28 
CLIMATE 
bring about a lowering and condensation a rise of temperature! Heat 
is lost during the night by radiation, and since there is greatest loss 
of heat in this way where the atmosphere is dry, clear, and rare, there 
are great extremes of heat by day and cold by night in the interior 
of continents, especially at high elevations. Low temperatures prevail 
at high altitudes, but it is to be remembered that these low tempera- 
tures are those of the air. There is no diminution, but the reverse, 
in the strength of the rays of the sun on any body directly exposed to 
them. 
59. 1t is important that students of economic geography should 
be clearin their minds as to the meaning of the diminution of mean 
temperature with altitude. This is not a phenomenon observable 
equally at all times of the day and year. A statement as to the rate of 
that diminution, usually given as equal to about 1° F. for every 300 feet 
of ascent, expresses the result of averaging differences of temperature 
in a vertical column of air or in adjacent vertical columns, at different 
times and in different situations, and in a great many cases it is of 
much more practical importance to observe that at certain times in 
certain situations the difference is the other way, the lower tempera- 
bures at the bottom, the higher on the upper slopes or even on mountain 
tops. This will be understood when it is borne in mind that various 
causes are at work affecting air temperatures. First it should be 
noted that the air is heated principally not by the direct rays of the 
sun, but indirectly through the warming of the surface of the earth, 
which then imparts its heat in various ways to the air above. 
Naturally, therefore, when the surface of the earth is warm, the air is 
all the warmer the nearer it is to the surface, and this difference is all 
the greater on account of what occurs in connection with one of the 
modes of conveying the heat from the ground to the higher strata of 
the air, namely, by means of convection currents. The air nearest 
the ground expands in consequence of its greater heat and so becomes 
relatively light and rises. But as it rises it becomes subjected to less 
pressure and expands still more, and this expansion is accompanied 
by an instantaneous lowering of temperature permeating the whole 
mass. As long as the air rises and there is no condensation of the water- 
vapour in it into cloud, rain, snow or any other form of water, this 
cooling goes on at the rate of 1° F. for every 180 feet of ascent, a 
figure which does not express an average but states a fact observed 
with every rise of air, whether by day or night, in summer or in winter. 
The rate of cooling, however, is checked when any condensation takes 
place. 
60. But if the heating of the air above takes place from the ground 
1 The conversion of water into vapour, like the conversion of ice or any other 
solid into the liquid state, involves the expenditure of heat. That is, heat (in the 
scientific sense of the term) is used in the conversion, and is not available for 
raising or maintaining temperature. Meanwhile, of course, temperature may be 
maintained. and even raised. bv external supplies of heat (as from the sun, or a fire).