CURRENTS OF AIR AND OF WATER 



changed by passing over one or more of 

 the Great Lakes. Other things also happen. 

 Thus a tropical Gulf air mass may be in 

 contact with the ground for the first few 

 hundred miles up the Mississippi valley; it 

 may then flow up and over one cold mass 

 and have another push in under it from 

 the west and so occlude the warm air from 

 contact with the ground. Even with such 

 complications, this concept of air masses 

 gives a framework on which much of our 

 knowledge of the winds and the weather 

 in the zone of prevaihng westerlies may be 

 arranged in manageable form. 



The cyclonic and anticylonic air circula- 

 tions, which are typical of the middle 

 latitudes, especially in the northern hemi- 

 sphere, are extremely comphcated phenom- 

 ena, and, despite recent, rapid advances 

 in knowledge, summarized by Byers (1944) 

 and Willett (1944), they have not yet been 

 adequately analyzed by meteorologists to 

 allow us to make a simple, truthful sum- 

 mary. Much more is known about them 

 than we have indicated, but the interactions 

 of mechanical and thermodynamic forces 

 are not yet understood completely, even 

 qualitatively. Meterological explanations re- 

 main, as Willett stated in 1931 (p. 211), 

 "at best only roughly qualitative and in 

 parts entirely hypothetical." 



It must not be forgotten that the weather 

 is a matter of prime importance in the lives 

 of many animals other than man. 



Monsoons and Local Winds 



A monsoon is a large-scale, periodic wind 

 circulation that has a direct thermal origin 

 and is characterized by a seasonal change 

 of direction. It is closely associated with 

 sizable land masses and affects neighboring 

 parts of the sea. Although recognized in 

 other continents by meteorologists, mon- 

 soons are best developed in Asia, where 

 they blow out over coastal waters of the 

 Pacific and even extend far across the 

 northern Indian Ocean. A high pressure 

 area exists in central Asia during the win- 

 ter with a resulting steady outflow of air 

 over much of the continent; this becomes 

 distinctly dry south of the Himalayan bar- 

 rier. Converselv, in summer, a strong mon- 

 soon area of low pressure exists north of the 

 southern mountain chain, causing an mflow 

 of air the effects of which reach bevond the 

 equator. The resulting summer monsoon in 

 India brings in much moisture and produces 

 the well-known monsoon rains. These rains 



145 



are heaviest on the southern slopes of the 

 Himalaya Mountains, where precipitation in 

 the Khasi Hills of Assam reaches an annual 

 average of more than 35 feet of rain, most 

 of which is brought in by the summer 

 monsoon. 



Although the Indian monsoon is the best- 

 known one in the world, actually the screen- 

 ing mountains prevent a typical winter 

 monsoon from reaching the Indian plains 

 as it does those of southeastern China. The 

 monsoon circulations dominate the climate 

 of India. The rain-bearing summer monsoon 

 may arrive early or late and may be strong 

 or weak, resulting in heavy or light croos or 

 in complete crop failure. Similar efi^ects are 

 produced on vegetation in general with 

 the result that animal life, including man, 

 is exposed now to plenty and again to 

 severe starvation (Willett, 1944; Byers, 

 1944). 



In many ways the Indian monsoon is a 

 local wind on a subcontinental scale. Much 

 smaller versions develop on a diurnal basis 

 along most coasts where onshore breezes 

 prevail during the day, when the land 

 warms more rapidly than the neighboring 

 water; these turn to ojBFshore breezes in the 

 late evening as the land radiates its heat 

 more rapidly than does water. A large lake, 

 like a miniature ocean, has its own set of 

 shore breezes, and forests show a similar, 

 though still fainter, set of air currents— out- 

 ward on warm, sunny days, and inward on 

 clear nights. Among mountains, the warmed 

 valley air creeps up the mountainside dur- 

 ing the day, and the chilled air of the upper 

 altitude flows down the mountain slopes at 

 night. Long, canyon-like valleys may chan- 

 nel these flows into near-gale force. 



The Chinook wind of the eastern, lee- 

 ward side of the Rocky Mountains, which 

 is similar to the foehn wind of the Alps, de- 

 serves special mention. A moist air mass, 

 saturated and rainy, ascends the western 

 slope of the Rockies. It cools relatively little 

 despite its increase in altitude, since it is 

 warmed by the latent heat of vaporization 

 of the water released by condensation to 

 form rain or snow. On the east slope of the 

 mountains the air mass, now dry, becomes 

 compressed as it flows down into regions 

 of greater pressure and is warmed by the 

 compression to a temperature well above 

 that it had before crossing the mountains. 

 The altered mass flows out onto the north- 

 em plains as a warm, dry Chinook wind 

 that may raise the temperature some 10° C 



