Solar radiation
Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum . The portion of this ultraviolet radiation that is not absorbed by the atmosphere produces a suntan or a sunburn on people who have been in sunlight for extended periods of time.
Solar radiation is thermal radiation emitted from the surface of the sun, which is powered by nuclear fusion.
Climate effect of solar radiation
The average energy density of solar radiation just above the Earth's atmosphere, in a plane perpendicular to the rays, is about 1367 W/m², a value called the solar constant (although it fluctuates by a few parts per thousand from day to day). Because the surface area of a sphere is 1/4 of the surface area of its cross-section, the time and spatially averaged insolation over the earths surface above the atmosphere is a quarter of this value, 342 W/m2. At any given location and time the amount received perpendicular at the surface depends on the state of the atmosphere and the latitude.
On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright yellow light (sunlight in the strict sense) and heat. The heat on the body, on objects, etc., that is directly produced by the radiation should be distinguished from the increase in air temperature.
The amount of radiation intercepted by a planetary body varies as the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time, sometimes achieving a nearly perfect circle, and at other times stretching out to an eccentricity of 5%. The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies (for example ). Changes associated with the redistribution of solar energy are considered a likely cause for the recent ice ages (see: Milankovitch cycles).
See also
