82 WORK OF RUNNING WATER 



moisture abundant. The climatic conditions favoring chemical 

 weathering are therefore different from those favoring mechanical 

 weathering (p. 25). 



So long as the water of the surface and that in the soil remain 

 unfrozen, temperature affects neither corrasion nor transportation. 

 But in middle and high latitudes the surface is frozen for some part 

 of each year. During this time corrasion is at a minimum, for 

 although the streams continue to flow, there is relatively little 

 water running over the surface outside the drainage channels, and 

 that little is relatively ineffective. Under some conditions, there- 

 fore, temperature affects both corrasion and transportation. 



The humidity of the atmosphere has an influence on the rate of 

 erosion. A moist atmosphere favors oxidation, carbonation, hydra- 

 tion, and the growth of vegetation, all of which promote certain 

 phases of rock weathering. On the other hand, humidity tends to 

 prevent sudden and considerable variations in temperature, thus 

 checking the weathering effected by this means. Precipitation, 

 the most important single factor in determining the rate of erosion, 

 is dependent on atmospheric humidity. Its amount, its kind (rain 

 or snow), and its distribution in time, are the elements which 

 determine its effectiveness in any given place. 



Other things being equal, the greater the amount of precipita- 

 tion the more rapid the corrasion and transportation. Much, how- 

 ever, depends on its distribution in time. A given amount of 

 rainfall may be distributed equally through the year, or it may fall 

 during a wet season only. The maximum inequality of distribution 

 would occur if all the rainfall of a given period were concentrated 

 in a single shower. With such concentration the volume of water 

 flowing off over the surface immediately after the downpour 

 would be greater than under any other conditions of precipitation, 

 and since velocity is increased with volume, and erosive power 

 with velocity, it follows that the erosive power of a given amount 

 of water would be greatest under these circumstances. Further- 

 more the largest proportion of the precipitation would run off over 

 the surface under these circumstances, for less of it would sink 

 beneath the surface, and less would be evaporated. 



If erosive power and rate of erosion were equal terms, the maxi- 

 mum concentration of rainfall would be the condition for greatest 

 erosion; but we have seen (p. 79) that erosive power and rate of 

 erosion do not always correspond. If the water falling in this way 



