THREE RAINFALL DISTRICTS 37 



experienced in harvestini; and tlircshinj,' ilu- imiiiciist' (Top urowu." Another com- 

 pensatinjr factor is that the rains in Montana durinj: the growing season are 

 rarely of a torrential character and practically all that, falls can be utilized in 

 plant development. 



"It is usually considered." .'Jays the Atlas of American Agriculture, "that 

 between 15 and 20 inches of annual precipitation, broadly speaking, determines the 

 dividing line between areas where farming operations can be successfully conducted 

 by ordinary methods and those where irrigation or other special methods are 

 necessary, but no hard and fast rule can be laid dovin in this connection. With 

 an annual precipitation of only l.j inches other conditions must be very favorable 

 for profitable agriculture by ordinary methods. There are in Montana, eastern 

 Washington, and elsewhere in the northwest, important grain-producing areas 

 which receive, on the average, less than l.l inches of precipitation annually. In 

 fact, wheat is grown in eastern AVashington on only nine inches' annual precipi- 

 tation, but in these areas special care is given to conserving the moisture of 

 the non-growing period for use during the following season." 



The greatest annual precipitation in Montana occiirs over that portion lying 

 west of the continental divide and in the higher mountain regions, and the least 

 over the northeastern plains. There is, however, at least one notable exception to 

 this rule in a considerable district at the eastern base of the main range in the 

 central portion with very light rainfall. About one-half of the area receives from 

 10 to 15 inches annually, and the remainder of the state from 15 to 25 

 inches. Amounts of 20 inches or more, at the lower altitudes, fall only over lim- 

 ited areas, principally in the northwestern portion. 



"The area of the state falls into three natural rainfall districts," states 



Circular 87, Montana Experiment Station. A Further Report on Montana Climate, 



"which may be terjned the eastern and the western mountain 



Three Rainfall and the plains districts, the plains being the region of later 



Districts settlement in an agricultural sense." 



The mountain district west of the main range shows an an- 

 nual rainfall of 20.37 inches, with extremes ranging from 11.29 inches to 34.25 

 inches. The average rainfall for the growing season of April. May, June, and 

 July is 6.23 inches or but 40 per cent of the annual. In the northwestern corner 

 of the state the seasonal rainfall is even a smaller proportion. Only 26 per cent 

 of the rain of the year comes in May and June. This district must make a special 

 effort to conserve the fall and winter precipitation. 



"In the mountain district east of the main range the average yearly rainfall 

 is 1&22 inches, varying from 11.09 inches to 23.53 inches. The rainfall for April, 

 May. June and July is 53 per cent of the annual and that for May and June 

 is 35 per cent of the annual. 



"The plains area of the state is drier than either of the mountain areas. The 

 average yearly precipitation is 13.63 inches, with a range from 12.06 inches to 

 15.51 inches. The rainfall for the growing season is about 55 per cent of the 

 annual and that for May and June is 37 per cent of the annual. 



"Contrasting the districts east of the continental divide, it is noted that while 

 the plains district has over two and one-half inches less annual rainfall it has 

 only one and two-tenths inches less diu'ing the growing season and about seven- 

 tenths of an inch less in May and June." 



Unusually favorable climatic conditions prevail in Montana during the har- 

 vest and fall-work season. August is the chief harvesting month with an aver- 

 age precipitation of only 1.10 of an inch distributed over a 

 Good Harvest period of three to six days. As a resvdt, practically no time 



Weather is lost in the fields and maximiun harvesting efficiency is in- 



sui'cd. September is a wetter month with an average of 1.41 



inches of precipitation distributed over a period of six to nine days. This in- 



