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 [1] (http://www.grida.no/climate/ipcc_tar/wg1/041.htm#121). 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.

Solar radiation is commonly measured with a pyranometer or pyrheliometer.

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 4 times the surface area of its cross-section, the temporally and spatially averaged insolation over the Earth's surface above the atmosphere is a quarter of this value, 342 W/m². At any given location and time, the amount received 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 see a graph (http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html)). For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles).

See also

• Solar neutrino problem: solar neutrino measurement problem
• Solar variation: variations in solar activity
• Solar wind: particles flowing from the Sun
  • Coronal mass ejection: large ejection of electrons and protons
  • Polar aurora: usually electrons hitting Earth's atmosphere
  • Solar flare: eruption creates increase of solar wind particles
  • Solar proton event: protons hitting Earth's atmosphere
  • Pyranometer: solar radiation sensor or pyranometer

External links

• Measuring Solar Radiation (http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Solar_Radiation) : A lesson plan from the National Science Digital Library.

• Websurf astronomical information  (http://websurf.nao.rl.ac.uk/surfbin/first.cgi): Online tools for calculating Rising and setting times of Sun, Moon or plane, Azimuth of Sun, Moon or planet at rising and setting, Altitude and azimuth of Sun, Moon or planet for a given date or range of dates, and more.da:Solindstråling

de:Solarstrahlung es:Radiación solar it:Radiazione solare pl:Promieniowanie słoneczne