Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



width and up to a mile or two in depth. Some- 

 times the Jetstream is a continuous band, but 

 more often it is broken or split at several 

 points. 



Jetstreams are found in both the Northern 

 Hemisphere and the Southern Hemisphere, but 

 much more is known about the predominant 

 one in the Northern Hemisphere. This is the 

 one normally referred to when only the term 

 "Jetstream" is used. It is located in the high 

 tropopause along the boundary of the polar 

 front zone where there is extreme horizontal 

 temperature contrast. Normally, there is a 

 break in the tropopause where the Jetstream 

 exists, or it may be said that it exists where 

 the tropopause has its greatest slope. 



The winds in the Jetstream occasionally 

 exceed 250 knots. Most of the time the winds 

 range from about 100 to 150 knots. However, 

 a band of winds is classed as a Jetstream only 

 when the winds in the band have a speed of 

 50 knots or more. The Jetstream is stronger 

 in winter than in summer. 



It is closely associated with migratory low- 

 pressure systems and the polar front. Most 

 of the time there are no signs of a Jetstream 

 on the surface. The Jetstream increases in 

 intensity with elevation to just below the tropo- 

 pause where its maximum speed is reached. 

 Thereafter, it decreases in intensity again. 



The Jetstream is very important both in 

 forecasting weather and in naval flight opera- 

 tions. In forecasting the weather, it is important 

 relative to the development and the movement 

 of fronts and low-pressure systems. In naval 

 flight operations, it is important as something 

 to be avoided when the flight plan goes against 

 it, and as something that can be used to gain 

 time when the flight plan is with the wind 

 direction. A 150-knot wind can increase or 

 decrease the ground speed of an aircraft to 

 a large extent, depending on the direction of 

 the flight relative to the wind. 



TERTIARY CIRCULATIONS 



Many regions have local weather phenomena 

 caused by temperature differences between land 

 and water areas or by local topographical 

 features. Those weather phenomena which show 

 up as circulations of air and are due to local 

 features are termed tertiary (third order) cir- 

 culations. A knowledge of these circulations 



which have a significant effect on the local 

 weather conditions is important for Aerog- 

 rapher's Mates. 



LAND AND SEA BREEZES 



There is a daily contrast of heating of local 

 water and land areas similar to the seasonal 

 variation. During the day, the land is warmer 

 than the water area, and at night the land area 

 is cooler than the water area. A slight variation 

 in pressure is caused by this temperature con- 

 trast. At night the wind blows from ^nd to sea 

 and is called a land breeze. During the day, 

 the wind blows from water areas to land areas 

 and is called a sea breeze. These breezes 

 are shallow and do not penetrate far inland. 

 Often in the middle and higher latitudes, these 

 breezes are not noticeable, due to stronger 

 winds of other character. (See fig. 13-12.) 



The sea breeze is most pronounced in late 

 spring and summer; the land breeze is most 

 pronounced in late fall and early winter. 



MOUNTAIN AND VALLEY WINDS 



During the day, mountain slopes are warmer 

 than the surrounding atmosphere at the same 

 level; this heating effect causes the wind to 

 flow upward from the valley area along the 

 mountain slopes and is called a "valley wind." 

 At night the situation is reversed, and the slopes 

 become colder than the surrounding atmosphere; 

 this cools the atmosphere in the lower levels 

 near the surface of the slopes and causes the 

 wind to flow downward into the valley. This 

 wind is referred to as a "mountain wind." 

 Winds ascending a mountain slope are called 

 ANABATIC winds. At times these anabatic winds 

 are unnoticeable due to the effects of vertical 

 convection. The reverse of the anabatic wind 

 is the KATABATIC wind. Hence, the katabatic 

 wind occurs when the wind flows down the 

 slope. (See figs. 13-13 and 13-14.) 



FOEHN WINDS 



The foehn wind is a warm dry wind with a 

 strong downward component on the leeward side 

 of mountains. When air flows up and over a 

 mountain barrier, it undergoes expansion and 

 cools at the dry adiabatic lapse rate (l°C per 

 100 meters) until the temperature drops to the 



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