CLIMATOLOGY 81 



collecting and standing in depressions and on the low ground, 

 and to the water-logging of the soil or the filling of the space 

 between the soil grains which thereby prevents the circula- 

 tion of air. Both flooding and water-logging are the results 

 of one heavy rain or several rains coming in rapid succession. 

 In looking at the data relating to the effect of excessive mois- 

 ture on crop production it is necessary to determine the 

 season of the year in which the greater part of the rain 

 falls, the quantity of water falling in any twenty-four 

 hours, and the rapidity with which one rain follows another. 

 A rainy period before the crops are planted in the spring 

 ordinarily causes no damage. The same amount of rain in 

 June or July might mean a crop failure. Comparisons of 

 the monthly rainfall or annual rainfall of an area in one 

 state with that of another are made for the purpose of 

 giving information about the drainage conditions or drain- 

 age channels. Such comparisons have no value unless the 

 individual rainfalls are taken into consideration. The run- 

 off to be disposed of in the growing season in the Red River 

 Valley of the North is practically the same as that in south- 

 ern Illinois, although the average annual rainfall in southern 

 Illinois is one-half greater than in the Red River Valley. 

 In Illinois the rainfall is distributed throughout the twelve 

 months of the year, while in the Red River Valley there is a 

 tendency for it to be limited to the spring and summer 

 months. 



Winds and sunshine. When winds are excessive they 

 make a region disagreeable and cause inconvenience during 

 the winter months by drifting the snow. In an arid country 

 they may be the cause of rapid evaporation, which is detri- 

 mental where the water supply is short. In prairie regions 

 the effect of the wind around buildings can be alleviated to 

 a certain extent by trees planted on the two sides from 

 which the prevailing winds blow. Sunshine has a bearing 

 on plant growth, but ordinarily where the conditions of 

 temperature and moisture are right the sunshine wift be 



