Page:Physical Geography of the Sea and its Meteorology.djvu/125

 Rh 2½ per cent.; that is, a column of atmosphere 100 feet high will, after its temperature has been raised 12°, be 102½ feet high. However, only about one-third of the direct heat of the sun is absorbed in its passage down through the atmosphere. The other two-thirds are employed in lifting vapour up from the sea, or in warming the crust of the earth, thence to be radiated off again, or to raise the temperature of sea and air by conduction. The air at the surface of the earth receives most heat directly from the sun; as you ascend, it receives less and less, and the consequent temperature becomes more and more uniform; so that the height within the tropics to which the direct rays of the sun ascend is not, as reason suggests, and as the snow-lines of Chimborazo and other mountains show, very great or very variable.

256. Hurricanes not due to direct heat of the sun.—Moreover, daily observations show most conclusively that the strong winds and the great winds, the hurricanes and tornadoes, do not arise from the direct heat of the sun, for they do not come in the hottest weather or in the clearest skies. On the contrary, winter is the stormy period in the extra-tropical regions of the north; and in the south, rains and gales—not gales and sunshine —accompany each other. The land and sea breezes express more than double the amount of wind force which the direct heat of the sun is capable of exerting upon the tradewinds. I say more than double, because in the land and sea breezes the wind-producing power acts alternately on the land and on the sea—in opposite scales of the balance; whereas in the trade-winds it acts all the time in one scale—in the sea scale; and the thermal impression which the solar ray makes through the land upon the air is much greater than that which it makes by playing upon the water.

257. The influence of other agents required.—From these facts it is made obvious that other agents besides the direct and reflected heat of the sun are concerned in producing the trade-winds. Let us inquire into the nature of these agents.

258. Where found.—They are to be found in the unequal distribution of land and sea, and rains, as between the two hemispheres. They derive their power from heat, it is true, but it is chiefly from the latent heat of vapour which is set free during the processes of precipitation. The vapour itself, as it rises from