Cloud cover

The power of solar radiation entering the atmosphere, or the “solar constant,” is 1367 W/m2 (50,169 Btu/hr/sq ft). Within the atmosphere, this power is reduced by absorption, scattering, and reflection effects to about 1000 W/m2 (36,700 Btu/hr/sq ft) on the earth’s surface if there is a clear sky (Figure 2.3). The solar radiation that reaches the earth’s surface is further reduced by clouds, which reflect part of the radiation back into space, and absorb another part. In addition, a part of the radiation is dispersed into diffuse radiation by multiple reflections. Diffuse irradiation on the earth’s surface consists of the irradiation coming from angles different than the solar incidence angle (i.e., the actual sun position). Therefore, cloud cover reduces the total amount of irradiation (global radiation), and (due to scattering), it also changes the relation between beam and diffuse radiation. As cloud cover changes with the seasons, these effects are also seasonal dependent. The diffuse fraction of the total annual global radiation can be higher than 50%, depending on the location.

Even on a clear day a significant amount (>10%) of solar radiation is scattered due to the effects of molecules in the atmosphere. This causes the sky at daytime to appear blue and colors the sun red at sunset, when the distant light waves that enter the atmosphere are longer.