Theory of the Winds. 15 



become heavier than the column AB, and the lower portions 

 of CD and CD' flow beneath the lighter column of central 

 •air, in the directions shown by the lower arrows. If the 

 differences of temperature between the columns persist, 

 we have a central area of low pressure with higher pressures 

 on either side, and a continuous circulation of air goes on 

 from the warm area to the cold area above, and from the cold 

 to the warm below. 



If the earth's surface consisted entirely of either land or 

 water, and there were no mountain ranges, the application 

 of Halley's theory would be much simplified. The earth's sur- 

 face, however, although mainly occupied by water, has large 

 land areas, and these land areas are peculiarly distributed. 

 In the Northern Hemisphere we have the continents o£ 

 North America and Eurasia, and portions of South America 

 and Africa occupying a large part of the area. In the 

 Southern Hemisphere, however, the only large land masses are 

 the Antarctic continent, Australia, and the narrow southern 

 portions of Africa and South America. Here, consequently, 

 we have a large water area and comparatively small land 

 areas. It is mainly to this distribution of land and water 

 that we owe the different meteorological conditions of the 

 two hemispheres. 



Although a portion of the sun's heat is intercepted bv the 

 atmosphere, a good deal of it reaches the earth's surface. 

 Over land areas it quickly heats the ground, and the heat is 

 passed in an upward direction by convection currents ; but 

 as the air rises it is cooled by expansion, and we get a vertical 

 gradient of temperature. Dry air rising and expanding in 

 this way cools at the rate of 1° C. in temperature for each 

 rise of 100 metres; but saturated air at the usual temperature 

 of 10° C. only cools at the rate of o, 54 0. for the same dis- 

 placement, the latent heat of the water-vapour being given 

 up to the air on condensation. When, however, the sun's 

 rays strike the surface of the sea, they penetrate it and raise 

 its temperature but slightly. It thus comes about that 

 during the summer the lower portions of the atmosphere are 

 warmer over the continents than they are over the seas, and 

 during the winter they are cooler. We thus get gradients of 

 temperature between the land and the sea as well as tempe- 

 rature gradients from north to south, and this is one of the 

 reasons why the great masses of land in the Northern 

 Hemisphere produce a climate which differs so much from 

 that met with in the Southern Hemisphere. 



Comparatively little has been done to determine the 

 vertical temperature gradients of the atmosphere in the 



