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Interesting Solar Radiation Facts


 Solar energy is the radiant energy emitted by the Sun in the form of electromagnetic waves.The sun emits vast amount of solar energy. The earth intercepts only a fraction of it.It is essential to drive directly or indirectly all biological and physical processes on the Earth.The earth is the only planet in the solar system, which receives an optimum amount of solar energy that makes life sustainable on it.

  • Solar spectrum resembles to that of a black body at approximately 6000 K. 98% of the total emitted solar energy lies in the spectrum ranges from 250nm to 3000 nm. 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.Solar radiation having wavelength less than 0.286 nm (called ultraviolet) is absorbed by ozone layer in stratosphere. The UV radiation not absorbed by the atmosphere is responsible for the change of color in skin pigments.
  • The solar radiation, that traverse the atmosphere further, is subjected to scattering, reflection and absorption by air molecules, aerosols and clouds.The radiation budget represents the balance between incoming solar energy from the Sun and outgoing thermal (long wave) and reflected (shortwave)solar energy from the Earth. Globally, the budget is balanced. Otherwise the temperature would rise constantly.
  • Locally, the budget is not balanced. Tropical areas get more than they release, while higher latitudes of the winter hemisphere release more than they receive.
    The radiation from the Sun travels in the space as EM waves. Above the earth’s atmosphere, sunlight carries 1367 watts of power per square meter. This is known as solar constant

Solar Constant is the amount of solar radiation received outside the earth’s atmosphere on a unit area perpendicular to the rays of the sun, at the mean distance of the earth from the sun.The Earth receives 1.8 x 10 W of incoming solar radiation continuously at top of its atmosphere. But only half of it reaches the earths’ surface.

  • Factors like absorption, scattering and reflection of light during its passage through the atmosphere are responsible for reduction of the amount of solar radiation available on the earth’s surface. Solar energy is, by nature, a low density energy source.

Solar Radiation Budget

The earth is surrounded by an atmosphere containing various gases,dust and other suspended particles, water vapour and clouds of various types.The solar radiation during its passage in the atmosphere gets partly absorbed, scattered and reflected in different wavelength bands selectively.Radiation gets absorbed in water vapour, Ozone, CO2 ,O2 in certain wavelengths.Radiation gets scattered by molecules of different gases and small dust particles known as well as by larger particles.There is a reflection of radiation due to clouds,particles of larger size and other material in the atmosphere.Considerable amount of solar radiation also gets absorbed by clouds which are of several types.

Basic terminology for solar radiation

  • Irradiance( W/m2): Amount of radiant energy incident on a surface per unit area per unit time
  • Direct Solar Irradiance: Solar irradiance on a surface held perpendicular to sun rays.
  • Global solar irradiance: Solar irradiance on a horizontal surface due to both direct sun rays and diffuse sky radiation.
  • Diffuse solar irradiance: Solar irradiance on a horizontal surface due to sky radiation only.
  • Reflected solar irradiance: Upward radiant exitance in the short wave range.
  • Net terrestrial irradiance: Upward radiant exitance minus downward irradiance in long wave range through a horizontal surface
    near earth surface.
  • Net total irradiance: Downward irradiance minus Upward radiant exitance in entire spectrum.
  • Insolation: it is a measure of solar radiation energy received on a given surface area and recorded during a given time. It is also called solar irradiation and expressed as “hourly irradiation” if recorded during an hour or “daily irradiation” if recorded during a day. The unit recommended by the World Meteorological Organization is mega Joules per square metre (MJ/m2)


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