Sun unit
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The sun unit (abbreviated SU) is a measure of sun intensity weighted for skin damage related to the UV index.
[edit] Basics of solar radiation
Solar radiation sustains life on earth and determines climate. The energy flow within the sun results in a surface temperature of around 5800 K, so the spectrum of the radiation from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in the cool peripheral solar gas (Fraunhofer lines).
[edit] Solar constant and "sun value"
The irradiance of the sun on the outer atmosphere when the sun and earth are spaced at 1 AU—the mean earth/sun distance of 149,597,870 km—is called the solar constant. Currently accepted values are about 1360 W/m² (the NASA value given in ASTM E 490-73a is 1353 ±21 W/m²). The World Metrological Organization (WMO) promotes a value of 1367 W/m².
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