SECTION IV 

 DROUGHT FREOUENCY^ 



Many devices have been used in presenting climato- 

 logical data to show the frequency of droughts. The 

 eflfect of a drought on the growth of vegetation, water 

 supplies, etc., has many angles, depending, to a con- 

 siderable extent, upon several modifying phases, such 

 as the season of the year in which the drought occurs 

 with reference to plant growth, conditions preceding 

 tlie period of deficient precipitation, and the prevailing 

 temperature. Very hnportant modifying factors are 

 the accumulated deficienc}^, or excess, in precipitation 

 for the months, or even years, immediately preceding 

 the droughty period, and the temperature conditions 

 prevailing during it. 



These supplemental factors, in addition to the fact 

 that successive droughts are never directly comparable 

 with regard to the actual deficiencies in precipitation 

 or as to magnitude of the area affected, make it difficult 



1 By J. B. Kincer, Weather Bureau, I'. S. Department of Agriculture. 



indeed to compare one drought with another in the 

 matter of relative severity. When all these things are 

 considered, however, there is no doubt that the 1934 

 drought was the severest in the climatological history 

 of the United States. 



From the standpoint of the effect of droughts on 

 agriculture and the growth of vegetation in general, 

 their frequency of occurrence is shown best through 

 important deficiencies in precipitation, in relation to 

 the normal, which occin- during the most important 

 growing months. Figure 7 is based on the frecpiency 

 of rainfall shortages of significant amounts occurring 

 during the 4 most important growing months, May to 

 August, inclusive. It shows the percentage of years 

 on the average, in which rainfall for these months is 

 less than two-thirds of normal, based on records for 

 600 well-distributed stations for a uniform period of 

 40 years from 1894 to 1933, inclusive. 



FinRE 7. -This map, prepare*! by the Weather Bureau in cooperation witti the National Resources Hoard, sliows the frenuency of rainfall shortace during the important grow- 

 ing months. May to .\ugust, inclusive. It indicates that droughts are relatively infrecjuent east of the .Missi.'isippi Kiver, and as a rule, become increasingly frenuenl west- 

 ward from the Mississippi Valley. In the Oreat Plains and intermountain regions droughts are esjiecially serious because of the generally light rainfall and lack of moisture 

 reserves. 



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