152
PHYSICS OF THE AIR
attain the velocity of a gale and become a veritable aerial torrent. This drainage flow is known indifferently as the mountain breeze, or mountain wind; also canyon wind, katabatic wind, and gravity wind.
For simplicity let there be no general wind; let the cross-valley profile be the arc of a circle, and let the covering of .the walls be everywhere the same. But even thus simplified the problem of air drainage still requires the consideration of temperature changes of the free air, of the surface air, and of the surface itself.
METE 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0 RS
V NB
\A \
V N W
\ \ /°
V ^x /
\N ^\ fr
$ \ \ \
/ \ \
/ \ c\ w
/ \ \ \
/ D \x \
fc \ \*
\ D ^
0 23456789 10 II 12 DEGREES CENTIGRADE
FIG. 50.—Assumed temperature gradients.
Free Air.—Largely, perhaps almost wholly, because of the dust and vapor always present, the lower atmosphere emits and absorbs radiation abundantly through much the greater portion of the spectrum. But during clear nights the loss, in the lower air at least, usually, if not always, is greater than the gain. However, even on such nights when radiation losses are greatest, the lower air, neglecting surface influences, cools too slowly and, on any given level, too nearly uniformly to produce more than very scattering and very feeble convection currents.
Suppose, though, that a limited mass of free air is cooled to a temperature below that of the adjacent atmosphere at the same level, as must happen at night within a small isolated cloud. What will be the result?
This problem, interesting within itself and essential to the present discussion, can easily be solved graphically. To this end let AB and A'B'