33 



summer may exceed 130° F. (55° C.)- According to Hosier (iQOj) 

 the average date of the last frost in spring is April 29 in northern 

 Illinois and April 21 in the central portion, while the first frosts in 

 autumn occur on October 6 and 10, respectively. The average length 

 of the growing season is accordingly from 160 to 172 days, depending 

 on the latitude. 



D 



M 



M 



N 



100 



75 

 50 

 25 





Fig. 2. Rainfall curves for Dubuque, Beloit, Keokuk, and Springfield, from 

 December to November, expressed in inches (left) and millimeters (right). 



The rainfall, as shown in Figure 2, is unequally distributed, the 

 greater portion falling during the growing season. The resulting 

 dry winters are probably somewhat favorable to the perpetuation of 

 the prairie formations (Schimper, i<^02,^. The number of days with 

 o.oi inch of rainfall or more varies from 75 per year at Beloit to 

 117 at Dubuque, and of these from 35 to 52 occur during the grow- 

 ing season. 



Comparing the preceding statements with the cun^e of total sun- 

 shine (Figure 3), it becomes evident that the comparatively rainy 

 summer months have the greatest proportion of sunshine. This im- 

 plies heavy rains separated by days of hot dry weather, and leading 

 to a generally xerophytic season in late summer. This climatic fea- 

 ture has already been commented upon (Schimper, 190^^ as in a 

 measure conducive to a prairie type of vegetation. 



Further climatological data might be included, but it is believed 

 that these will give a sulficiently complete idea of the general climate 

 of the region. The details of plant distribution are in nowise af- 

 fected by the broad features of climate. 



