Relation of Weather to Crops 47 



factors influencing results 



In considering the effects of the weather on different crops some 

 difficulty is experienced in distinguishing with certainty between 

 the results caused by the different phases of the weather and those 

 caused by soil conditions. Differences in the physical and chemical 

 conditions of the soil, especially differences in the amount of alkali, 

 cause more or less marked differences in the success with which crops 

 resist unfavorable weather conditions. These facts have been given 

 due consideration and an attempt has been made to discriminate 

 as accurately as practicable between those results due to differing 

 soil conditions and those due to the effect of the weather. 



In the study that has been made of the effects of the weather 

 upon crops, five factors have been considered — temperature, direct 

 sunshine, relative humidity, rainfall, and wind. Of these five the 

 first has the greatest influence and the last the least influence. 

 High temperatures limit crop production in southern Arizona con- 

 siderably more than low temperatures. Relative humidity has a 

 greater influence on crops than the local rainfall, the latter being 

 too scanty most years to affect results very much. Most of the 

 wind that occurs in the region affects vegetation principally by 

 influencing the rate of evaporation of water from it, the velocity 

 seldom being great enough to directly damage crops. 



General Effects of Temperature 



High and low temperatures affect crops in various ways, the 

 principal ones being by preventing germination, by checking 

 growth, by kilHng part or all of the vegetative parts, by injuring 

 the blossoms, and by damaging the maturing product. The most 

 pronounced effects are brought about in the first two ways and the 

 least injury through the last. Crops affect considerably the tem- 

 peratures about and among them. Through the cooling effects of 

 evaporation and radiation combined, the temperature becomes 

 lower among growing plants during cool nights than it is over bare 

 ground, the difference varying from four to eight degrees. During 

 the day also the rapid evaporation of moisture from vegetation 

 causes the temperature to be a few degrees lower among plants 

 than elsewhere. The temperatures to which crops are subjected are, 

 therefore, more trying during frosty nights and less trying during 



