136 IOWA DEPARTMENT OF AGRICULTURE. 



through the months when it is needed for irrigation, while in the balance of 

 the year it is relatively dry. This feature of the climate is more in evidence 

 iu the western districts than in the balance of the state. By districts the 

 percentages of the rainfall in the six crop months are as follows: Northeast 

 district, 70 per cent; North Central district, 74 per cent; Northwest .district, 

 77 per cent; West Central district, 74 per ceat; Central district, 72 per cent; 

 E ist Central district, 68 per cent; Southwest district, 66 percent; South 

 Central district, 70 per cent; Southwest district, 73 per cent. The Mis- 

 souri valley receives the least amount, but gets a greater percentage 

 in the crop season. In other words, the fall and winter precipitation is 

 much lighter in the west than in the east. So there is in this state a wet and 

 dry season, about as well defined as in some of the tropical countries. 



Professor Blodgett, in his American Climatology, referring to this feature 

 in this climate, said: ' 'For the whole period of the warm months the quan- 

 tity of rain distributed over the Mississippi valley is very great, and there is 

 no great area so far in the interior which presents a similar result. The 

 quantities are absolutely as well as relatively large, and they considerably 

 exceed those of the plains of the Atlantic coast in the same latitude." 



VARIATION OF RAINFALL. 



Meteorological records in all parts of the United states show marked 

 variation in the seasonal rainfall, and a perpetual succession of wet and dry 

 periods, though the general averages are steadily maintained through long 

 periods. There are some faint suggestions of periodicity in the occurrence 

 of wet and dry seasons, but the complex problems relating to the variable- 

 ness of the weather have not been solved. All long-time tables of monthly 

 and annual precipitation show that the distribution is exceedingly erratic, 

 though the totals for the continents and hemispheres may be about the same 

 from year to year. During the past thirteen years the lowest yearly average 

 for this state was 21.91 inches in 1894, and the largest amount was 43.82 

 inches in 1902. At single stations the range in total rainfall is much greater 

 tnan for the state at large. It has occurred quite frequently that considerable 

 portions of the state suffered from excess of moisture, while other districts 

 were complaining of drought. 



In 1894 the state average for the four critical crop months (May-August) 

 was only 6.75 inches, or a monthly average of 1.68 inches. In 1902 the total 

 for that period was 27.80 inches, or 6.95 inches per month. And yet por- 

 tions of the state received about the normal amount of rainfall. Evapora- 

 tion acd precipitation are constants, but we have no means of determining in 

 advance where the vapor will be precipitated, for that is subject to vicissi- 

 tudes in the ebb and flow of the great atmospheric currents of the continent. 



Since the early settlement of this section the records show that quite 

 severe midsummer droughts have occurred at irregular intervals, averaging 

 from one to three in each decade, 'ine normal amount for the four critical 

 months is 16.21 inches. During the past thirteen years 'his was exceeded 

 Siven times, and the average fell below the normal six times. There has 

 been, in fact, a greater liability toward excess than deficiency in the crop 

 mouths, and more real damage to crops in this state has been caused by 

 excess in the season of planting and growth than by the reverse. 



In this connection the fact may be noted, esi ecially in seasonal rainfall, 

 that thp-e is a tendency in nature which causes oue extreme to be followed 



