PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



Figure VI-1 — ANNUAL WORLDWIDE PRECIPITATION 



I Over 80 inches annual mean rainfall 



I j\ I I -Mi 4I.I In SO in hi", 



[^] From 10 to 40 inches 



j Under 10 inches 



The map shows annual precipitation over the world compiled from land-station data 

 and some ship and island observations. Isopleths over the ocean areas, which 

 show large "dry" patches off western continental coastlines, are best guesses. 



coasts (e.g., Florida), while dryness 

 typically prevails along subtropical 

 west coasts (e.g., southern California) 

 and in adjacent continents. Finally, 

 the mid-latitude belt of wetness will 

 be disrupted where mountain ranges 

 (e.g., the Rocky Mountains) provide 

 shelter against rain-bearing winds 

 from nearby oceans. 



Between the semi-permanent cli- 

 matic patterns, which do not change 

 perceptibly, and the rather lively 

 short-term patterns associated with 

 traveling disturbances and storms, 

 there exist regimes of long-lived 

 anomalies superimposed on the gen- 



eral circulation. These anomalies are 

 quasi-stationary or move very slowly, 

 and their duration and intensity may 

 vary within wide limits. Anomalies of 

 this kind are always present, and 

 when their duration and intensity ex- 

 ceed certain limits of dryness, they 

 become recognized as droughts. Most 

 national weather services have estab- 

 lished definitions of drought; al- 

 though these are useful for record- 

 keeping, administrative actions, and 

 such, they do not reflect scientific 

 principles. In the following, the word 

 drought will be used in the meaning 

 of an extensive period of excessive 

 dryness. 



Research Findings 



There is some indication that cer- 

 tain time-lag relationships exist. For 

 example, Namias found that many 

 summer droughts in the United States 

 appear to be associated with changes 

 in the upper atmosphere that begin 

 to develop in the foregoing spring. 

 There is a need here for more research 

 to determine whether reliable two- 

 way statistical relationships exist and 

 are applicable to independent sets of 

 data; if this should prove to be so, 

 techniques for predicting the onset of 

 individual droughts might be devel- 

 oped. 



166 



