# The Azimuth Project Solar radiation (Rev #4)

## 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$\pm$2 W/m2, a value known as the solar constant. About 7% is ultraviolet (wavelength 0.2-0.4 $\mu$m), 41% visible light (0.4-0.7 $\mu$m) and 51% near-infra-red ($\gt$0.7$\mu$m).

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 $\mu$m while those longer than 0.295 $\mu$m 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 156 W/m2 from the sun (as a global average) and emits 55 W/m2 long-wave energy to the atmosphere. The atmosphere receives 84 W/m2 and emits 185 W/m2 to space. (The figures here are from Barry and Chorley, 2003. The account in Kiehl and Trenberth’s paper is more complicated.)

## References

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.