Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



is opposite, or clockwise, in the Southern Hemi- 

 sphere. It is commonly called a low or a 

 denression. This use of the word '"cyclone" 

 should be distinguished from the colloquial use 

 of the word as applied to the tornado. (See 

 fig. 13-10.) 



The formation of a new cyclone or the 

 intensification of an existing one is called 

 CYCLOGENESIS. Cyclogenesis usually occurs 

 with DEEPENING (a decrease in atmospheric 

 pressure), but the terms are not synonymous. 

 The decrease and eventual extinction of a 

 cyclone is called CYCLOLYSIS. Cyclolysis refers 

 to circulation in the atmosphere and should not 

 be confused with FILLING (an increase in 

 atmospheric pressure), although the two pro- 

 cesses usually occur together. 



Cyclones in middle and high latitudes are 

 referred to as extratropical cyclones, and the 

 tropical cyclones are referred to as hurricanes, 

 typhoons, baguios, or willy-willies, depending 

 on their geographical location. (Tropical cyclones 

 are discussed in chapter 14.) 



cyclonic 



circulation 



Figure 13-10. — Cyclone. 



209.41 



MONSOON WINDS 



The term "monsoon" is of Arabic origin 

 and means season. The monsoon wind is a 

 seasonal wind that blows from continental 

 interiors (or large land areas) to the ocean in 

 the winter, and in the opposite direction during 

 the summer. The monsoon wind is most pro- 

 nounced over India, although there are other 

 regions with noticeable monsoon winds. Over 



India the monsoons blow from the northeast 

 during January and from the southwest in July, 

 and are caused in the winter by the air blowing 

 out of the high-pressure area of Siberia and 

 in the summer by the wind flowing into the 

 low-pressure area over central Asia. (See 

 fig. 13-11.) 



In summer the weather associated with 

 monsoon winds is almost constant rain. This 

 condition is caused by mass motion of air 

 from the relatively high-pressure area over the 

 ocean to a low-pressure area over land. When 

 the air leaves the ocean, it is warm and moist. 

 As the air travels over land toward the low- 

 pressure area, it is also traveling from a lower 

 altitude to a higher altitude. The air is lifted 

 mechanically and cooled to its condensation 

 point by this upslope motion. 



In winter the weather situation is the reverse 

 of summer. Clear skies predominate during 

 this season. This is caused by the mass motion 

 of air from a high-pressure area over land to 

 an area of lower pressure over the ocean. As 

 the air leaves the high-pressure area over 

 land, it is cold and dry. As it travels over 

 land toward the ocean, there is no source of 

 moisture to induce precipitation. The air is 

 also traveling from a higher altitude to a 

 lower altitude; consequently, this downslope 

 motion causes the air to be warmed at the 

 adiabatic lapse rate. This warming process 

 has a still further clearing effect on the skies. 



JETSTREAM 



The Jetstream is a special circulation; how- 

 ever, since it is so closely associated with 

 secondary circulations of the migratory type, 

 it is discussed here. 



The term, Jetstream, is defined as relatively 

 strong winds concentrated within a narrow stream 

 in the atmosphere. While this term may be 

 applied to any such stream regardless of 

 direction, it is commonly interpreted to mean 

 a band or belt of winds with a strong westerly 

 component which meanders around the globe. 

 By saying that it has a strong westerly com- 

 ponent, it is meant that it flows primarily 

 from the west or from adjacent directions such 

 as northwest or southwest. By saying that it 

 meanders, it is meant that it is not found at the 

 same latitude or elevation all around the earth 

 at the same time, but it has a wave like tra- 

 jectory. It may range from 25 to 100 miles in 



305 



