The Azimuth Project
Solar radiation

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 0.2-0.4 μ\mum), 41% visible light (0.4-0.7 μ\mum) and 51% near-infrared (>\gt0.7μ\mum).

Because the radiation hits the Earth at an angle, and not at all at night, the average global power density is 342 W/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 0.285 μ\mum while those longer than 0.295 μ\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 absorbs 156 W/m2 from the sun (as a global average) and emits 55 W/m2 long-wave energy to the atmosphere. The atmosphere absorbs 84 W/m2 and emits 185 W/m2 as longwave radiation to space.

The above figures are from Barry and Chorley, 2003. The account in Kiehl and Trenberth’s 1997 paper is more complicated, but it can be partially summarized by this picture:

In particular, note that they list:

  • 342 W/m2 (341 W/m2) average power hitting the Earth’s atmosphere.

  • 198 W/m2 (184 W/m2) average power hitting the Earth’s surface.

  • 168 W/m2 (161 W/m2) average power being absorbed by the Earth’s surface.

Bracketed figures are from the Trenberth et al 2009 paper.


See also Insolation, which concentrates on calculating the daily average of power in the form of solar radiation hitting the top of the Earth’s atmosphere, as a function of latitude and time of year.

Also see Solar power for information on the amount of solar radiation available for solar power in various locations.

category: energy