Sun unit

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Not to be confused with the traditional Japanese unit of length, see cun (length)

The sun unit (abbreviated SU) is a measure of sun intensity weighted for skin damage related to the UV index.

Basics of solar radiation[edit]

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).

Solar constant and "sun value"[edit]

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.