CLIMATE. 



The rainfall of the State ranges from 30 inches in the immediate 

 vicinity of the Missouri River to 32.5 inches along the Mississippi. 

 There is a gradual decrease in the average annual rainfall from south 

 to north and from east to west. The rains are usually the result of 

 the meeting of warm, moisture-laden winds from the Gulf of Mexico 

 and cooler air currents from the north and west. There are no places 

 in the State where the rainfall is insufficient for the perfect develop- 

 ment of hardy trees. Although droughts do occur in Iowa, they come 

 only at long intervals, generally in the midsummer months; yet it is 

 probable that a greater loss to the State has resulted from excessive 

 moisture during the agricultural growing season than from insufficient 

 rainfall. These periods of excessive moisture favor the growth of 

 trees. 



The mean annual temperature is about 47.5. The transitions 

 between winter and summer are much more rapid than in States 

 nearer the Atlantic coast. This influences to a marked degree the 

 distribution and general growth of trees. Seedlings and often mature 

 trees are seriously injured or even killed by frosts in the fall before 

 the new wood has become thoroughly ripe. The average date of the 

 latest killing frost in the spring has been April 20, and that of the 

 earliest in autumn, October 9. 



The prevailing summer winds, which are usually hot and dry, are 

 from the south, while those at other seasons are from the west. These 

 winds are almost constant and have a velocity of from 8 to 9 miles an 

 hour. The distribution of many tree species is determined largely 

 by these constant drying winds, which probably are responsible for 

 the treeless condition of the plains. Their injurious effect, which 

 can be seen on any exposed windbreak or orchard, may be minimized 

 by planting protective borders of low-growing, hardy trees on the 

 exposed sides. Planting of windbreaks and groves in the State 

 during the past fifty years has probably modified the severity of 

 winds, but to what extent is not known. These strong winds stimu- 

 late transpiration from the leaves of the trees during the growing 

 period, and increase the evaporation from the soil during the dormant 

 season. This latter causes damage by winter killing. 



DRAINAGE. 



The imperfect natural drainage of parts of the State has resulted in 

 scattered, swampy areas, some of which are of very large extent. 

 Since land is rapidly increasing in value these areas are gradually 

 being drained and utilized for farming purposes. On swampy areas 

 which are difficult to reclaim by drainage, arborvitse, tamarack, willow, 

 and cottonwood can be grown successfully, and in addition to the 



[Cir. 164] 



