ATMOSPHERE AXD CLIMATE, ETC. 17 



a total evaporation for the vsame period of 198.70 cms. 

 when exposed to wind and sun. (See Appendix). 



Moisture occurs in the atmosphere either in the form of 

 water vapour or minute drops. The moisture in the air 

 is derived from the water which is evaporated from the 

 earth's surface and from the water vapour given off from 

 animals and plants. Moisture, as vapour in the atmosphere 

 is invisible and the amount of vapour which may exist in 

 the atmosphere depends upon the temperature. Thus 

 1 cubic metre of air may contain at C. 4*871 grams of 

 invisible water vapour, whereas at 10 C. it may contain 

 9-362 grams, at 20 C. 17'157 grams, at 30 C. 30'095 

 grams, and at 40 C. 50' 70 grams. If the air contains a 

 sufficient quantity of water vapour, and the temperature 

 falls, the vapour in excess of what the air can contain at 

 the lower temperature is condensed into small drops 

 forming clouds, fog, snow, rain, or dew. The moisture in 

 the atmosphere exerts great influence on animal and plant 

 life. Too much or too little moisture is equally detrimental 

 to the life and growth of plants. A dry atmosphere causes 

 excessive plant transpiration and evaporation, and crops 

 and soil require more irrigation. Generally it is advan- 

 tageous to have a dry atmosphere when crops are ripening, 

 but in the early stages of growth, if the temperature is 

 fairly high, a fair amount of moisture facilitates the 

 growth of the plants. In Egypt, crops are practically 

 entirely dependent upon irrigation for their water supply, 

 and the Nile obtains its water from the rains falling at 

 its sources. So far as agriculture is concerned the rainfall 

 of Egypt is of little importance, 



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