FOURTH ANNUAL YEAR BOOK— PART II. 135 



collected by the Iowa Weather and Crop Service show the state average to 

 have been 31.07 iuches. 



Prof. Lorin Blodgett's hyetal chart of the continent, published in 1855, 

 placed Iowa in the belt having a range of 25 to 40 inches, the southeastern 

 counties showing a mean of about 40 inches, the central belt from south- 

 west to northeast, 30 inches, and the northwestern fifth of the state, about 

 25 inches. The more recent observations do not show so great difference in 

 the yearly rainfall of these sections. A bulletin issued by the Weather Bu- 

 reau in 1897 contained a rain chart in which Iowa was placed in the belt 

 having an average of 30 to 40 inches, except an area of a few thousand 

 square miles in the belt ranging from 20 to 30 inches. Rainfall tables in the 

 following pages show that no single station having a record for more than 

 ten years has an average as high as 40 inches, and no station for alike 

 period has an average below 23 inches per year. 



RAINFALL DATA BY DISTRICTS. 



For convenience of reference and comparison, the state is divided into 

 three belts, or sections, on lines running east to west, each section three 

 counties in width. These sections may be subdivided into three districts, 

 following county lines, giving us nine districts, designated as follows: 

 Northeast district, seven counties; North Central district, fourteen counties; 

 Northwest district, nine counties; West Central district, nine counties; Cen- 

 tral district, fifteen counties; East Central district, fourteen counties; South- 

 east district, ten counties; South Central district, thirteen counties; South- 

 west district, nine counties. The tables show the following yearly averages 

 by districts: Northeast, 32.25 inches; North Central, 29.40 inches; North- 

 west, 28.16 inches; West Central, 29.36 inches; Central, 31.66 inches; East 

 Central, 32.61 inches; Southeast. 33.65 inches; South Central, 32.53 inches; 

 Southwest, 32.60 inches, it will be seen that the Southeast district has a 

 yearly average of 5,49 inches more than the Northwest district, and only 

 1,05 inches more than the Southwest district. 



The annual average rainfall of the three eastern or Mississippi river dis- 

 tricts is 32,50 inches; three Missouri valley districts, 30.04 inches — a differ- 

 ence of 2.46 inches between the eastern and western slopes of the state. 

 The central belt on nDrth and south line has an average of 31.51 inches, or 

 very nearly the state average. On the east and west line of division the 

 averages are as follows: Northern section, 29.93 inches; central section, 

 31,21 inches; southern section, 32.92 inches. These figures show a quite 

 regular gradient of decrease in yearly amount from south to north, as well 

 as from east to west. 



RAINFALL IN THE CROP SEASON. 



From an agricultural point of view the most important feature of the 

 climate of Iowa is that its maximum of rainfall comes in the crop season, 

 April to September, inclusive. The average winter precipitation is 3 30 

 inches, or 10 per cent of the yearly amount; spring, 8.85 inches, 28 per 

 cent; summer, 12.15 inches, 39 per cent; autumn, 7.10 inches, 23 per cent. 

 In the six crop months the average rainfall is 22.48 inches, or 71 per cent of 

 the annual total. And in the four most critical crop months. May 1st to 

 September 1st, the average for the state is 16.29 inches, or 51 per cent. It 

 will be seen from these figures that the bulk of precipitation is distributed 



