# The Azimuth Project Solar radiation (Rev #1)

### 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’s at an angle, and not at all at night, the average global power density is 342W/m2 at the top of the atmosphere.

### 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, leaving 70% to heat the surface (approx 50%) and atmosphere (approx 20%).

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 in the last para are from Barry and Chorley, 2003. The Kiehl and Trenberth account is more complicated.

### References

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

J. T. Kiehl and Kevin E. Trenberth, Earth’s Annual Global Mean Energy Budget, 1997, Bull. Amer. Meteor. Soc., 78, 197-208.