surface immediately 

 captured an insect which was dropped into the pit. 



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