Solar radiation
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Solar irradiance spectrum at top of atmosphere. |
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]. 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 commonly measured with a
pyranometer or pyrheliometer.
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). The Earth receives a total amount of radiation determined by its
cross section (π R
2), but as the planet rotates this energy is distributed across the entire
surface area (4 π R
2). Hence, the average incoming solar radiation (known as "
insolation") is 1/4th the solar constant or ~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.
 |
Solar irradiance spectrum above atmosphere and at surface |
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). 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).
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Solar neutrino problem: solar neutrino measurement problem
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Solar variation: variations in solar activity
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Solar wind: particles flowing from the Sun
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Coronal mass ejection: large ejection of electrons and protons
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Polar aurora: usually electrons hitting Earth's atmosphere
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Solar flare: eruption creates increase of
solar wind particles
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Solar proton event: protons hitting Earth's atmosphere
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Pyranometer: solar radiation sensor or pyranometer
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A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems, Rivington et al.
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Evaluation of three model estimations of solar radiation at 24 UK stations, Rivington et al.
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High resolution spectrum of solar radiation from
Observatoire de Paris*
Measuring Solar Radiation : A lesson plan from the National Science Digital Library.
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Websurf astronomical information : 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.
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Daylength - Formulas to calculate the daylength depending from latitude and day of year.
