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).