Solar radiation
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Description
Solar radiation is electromagnetic energy, that may be exploited.
Detailed description
The above curve, adapted from a NASA document[1], shows the solar spectral irradiance from outside the earth's atmosphere (orange) and at the earth's surface (red). The shaded areas signify spectral absorption of the energy by specific gases in the passage through the atmosphere. It will be noted that the radiation is most intense in the visible spectrum in the wavelength octave from 0.4 µm (violet) to 0.8 µm (red). Below 0.4 µm, the cut-off is fairly rapid, partially because the black-body spectrum attenuates at 6000 K but mainly because of the presence of ozone (O3) in the ozone layer. Above 0.8 µm, in the infra-red, less than 40 percent of the total energy is integrated under the red curve.
The so-called solar constant is the total electromagnetic radiation outside the earth's atmosphere and is the integration of the orange curve. It averages about 1.37 kW/m² ±0.12 kW/m², normal to the direction of radiation, these variations being due to a number of factors, such as the distance from the sun, weather on the sun, the sunspot cycle etc. The radiation received on the earth's surface is even more variable, even from a cloudless sky, because of latitude, dust, water vapour, altitude etc. For convenience, it is common to assume 1.0 kW/m² although the true average is probably slightly higher.
