Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 80

The first, and very vital, screening of solar energy takes place in the stratosphere,
where ozone absorbs most of the ultraviolet radiation from the sun. Greenhouse gases
absorb infrared solar radiation, but visible light passes through to the surface except
when there is cloud cover. The visible light is scattered as it strikes molecules of air,
water, and dust. Scattering is a selective process, principally affecting the wavelengths
of blue light. The sky looks much bluer or darker in high mountains than at lower el-
evations, where more water and pollutants scatter light of other wavelengths, diluting
the blue color. Clouds, of course, are the single most important factor controlling the re-
ceipt of solar energy at any given latitude and in mountains (Germino and Smith 2000).
Since mountains stand above the lower reaches of the atmosphere, the solar radiation
is much more intense since it has passed through less atmosphere (Fig. 3.6).
The solar constant is defined as the average amount of total radiation energy re-
ceived from the sun at the top of the atmosphere on a surface perpendicular to the sun's
rays (Fig. 3.5). This is approximately 1,365 Wrrr 2 (2 calories per square centimeter per
min). At midday under clear skies, the total energy flux from the sun in high moun-
tains may approach the solar constant, but several field investigations have recorded
readings even slightly above the solar constant (Gates and Janke 1966; Terjung et al.
1969). Astonishingly high values of irradiance (up to 1,832 Wrrr 2 , 134 percent of the
solar constant) incident upon a horizontal surface have been observed in the southern
Ecuadorian Andes (4°S, 1,500 and 3,400 m). Cloud radiative effects were identified as
the exclusive source of the “superirradiance” (Emck and Richter 2008).