Mountain Sun

MOUNTAIN SUN, that is, solar radiation at a great height above sea level, has a number of features that distinguish it from solar radiation in valleys and plains. This difference is especially felt in relation to the content of ultraviolet rays. As ultraviolet rays pass through a heterogeneous and differently composed earth's atmosphere at different heights, their intensity is significantly weakened due to scattering and absorption. The latter in the upper layers of the atmosphere occurs due to ozone, which greedily absorbs ultraviolet rays, as a result of which short ultraviolet rays do not reach the earth's surface. The shortest ultraviolet rays found on the earth's surface have a length of 289.6 mμ. In the lower layers of the atmosphere, intense absorption of ultraviolet rays (see Heliotherapy) occurs due to a decrease in the transparency of the air (dust, smoke, water vapor). Ultimately, in solar radiation on high mountains, ultraviolet rays are found in a significantly larger quantity than in the radiation reaching sea level, and there are rays with a shorter wavelength. If we take the incident radiation as a unit, then, depending on the height above sea level and the angle of incidence of the rays, the following fraction of radiation reaches the places listed below: Height above sea level 90° 60° 30° 20° 10° Potsdam, 100 m 0.483 0.432 0.234 0.121 0.017 above sea level 324 m 0.263 0.214 0.070 0.021 0.001 Teide Peak, 324 m 0.636 0.593 0.405 0.269 0.076 3,280 m above sea level 324 m 0.446 0.394 0.200 0.096 0.011 On high mountain peaks (Monte Rosa, 4,368 m), the shortest waves in the solar spectrum are equal to 290.24 mμ. In the mountains at an altitude of 2,000 m, ultraviolet rays with a wavelength up to 295 mμ are detected. In solar radiation reaching the plains, ultraviolet rays are detected only with a wavelength from 310 mμ and greater. Thus, the presence in mountain sun of ultraviolet rays with a large wavelength and rays with a wavelength of 290–310 mμ, that is, rays possessing a strong action on a number of biological processes (development of inflammatory phenomena in the skin—erythema, pigmentation, anti-rachitic action, to a certain extent bactericidal action, etc.), in a significant degree characterizes mountain sun as opposed to plain sun. In relation to the indicated processes, rays with a wavelength from 320 mμ and above exert only a very weak action, and only with prolonged exposure. The greater content of ultraviolet rays in solar radiation in the mountains is also attributed to the more energetic therapeutic action of sun treatment conducted there. From this arose the idea of creating an artificial source of light with a spectrum rich in ultraviolet rays. Such sources appeared under the name of "artificial mountain sun" (see Bahr mercury-quartz lamp). The spectrum of the latter, however, differs from the solar spectrum by a significantly greater content of ultraviolet rays, and moreover rays with a shorter wavelength than those encountered in solar radiation. Along with this, the spectrum of radiation of "artificial mountain sun" lacks infrared, red, and orange rays. Mountain sun natural, see Heliotherapy. - Mountain sun artificial, see Bahr mercury-quartz lamp.

g. Varshaver.