CLIMATIC FACTORS : THE AIR 



97 



the snow, the ratio may range from 5:1 to 50:1, but 10:1 is fairly 

 average. Careful records of snowfall and water equivalents have 

 not been generally kept until recently. In the western mountains, 

 where melting snow may be the only source of water for distant 

 low country, such records make possible forecasting of floods 

 and, more particularly, the supply of water available for irriga- 

 tion. 54 



FlG. 44. Precipitation-evaporation ratios for the United States calculated 

 according to Transeau. 255 — By permission from Jenny, Factors of Soil Forma- 

 tion, copyrighted 1941, McGraw-Hill Book Co. 



Atmospheric Moisture and Vegetation.— It should be clear that 

 any single atmospheric factor is insufficient in itself to explain the 

 distribution and survival of species or plant communities. Pre- 

 cipitation records are only suggestive, for they must be inter- 

 preted in terms of seasonal distribution, and they are not at all 

 indicative of soil moisture conditions or of the evaporating power 

 of the air to which a plant must be adjusted if it is to survive. 

 The variation in the seasonal pattern of precipitation from place 

 to place becomes particularly apparent when illustrated with 

 twelve-point polygonal diagrams, 256 which make possible easy 

 comparison of amount and time of rainfall by months. Evapora- 

 tion alone is a poor criterion of ecological conditions since it does 

 not take into account the amount of water supplied to the soil. 



