69 



state is highest during spring. During the summer, the southwest 

 winds predominate in the northern and central parts, and in the south- 

 ern part 82 per cent, of the winds are southerly. The velocity of the 

 wind is least during the summer, and the greatest stagnation occurs 

 in August. During autumn there is a falling off of the southerly 

 winds and an increased velocity as winter conditions develop. The 

 transition in the fall is in marked contrast with the vigor of the 

 spring transition. The cooler seasons are more strongly influenced 

 by northerly winds, and the warmer seasons by southerly winds. 



j. Climatic Centers of Influence 



In the preceding section the average conditions of temperature, 

 precipitation, and the direction and velocity of the winds have been 

 summarized, but little effort was made to indicate the mode of opera- 

 tion of the determining factors which produce and maintain these aver- 

 age conditions. It is often true that the main factors which explain 

 the conditions seen in some restricted locality can not be found within 

 it because the local sample is only a very small part of a much larger 

 problem. Thus no one attempts to find an explanation of the through- 

 flowing upper Mississippi system within the state of Illinois; a larger 

 unit of study is necessary. The region examined must extend to the 

 headwaters. So, also, with most of the climatic features of Illinois ; 

 their approximate sources must be sought elsewhere. Let us there- 

 fore consider some of the broader features which influence the climate 

 of North America, particularly that of the eastern part. 



The climates of the world have been divided into two main kinds, 

 depending primarily upon the controlling influence of temperature. 

 This is due to the relative specific heat of land and water, that of water 

 being about four times that of land. The sea, which covers three 

 fourths of the earth's surface, is thus an immense reservoir of heat, 

 which is taken up and given off slowly, at a rate one fourth that of the 

 land. It is therefore relatively equable. The northern hemisphere 

 contains the largest amount of land, and is therefore less under the 

 control of the sea than the southern hemisphere ; yet the sea's influence 

 is very powerful, particularly near the shore. The large land masses, 

 on the other hand, on account of their lower specific heat, receive and 

 give off heat more rapidly to the air above. For this reason the tem- 

 perature changes, as between day and night or summer and winter, 

 are much more rapid and much more extreme over land than over 

 the sea. A climate dominated by the equable sea is oceanic ; that 

 dominated by the changeable lands is continental. Illinois lies far 



