Even when the sky is clear, the intensity of the solar beam is significantly reduced during its passage through the atmosphere. This reduction in intensity is due partly to scattering by air molecules and dust particles and partly to absorption by water vapour, oxygen, ozone and carbon dioxide in the atmosphere. With the Sun vertically overhead, the total solar irradiance on a horizontal surface at sea level is reduced by about 14% with a dry, clean atmosphere and by about 40% with a moist, dusty atmosphere, compared to the value above the atmosphere.931 The proportion of the incident solar flux removed by the atmosphere increases as the solar elevation (the angle of the Sun's disc to the horizontal) decreases, in accordance with the increase in pathlength of the solar beam through the atmosphere. The atmospheric pathlength is approximately proportional to the cosecant of the solar elevation: it is, for example, twice as long with a solar elevation of 30 ° as with the Sun at the zenith.
Was this article helpful?