The Azimuth Project
Solar radiation (Rev #2)

Solar output

The total solar output to space is 3.84 ×\times 1024 watts, but only a tiny fraction hits the Earth. At the top of the atmosphere, energy is received with a flux, or power density of 1366±\pm2 W/m2, a value known as the solar constant. About 7% is ultraviolet (wavelength 0.2-0.4μ\mum), 41% visible light (0.4-0.7μ\mum) and 51% near-infra-red (>\gt0.7μ\mum).

Because the radiation hits the Earth at an angle, and not at all at night, the average global power density is 342W/m2 at the top of the atmosphere. (This is one quarter of 1366 W/m2, since the area of a sphere is four times the area of its circular shadow.)

Surface receipt of solar radiation

About 18% of the incoming energy is absorbed directly by ozone and water vapour. This almost entirely removes wavelengths shorter than 0.285μ\mum while those longer than 0.295μ\mum reach the ground. About 30% of incoming solar radiation is reflected directly back into space by the atmosphere, clouds, and the earth’s surface. The remaining 70% heats the surface (approximately 50% goes there) and atmosphere (approximately 20% goes there).

The earth’s surface receives 156W/m2 from the sun (as a global average) and emits 55W/m2 long-wave energy to the atmosphere. The atmosphere receives 84W/m2 and emits 185W/m2 to space. (The figures here are from Barry and Chorley, 2003. The account in Kiehl and Trenberth’s paper is more complicated.)


  • R. G. Barry and R. J. Chorley, Atmosphere, Weather and Climate, Routledge, London, 2003.

  • J. T. Kiehl and Kevin E. Trenberth, Earth’s annual global mean energy budget, Bull. Amer. Meteor. Soc. 78 (1997), 197–208.