PART VI — PRECIPITATION AND REGIONAL WEATHER PHENOMENA 



4. TROPICAL WEATHER 



Monsoon Variations and Climate and Weather Forecasting 



The monsoon area extends from 

 western Africa to northeastern Aus- 

 tralia, being bounded to the north by 

 the great mountain ranges of southern 

 Asia. In the southern hemisphere it 

 encompasses southeastern Africa and 

 northern Australia but does not ex- 

 tend beyond the equator over the 

 central Indian Ocean. (See Figure 

 VI-10) Its peculiarity, distinguishing 

 the area from all others, is the marked 

 difference in prevailing surface wind 

 directions between winter and sum- 

 mer. Winds blow predominantly from 

 continent to sea in winter and from 

 sea to continent in summer. 



Thus, in general, since moist air 

 covers the continents in summer and 

 dry air in winter, the summers are 

 usually wet and the winters dry. 

 Over the northern hemisphere this 

 pattern is significantly distorted by 



the huge, elevated mass of Himalaya- 

 Tibet which, through its thermo- 

 mechanical effect on the atmospheric 

 circulation, supports the vast perma- 

 nent deserts east of 70° E. longitude, 

 insures that India, Burma, and Thai- 

 land experience arid winters and very 

 wet summers, and keeps China rela- 

 tively cloudy and moist throughout 

 the year. 



Except for destructive winds asso- 

 ciated with relatively rare tropical 

 cyclones in the China Seas and Bay 

 of Bengal, the attention of meteor- 

 ologists in the monsoon area is 

 focused on only one phenomenon — 

 rain. Accurate long-range forecasts 

 for agricultural planning, or short- 

 range forecasts for irrigation or of 

 floods would be invaluable to the 

 economy of every country in the area. 

 But rainfall variability on every time- 



Figure VI-10 — MONSOONAL AREAS 



The map delineates the regions of the world that are monsoonal — i.e., where the 

 prevailing wind direction shifts at least 120 degrees between January and July. By 

 sharpening the definition according to principles developed by Ramage, it is possible 

 to define the true monsoon area as that included in the rectangle shown covering 

 large parts of Asia and Africa. 



and space-scale — from inter-annual 

 to diurnal and from intercontinental 

 to mountain/valley — render clima- 

 tology of limited use in providing the 

 necessary planning information. 



Status of Tropical Meteorology 



Long-Range Forecasting — Most 

 existing work was done in India and 

 Indonesia before World War II. Mul- 

 tiple-regression equations based on 

 lag correlations were first used in 

 the lQ20's to forecast seasonal rain- 

 fall. Unfortunately, performance was 

 disappointing — droughts and floods 

 were never anticipated and predictor/ 

 predictand correlations proved to be 

 most unstable. Apart from a modest 

 continuing search in India for new 

 correlations, little effort is now being 

 made. 



Unless the deterministic forecast 

 methods to be tested in the Glo- 

 bal Atmospheric Research Program 

 (GARP) perform much better than 

 even their most optimistic proponent 

 expects, there is little chance of use- 

 ful developments in forecasting sea- 

 sonal rainfall extremes. 



Short-Range Forecasting — For the 

 past fifty years the practice of tropical 

 meteorology has been distorted (usu- 

 ally unfavorably) by uncritical graft- 

 ing of hypotheses and techniques 

 developed in middle latitudes. As one 

 scientist has observed: 



We have again and again ob- 

 served very reputable and highly 

 specialized meteorologists from 

 higher latitudes who were deter- 

 mined to solve the problems of 

 tropical meteorology in a very 

 short time by application of mod- 

 ern scientific methods and use of 

 new scientific resources such as 



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