Theory of the Winds. 31 



Trade Winds. If, however, the Trade Winds are due to 

 temperature gradients, we must assume that over the high 

 pressure belts in latitudes #0°N.and S.the air column is still 

 colder than at the equator, in spite of the fact that along these 

 latter belts the air is descending. 



Europe is not a very satisfactory area for studying the 

 effects of the temperatures of the atmosphere at different 

 heights as to their effects upon the winds. Owing to its being 

 a large land area, and the presence of Asia to the east, there 

 are great variations in the pressure gradients throughout 

 the year. Such an area as South America, where the mean 

 pressure gradients are constant throughout the year, would 

 be a more suitable one for the purpose. 



The fact that in high latitudes the stratosphere is warmer 

 than in low latitudes requires explanation. That it might be 

 warmer in the summer, even at the poles, is not so strange; 

 for the length of the day in summer more than compensates 

 for the low altitude of the sun. In the winter, however, the 

 air receives practically no heat directly from the sun within 

 the Arctic Circle; yet at Pavlovsk, even in winter, it does 

 not fall below the temperature at Batavia. According to 

 Gold's theory the upper atmosphere, when the sun's rays 

 pass vertically through it, only intercepts about 12 per cent, 

 of the sun's heat. It is to the long waves reflected back 

 from the lower atmosphere and earth that the stratosphere 

 owes its comparatively high temperature; and in high latitudes 

 the earth's surface receives less heat from the sun than it does 

 in low latitudes, and should be cooler on this account. It 

 may be, however, that a very appreciable amount of the sun's 

 heat is intercepted in the stratosphere by solid matter, and 

 that this amount becomes considerable when the sun's rays 

 pass obliquely through great thicknesses of it. Indeed, the 

 amount thus absorbed may more than compensate for the less 

 quantity received by radiation from the earth and air below. 

 Such solid matter could not be ice deposited by the slow rise 

 which must result in the polar area as a result of the incoming- 

 winds from lower latitudes, for the temperature does not fall. 

 Meteoric dust and ions may, however, intercept some heat. 



All the sun's rays falling on the polar regions would pass 

 through the atmosphere obliquely the year round; and it is 

 conceivable that the stratosphere in these regions is actually 

 more heated by the sun's rays and the rays from the seas 

 and lands below than is the case in equatorial regions, where 

 the sun's rays pass entirely, or nearly so, through the thin 

 atmospheric covering of the earth. 



