Solar radiation

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 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 turd.

Solar constant

A 1903 Langley bolograph with an erroneous solar constant of 2.54 calories.

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, (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.

The solar constant includes all types of solar radiation, not just the visible light. (See electromagnetic spectrum for more details) It is linked to the apparent magnitude of the Sun, −26.8, in that the solar constant and the magnitude of the sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun.

In 1884 Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California, and (by taking readings at different times of day) attempted to remove atmospheric absorption effects. However he obtained the incorrect value of 2903 W/m², perhaps due to mathematical errors. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m². Abbott proved that one of Langley's corrections was erroneously applied, and his results varied between 1.89 and 2.22 calories, and the variation appeared to be solar, not terrestrial.

The angular diameter of Earth seen from the sun is ca. 1/11,000 radian, so the solid angle of Earth seen from the sun is ca. 1/140,000,000 steradian. Thus, the sun emits about 2 billion times the amount of radiation that is caught by Earth, or about 3.86×10²⁶ watts.

Climate effect of solar radiation

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).

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 radiation sensor

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 planet, 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.
• An Excel workbook with a solar position and solar radiation time-series calculator; by Greg Pelletier

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