Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 79

at lower elevations through a powerful magnifying glass. The alpine environment has
perhaps the most extreme and variable radiation climate on Earth (Emck and Richter
2008). The thin, clean air allows very high solar intensities, and the topographically
complex landscape provides surfaces with a range of different exposures, shadowing,
and reflection from nearby peaks. Although the air next to the ground may heat up very
rapidly during daylight, it may cool just as rapidly if the sun's rays are blocked. Thus, in
the sun's daily and seasonal march through the sky, mountains experience a continually
changing pattern of sunshine and shadow, influencing the energy flux in the ecosystem
(Germino and Smith 2000). The factors to consider are the amount of sunlight received,
the quality or kinds of radiation, and the effect of slopes upon this energy.
AMOUNT OF SOLAR RADIATION
The most striking aspect of the vertical distribution of solar radiation in the atmosphere
is the rapid depletion of short-wavelength energy at lower elevations. This attenuation
results from the increased density of the atmosphere at lower altitudes (Tables 3.1, 3.2).
The atmosphere acts as a filter, reducing the intensity of some wavelengths and screen-
ing out others altogether. Consequently, the amount of energy reaching the surface at
sea level is only about half that at the top of the atmosphere (Fig. 3.5). High mountains
protrude through the lower atmospheric blanket and thus have the potential to receive
much higher levels of solar radiation, as well as cosmic-ray and ultraviolet radiation
(Emck and Richter 2008).
FIGURE 3.5 Spectral distribution of direct solar radiation at the top of the atmosphere and at sea
level. Calculations are for clear skies with the sun directly overhead. Also shown is the spec-
tral distribution of cloud light and sky light. The graph is plotted on a wave number scale in
cm” 1 —that is, the reciprocal of the wavelength, directly proportional to the frequency of light—to
allow display of the full spectrum (a wavelength plot has difficulty including the visible and in-
frared together). The total area under the upper curve is the solar constant, 2.0 cal. cm” 2 min” 1
(1,365 W/m 2 ). (After Gates andjanke 1966: 42.)