Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 81

FIGURE 3.6 Spectral transmissivity of the atmosphere at 4,200 m (14,000 ft) and at sea level for lat-
itude 40°N at summer and winter solstice. The attenuation shown here is for clear skies and is
due entirely to ozone absorption. When the effects of dust, water vapor, and other impurities are
included, the difference in transmissions between high and low elevations becomes considerably
greater. (After Gates and Janke 1966: 45.)
QUALITY OF SOLAR RADIATION
The alpine environment receives considerably more ultraviolet radiation (UV) than low
elevations. If only wavelengths shorter than 320 um are considered, alpine areas re-
ceive 50 percent more UV during the summer solstice than areas at sea level. During
the winter season, when the sun is lower in the sky (and therefore passes through dens-
er atmosphere), alpine areas receive 120 percent more UV than sea level (Gates and
Janke 1966). Many local influences, especially solar elevation, cloud coverage and type,
albedo, and aerosol properties strongly affect the amount of UV received (Pfeifer et al.
2006). Clouds are a major source of variation in solar radiation and UV received, and
can even increase UV levels compared to a cloud-free day (Parisi and Downs 2004). The
relatively greater quantity of UV at high elevations has special significance for human
comfort and biological processes. A proverb in the Andes says, “Solo los gringos y los
burros caminan en el sol” (“Only foreigners and donkeys walk in the sunshine”), indic-
ating the respect that local people give to the strength of the sun at high altitudes (Pro-