134 IOWA DEPARTMENT OF AGRICULTURE. 



rain-bearing winds that blow inland from the Pacific Ocean; as a result the 

 eastern slope of the Rockies receives a scant and irregular supply of rain- 

 fall, and the Mississippi valley practically receives no moisture from that 

 source. The western and northwestern winds in this section are cool and 

 dry, while the southerly and easterly winds are warm and moist, affording 

 generally an ample supply of rainfall. If the great mountain ranges had 

 been stretched diagonally across this continent, cutting off this region from 

 the rain-bearing wind currents from the Gulf, this section would be in reality 

 the great American desert, instead of the richest domain of Ceres. 



It appears, then, that the essential features of the climate of this region 

 are determined by the size and general topography of the continental area 

 at the westward, the height and location of the mountain ranges, the direc- 

 tion of the prevailing winds, and the general movement of the "highs" and 

 "lows" that cross the valley. 



PRECIPITATION . 



Nearly the entire amount of moisture precipitated over Iowa and contig- 

 uous portions of the Mississippi valley comes directly or indirectly from the 

 Gulf of Mexico. The mechanics of this irrigation process may be under- 

 stood quite readily. By cyclonic force, or the powerful suction of low area 

 storms of a rotary character, the warm, moist winds from the south are 

 drawn up into the valley, and by dynamic cooling are made to deposit a 

 goodly portion of their burden of moisture. It may be said, therefore, that 

 this valley is watered by cyclones, which in their mechanical action and ef- 

 fect may be termed vast rotary pumps, and condensers of atmospheric va- 

 pors. This great central depression, which may be called the.' 'trough of 

 the continent," extending from the Gulf to the Arctic Sea, gives an unob- 

 structed pathway for the warm and moist south winds and the cool waves 

 from the north, which here commingle in the atmospheric eddies, and re- 

 fresh the earth with copious showers. 



The heaviest annual precipitation is deposited in the region near the 

 Gulf , and there the bulk of it comes in the fall, winter and early spring, 

 frequently in excessive downpours. In considerable portions of the Gulf 

 region the mean annual rainfall is double the average in Iowa, and as a re- 

 sult commercial fertilizers are in demand to restore some measure of the loss 

 of fertility caused by the washing and leaching process. This state is more 

 fortunately located, in the region of the golden mean between the extremes 

 of heavy precipitation at the south and east, and general deficiency at the 

 west and northwest. In other words, the people of Iowa suffer less damage 

 from excessive rains than their neighbors at the east and south, and very 

 much less harm from drouth than their neighbors in the western and north- 

 western part of the interior valley. 



At an early day in various historic and scientific publications this' state 

 was credited with a mean annual precipitation of 40 to 47 inches. This 

 hisjh average was obtained from insufficient climatic data, collected at a few 

 stations in the extreme east and southeast parts of the state, where the year- 

 ly average is somewhat greater than in the west and northwest districts. 

 Since that early period stations have been established in all parts of the 

 state, and from the mass of observations obtained the true mean is found to 

 bs 31.40 inches. During the past thirteen years, the voluminous records 



