68 



brings the state within that influence. And, finally, it lies in the zone 

 of the prevailing westerly winds, and directly across the path of one 

 of the main storm tracks, along which travel in rapid alternation the 

 highs and lows which cause rapid changes of temperature, wind, and 

 precipitation, and thus produce the extremely variable weather condi- 

 tions. 



The state is 385 miles long, and as latitude has much influence 

 upon climate, the climate of Illinois differs considerably in the extreme 

 north and south. This is clearly shown in the average annual tempera- 

 ture, which in the northern part is 48.9° F., in the central part is 

 52.70°, and in the southern part is 55.9° (Hosier, '03). These aver- 

 ages probably closely approximate the soil temperatures for these re- 

 gions. The average date of the last killing frost in the northern part 

 is April 29 ; in the central part, April 22 ; and in the southern part, 

 April 12. The average date of the first killing frost for the northern 

 part is October 9, central part, October 11, and the southern part is 

 October 18 (Henry). The growing season for vegetation in the 

 northern half of the state averages from 150 to 175 days and for the 

 southern half from 175 to 200 days (VVhitson and Baker, 12:28). 

 The precipitation shows similar differences, increasing from north to 

 south. The annual average for the northern part is 33.48 inches, in- 

 creasing to 38.01 in the central and to 42.10 inches in the southern 

 part (Hosier, '03:62). Hosier has shown that the Ozark Ridge, 

 with an average elevation of about 800 feet, condenses the moisture 

 on its south slope so that it has a precipitation of 7.15 inches more 

 than do the counties just north of the ridge. This same humid area 

 appears to extend up the Wabash Valley to Crawford county, and 

 gives the valley counties a rainfall 3 inches in excess of the adjacent 

 counties to the west. The average annual rainfall for the state is 

 37.39 inches — nearly one third of it during April, Hay, and June, 

 and if July is included, more than half. The heaviest precipitation, 

 8.23 inches, is in Hay and June. 



As previously mentioned, the state lies in the zone of prevailing 

 westerly winds and across the path of storms. These have a dominant 

 influence upon the direction of the winds. In the northern part of the 

 state, they are, by a slight advantage, southerly — a tendency which 

 progressively increases toward the south, for in the central part the 

 southerly winds reach 55 per cent., and in the southern part 62 per 

 cent. During the winter the northwest winds predominate throughout 

 the state, to a marked degree in the central part, where they reach 

 60 per cent., and where also the velocity is greatest, reaching an av- 

 erage of 10.3 miles an hour. The velocity of the wind for the entire 



