TEMPERATURE 83 



The correlations of yields and rainfall for July and August, espe- 

 cially the latter, shown by charts 10 and 11, illustrate the importance 

 of rainfall to corn during these months. The August precipitation fol- 

 lows the yield, even more closely than does the total seasonal rainfall. 



Rainfall affects not only yield, but habit of growth as well. A wet 

 season favors larger and continued growth, while a dry one induces 

 smaller growth and earlier ripening. Therefoie, in wet seasons, the 

 corn is liable to be injured by early autumn froi;ts. 



The peculiar adaptability of the climate of the corn belt states to 

 the growing of this cereal is accounted for by the fact that the greater 

 part of the rainfall occurs during the crop season. For example, in 

 Iowa 71 per cent of the total precipitation, or 22.48 inches, occurs dur- 

 ing the six crop months, while 51 per cent, or i6.2g inches, falls during 

 the four most critical crop months of May to August, inclusive.* Dur- 

 ing the three spring months 28 per cent of the precipitation occurs ; in 

 the summer and autumn respectively, the percentages are 39 and 23, 

 while but 10 per cent falls during the winter.** 



RELATION OF TEMPERATURE TO CORN GROWING 



Corn is a semi-tropical plant and requires for its maximum growth 

 moderately large rainfall, well distributed through the growing sea- 

 son, together with a large amount of sunshine and a relatively high 

 temperature. An examination of the accompanying charts will show 

 that in Iowa the combination of large precipitation and high average 

 temperatures is rarely found. In fact, these two seem to be opposed 

 to each other. Heavy rainfall is accompanied by a low average tem- 

 perature. Low rainfall and high temperatures (e. g. 1894 and 1901) 

 are found together. For these reasons no very direct relationship be- 

 tween yields and average temperatures can be traced. 



Another feature of temperature, that of frosts, is not shown by the 

 charts. Late spring and early autumn frosts decrease the yields, but 

 such influences cannot be plotted. Unseasonable frosts shorten the 

 growing season, the importance of which is obvious. 



'Roporf Iowa Weather and Crop Service (1902) 



**The effect of moisture and its relation to the corn crop will be fvirtlier discussed In Chap. VIII. 



