POLAR INFLUENCES ON WORLD WEATHER 



29 



4000 meters computed by Teisserenc de Bort.^ The effect of the 

 circulation is to create a centrifugal force by virtue of which the 

 pressure decreases in the Arctic Basin and increases at 30° latitude, 

 where a ring of high pressure tends to build up under the combined 

 influence of the drift from the east in the equatorial belt arid~thfe\^ 

 drift from the west in higher latitudes. This lowering of the pressure 

 in the polar area cannot go so far as to make a low-pressure area at 

 the pole, because in that 



, ^ r • Equator North Pole Equator 



case the now 01 air away utituda cf 10° 20° 30° 4o° bo° 6o° 70° 8o° 9o°.8o° 7o« 6o° 50° 4o° 30° 20° 10° ° 



Fig. 3 — Mean distribution of air pressure by latitude from 

 the equator across the north pole. 



1 — [ — [ — I — I — I r 

 from the pole would 



cease and the circulation 

 disappear. There re- 

 mains on the average an 

 area of high pressure 

 over the Arctic Basin 

 due to the cold and a 

 ring of higher pressure 

 at about 30° latitude 



due to dynamic causes. A cross section of the mean pressure at each 

 degree of latitude running from the equator across the north pole is 

 given in Figure 3. 



Decreasing Density of Air with Increasing Height 



The fourth reason why the simple circulation represented in 

 Figure i is interfered with is the decrease of density of the air with 

 height. Ascending air expands and, as a result of expansion, chills. 

 This causes condensation of the moisture always present in the air. 

 The latent heat of the condensing moisture retards the cooling of 

 the air, and part of the condensed moisture falls as rain. When this 

 air begins to descend in the Arctic Basin or within an area of high 

 pressure the air is heated at the rate of one degree centigrade for each 

 102 meters of descent and soon becomes much warmer than the air 

 it replaces, so that further descent would cease were it not for the 

 cooling of the air by radiation into space. 



Absorption and Radiation of Heat 



The fifth influence which modifies and changes the simple circu- 

 lation described at the beginning is the absorption of heat by water 

 vapor in the atmosphere and the radiation of heat from the 

 atmosphere. 



1 Leon Teisserenc de Bort: Etude sur la circulation generale de I'atmosphere, Annales du Bureau 

 Central Miteorol. de France, IV: MeUorol. Gen. 1885, Part II, pp. 35-44, and maps, Pis. 6—14; reference 

 on Pis. 9 and 13, Paris, 1887. 



