144 



ANALYSIS OF THE ENVIRONMENT 



ment is much simpler. The actual wind 

 system of the lower atmosphere is more 

 complex, especially over the continents. The 

 winds that blow over the oceans during the 

 northern midsummer season are shown 

 somewhat realistically schematized in Fig- 

 ure 27, In July the sun shines vertically 

 some distance north of the equator, and the 

 global wind system is then shifted to the 

 north of its average position. Northeast 

 trade winds occur in the Atlantic and east- 

 ern Pacific, and southeast trades are typi- 

 cally developed in all three oceans of the 

 southern tropics. The polar easterlies blow 

 sparingly at the latitudes that are clearly 

 shown in Figure 27. The southern westerhes 

 are charted as the winds that blow around 

 the world, and characteristic cyclonic 

 whorls occur in the corresponding regions 

 of the northern hemisphere. The Indian 

 monsoon is well developed. 



We cannot examine the climatic effects 

 produced by these winds in detail. In gen- 

 eral, the situation is as follows: The ascend- 

 ing moist air in the equatorial region, when 

 chilled, furnishes the downpour of tropical 

 rain characteristic of the area. The seasonal 

 shift of the heat equator with the position 

 of the sun gives the simplest cause of rainy 

 and dry seasons that are characteristic of 

 the north and south borders of this tropical 

 rainy belt. On both sides of these equa- 

 torial doldrums are the descending, dry, 

 easterly trade winds. When they develop 

 over land they tend strongly to help pro- 

 duce the circumtropical arid regions; the 

 Sahara Desert is a notable example. As the 

 trade winds pass over extensive bodies of 

 water they pick up moisture that is precipi- 

 tated as rain on the windward side of any 

 mountains occurring in the trade wind belt. 

 In India, the onshore monsoon brings the 

 rains, and the offshore monsoon of winter 

 establishes the dry season. 



Poleward from the dry trade winds, the 

 prevaiUng westerlies bring much rain, par- 

 ticularly to the western side of the conti- 

 nents or islands in their path. They are not 

 a steady current, especially in the northern 

 hemisphere, and the accompanying rains 

 are usually intermittent. The great north- 

 to-south mountain ranges— the American 

 coastal ranges or Rockies, for example— cast 

 a decidedly dry "wind shadow" to the east. 

 Precipitation is relatively slight in the cold 

 polar regions. This analysis of global con- 

 ditions reinforces an earlier, more gen- 

 eralized account of world rainfall (p. 79). 



The PrevaiUng Westerlies and Air-Mass 

 Analysis 



The conditions over the continents in 

 the region of the northern prevaiUng west- 

 erhes are of especial interest to ecologists, 

 both because of their complexity and be- 

 cause much of the more detailed ecological 

 study has been done in this world belt. 

 Here, particularly in winter, large cyclonic 

 storms move eastward in almost regular 

 weekly progression. The rate of transloca- 

 tion of the whole storm system is about 15 

 miles per hour in summer and about 25 

 miles per hour in other seasons. According 

 to air-mass analysis, this succession of 

 weather is based on five relatively simple 

 relationships. These are summarized here 

 in somewhat simplified form from an al- 

 ready overschematized statement by Wen- 

 strom (1942); they may well be compared 

 with a more generafized account by Willett 

 (1944). 



1. Air masses that remain for some time 

 in a given region become air conditioned in 

 temperature and humidity relations and can 

 be identified by temperature-humidity 

 characteristics. 



2. These large masses tend strongly to 

 retain their characteristics even when they 

 move to wholly different conditions. 

 Changes begin near their contact with the 

 ground and only gradually affect the upper 

 parts of a given air mass. These masses are 

 moved about by various forces that, in the 

 large, are under the control of the planetary 

 wind system. 



3. When two dissimilar air masses come 

 in contact, they do not mix immediately, 

 but retain a more or less definite boundary 

 or "front" where mixing takes place. Warm- 

 er, lighter air flows up and over a sloping 

 mass of colder air as it would over sloping 

 land, or the cooler air undernms the 

 warmer mass. 



4. The contact and mixing of air masses 

 along a "front" produce clouds, rain, and 

 other types of weather, often in fairly rapid 

 succession. 



5. Weather changes can occur within the 

 air masses either as a result of the internal 

 characteristics of the mass itself or because 

 of the movement of the mass to a new 

 location. 



Air masses become modified as they 

 travel across the continent so that they lose 

 their originally distinct characters just out- 

 lined; for example, a polar air mass is much 



