Solar Constant

  • The average value of incoming solar radiation (INSOLATION) received at the thermopause ie. 480km above the earth’s surface, when the earth is at average distance from the sun is called solar constant.
  • The average value of solar constant is estimated to be 1.968 calories per cm2 per minute.
  • The energy emitted by the sun is received by the earth in the form of electromagnetic radiations.
  • The quantity of radiations is about 1.968 calories/cm2/ minute.
  • A calorie is that amount of energy which is required to raise the temperature of one gram of water by one degree Celsius.

Several processes deplete the solar radiation as it passes through the earth’s atmosphere like


  • The radiations are reflected back in the space by the surface and atmosphere of earth.
  • The total reflection of the incoming solar radiation is called albedo and is expressed in terms of percentage of insolation.
  • Clouds are the most important reflectors by far.
  • Their reflectivity ranges from 40 to 90% depending upon the thickness and type of cloud.


Some of the energy is absorbed and raises the temperature of the surface.


  • It is the process by which small particles, with size comparable to the wavelength of the radiations deflects the radiations in different direction.
  • The direction of radiation changes as it keeps on scattered by the particles.


Some radiation passes through the atmosphere without reflection, refraction, absorption, or scattering. This is called transmission.

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