Page:Physical Geography of the Sea and its Meteorology.djvu/488

462 mount up towards the north in consequence of the heavy winter precipitation upon the western slopes of these mountains. The heat which is required to convert the water of the Columbia and other rivers into vapour is set free on the mountain range, and the upper Missouri is by this heat kept open for navigation long after the lower and more southern portion of it is frozen up.

868. The influence of aqueous vapour upon winds and climates.—The average evaporation of water from sea and land is estimated to be from one third to one half as much daily as is contained in the great chain of American lakes. The average precipitation equals the evaporation. The heat that is absorbed and evolved in the process of lifting up and letting down such a body of water has a powerful influence upon climates as well as upon winds; it is the chief source of that motive power which gives to the winds their force, to the storm its violence. Six hundred and twenty pounds of aqueous vapour occupy in the open air the space which it takes one thousand pounds of dry air at the same temperature to fill. Now to appreciate the wind begetting power of this vapour, and its heat, let us imagine the air over an area of considerable extent to be saturated with vapour from the sea, and that from some cause, as in a thunder-storm, this vapour is suddenly, or even rapidly, condensed:—The aerial rarefaction over such an area, and consequently the wind-begetting power within it, would be immense, merely on account of the condensation of this vapour; but if we take into the account the rarefying effect of the heat that is set free during the process of condensation and precipitation, we may cease to marvel either at the force of the wind, or the violence of the rain which marks the hurricane; nor need we wonder at the low range of the barometer or the mildness of temperature in all rainy latitudes.

869. How the temperature of air may he raised by crossing mountains,—In the preceding chapter the circumstances have been considered which favour the idea that most of the unknown surface of the antarctic circle is not only land, but that its coasts are probably highlands; that in its topographical features it presents all the conditions that are required for the rapid condensation of the vapour with which the impinging air from the sea is loaded, and that in the valleys beyond mild climates may be expected. The aqueous vapour which the air carries along is