Solar elevation

Rainbows may be seen during showers when the sun is behind the observer the direction of the sunlight determines the forward direction (or line of sight). The angular positions of the primary and secondary rainbows relative to the observer s line of sight are 180° — 137.9° = 42.1° and 180° — 129.1° = 50.9°, respectively. The fraction of the total rainbow that can be seen depends on the solar elevation. When the sun is greater than about 51° above the horizon, no rainbow can be seen even though conditions are otherwise favorable. On the other hand, the complete rainbow—one that forms a complete circle—may be seen from an airplane. 

Blumthaler, M., Ambach, W., and Huber, M. (1993) Altitude effect of solar UV radiation dependent on albedo, turbidity, and solar elevation, Meteorol. Z, N.F. 2,116-120. 

Solar Radiation. Of all the factors which collectively determine the amount and spectral distribution of the radiation entering a surface layer of the atmosphere, the best established appear to be the spectral irradiance outside the atmosphere and the attenuation by molecular scattering. The absorption coefficients of ozone are well established, but no easy method exists for determining the amount of ozone in a vertical profile of the atmosphere at a given time. The measurement of the particulate content of the atmosphere and its correlation with atmospheric transmission is a field in which much remains to be accomplished. Surprisingly few data exist on the spectral distribution of sky radiation and its variation with solar elevation and atmospheric conditions. The effect of clouds is of secondary importance, as intense smog generally occurs under a clear sky. 

Solar elevation The dependence of solar UVR on solar elevation is shown in... 

Figure 7. Dependence of global erythemal irradiance (solid curve) on solar elevation for a clear sky day (18 July, 1995) at Izana Observatory (Canary Islands, Spain, 28.3°N, 16.5°W, 2367 m above sea level) with total ozone 282 DU and aerosol optical depth 0.06 at 350 nm. Spectral irradiances at 302 nm and 320 nm (dashed curves) on the same day are given in relative units, normalized to the maximum of erythemal irradiance.

From Figure 7 the seasonal variation of UVR at noon can also be estimated, remembering that noon solar elevation increases from winter solstice to summer solstice by about 47°. Annual maximum noon solar elevation (se(max)) can be calculated for any latitude higher than 23.5° as (se(max))= H3.5° —latitude, while for latitudes lower than 23.5° the maximum noon solar elevation is of course 90°. As already mentioned, the absolute values of erythemal irradiance shown in Figure 7 correspond to a situation with extremely high irradiance however, relative variations of irradiance with solar elevation can be estimated from these measurements. 

The change in solar elevation can also be interpreted in terms of changing latitude. At 20° latitude northwards of the station Izana, the noon intensity would be (under all other identical conditions) 100 mW m less, which is about 30% smaller. Moving another 20° northwards would reduce erythemal irradiance again by about 125 mW which means that irradiance at 68°N is about 50% less than at 48°N. 

For more than a century, the presence of thin clouds has been reported at high altitudes in the polar regions. Because of their colorful appearances, especially at low solar elevation, these clouds have been named mother-of-pearl or nacreous clouds, or more recently in the context of polar ozone research -polar stratospheric clouds (PSCs). 

Woolf, H. M. 1968, On the Computation of Solar Elevation Angles and the Determination of Sunrise and Sunset Times. In manuscript. Goddard Space Flight Center, 1968. 

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