1. DROUGHT 



The Causes and Nature of Drought and its Prediction 



Drought is one of the manifesta- 

 tions of the prevailing wind patterns 

 (the general circulation). A few spe- 

 cial remarks may clarify this mani- 

 festation, and suggest further work 

 necessary to understand and predict 

 droughts. 



Virtually all large-scale droughts 

 (like the Dust Bowl spells of the 

 1930's or the 1962-66 New England 

 drought) are associated with slow and 

 prevailing subsiding motions of air 

 masses emanating from continental 

 source regions. Since the air usually 

 starts out dry, and the relative hu- 

 midity declines as the air descends, 

 cloud formation is inhibited — or, if 

 clouds are formed, they are soon 

 dissipated. 



The atmospheric circulations that 

 lead to this subsidence are certain 

 "centers of action," like the Bermuda 

 High, which are linked to the plan- 

 etary waves of the upper-level wester- 

 lies. If these centers are displaced 

 from their normal positions or are 

 abnormally well developed, they of- 

 ten introduce anomalously moist or 

 dry air masses into certain regions 

 of the temperate latitudes. More im- 

 portant, these long waves interact 

 with the cyclones along the polar 

 front in such a way as to form and 

 steer their course into or away from 

 certain areas. In the areas relatively 

 invulnerable to cyclones, the air de- 

 scends, and if this process repeats 

 time after time, a deficiency of rain- 

 fall leading to drought may occur. 

 In other areas where moist air is 

 frequently forced to ascend, heavy 

 rains occur. Therefore, drought in 

 one area is usually associated with 

 abundant precipitation elsewhere. 

 For example, precipitation was heavy 

 over the Central Plains during the 



1962-66 drought in northeastern 

 United States. 



After drought has been established 

 in an area, it seems to have a tend- 

 ency to persist and expand into ad- 

 jacent areas. Although little is known 

 about the physical mechanisms in- 

 volved in this expansion and per- 

 sistence, some circumstantial evi- 

 dence suggests that numerous 

 "feedback" processes are set in mo- 

 tion which aggravate the situation. 

 Among these are large-scale inter- 

 actions between ocean and atmos- 

 phere in which variations in ocean- 

 surface temperature are produced by 

 abnormal wind systems, and these in 

 turn encourage further development 

 of the same type of abnormal circu- 

 lation. Then again, if an area such 

 as the Central Plains is subject to 

 dryness and heat in spring, the 

 parched soil appears to influence sub- 

 sequent air circulation and rainfall 

 in a drought-extending sense. 



Finally, it should be pointed out 

 that some of the most extensive 

 droughts, like those of the 1930's 

 Dust Bowl era, require compatibly 

 placed centers of action over both 

 the Atlantic and Pacific oceans. 



In view of the immense scale and 

 complexity of drought-producing sys- 

 tems, it is difficult for man to devise 

 methods of eliminating or ameliorat- 

 ing them. However, given global 

 data of the extent described previ- 

 ously, and the teamwork of oceanog- 

 raphers, meteorologists, and soil 

 scientists, it should be possible to 

 understand the interaction of con- 

 tinent, ocean, and atmosphere suf- 

 ficiently so that reasonably accurate 

 estimates of the beginnings and end- 

 ings of droughts are possible. 



Ability to predict droughts would 

 be of tremendous planning value. 

 Unfortunately, encouragement for 

 drought research comes only after a 

 period of dryness has about run its 

 course, because the return of normal 

 or abundant precipitation quickly 

 changes priorities to more urgent 

 matters. Without continuing in- 

 depth drought studies, humanity will 

 always be unprepared to cope with 

 the economic dislocations induced by 

 unpredictable long dry spells. 



It has long been known that the 

 general circulation of the atmosphere 

 is such that alternating latitude belts 

 of wetness and dryness tend to domi- 

 nate the world system of climates. 

 (See Figure VI-1) In connection with 

 droughts, the important belts are: 



1. The equatorial belt of wetness 

 associated with ascending cur- 

 rents in the zone where the 

 trade winds from the southern 

 and the northern hemisphere 

 meet; 



2. The subtropical belt of dryness 

 associated with descending air 

 motions in the so-called sub- 

 tropical anticyclones; 



3. The mid-latitude belt of wetness 

 associated with traveling de- 

 pressions and storms that de- 

 velop in the zone of transition 

 between warm and cold air 

 masses — i.e., the "polar front." 



While the equatorial belt of wet- 

 ness is more or less continuous around 

 the world, the subtropical belt of dry- 

 ness is disrupted by monsoon-like 

 winds in the warm seasons and by 

 polar-front disturbances in the cold 

 season. As a result, rainfall is gen- 

 erally adequate along subtropical east 



165 



