Solar radiation

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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 size 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.

[edit] Solar constant

Main article: solar constant

The solar constant is the amount of incoming solar radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter,<ref>Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present. Retrieved on October 5, 2005.</ref> (although it fluctuates by a few parts per thousand from day to day). Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×10¹⁷ W. The solar constant is not quite constant over long time periods; see solar variation.

The solar constant is relatively constant, but varies according to sunspot activity. It affects mainly long-term climates, rather than short-term weather. The Earth receives a total amount of radiation determined by its cross section (π R²), but as the planet rotates this energy is distributed across the entire surface area (4 π R²). Hence, the average incoming solar radiation (known as "insolation") is one fourth 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.

[edit] Climate effect of solar radiation

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).

[edit] Notes

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[edit] See also

• Solar heating : Predicting solar heating effects.
• 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 Insert non-formatted text here

[edit] External links

• A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems, Rivington et al.
• Evaluation of three model estimations of solar radiation at 24 UK stations, Rivington et al.
• High resolution spectrum of solar radiation from Observatoire de Paris
• Measuring Solar Radiation : A lesson plan from the National Science Digital Library.
• 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.
• Daylength - Formulas to calculate the daylength depending from latitude and day of year.

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