Total solar irradiance

Solar irradiance reaches the earth surface directly or after diffusing on particulates which are in the atmosphere. The sum of direct solar radiation and diffuse sky radiation is called total solar irradiance and its concentration (energy per unit time) is expressed in W/m2 and measured by pyranometer. Its value depends mainly on the height of the sun over the horizon and cloudiness. When the sun is in the zenith, the optical path length of sun rays is the shortest – then the air layer, through which rays go, is the smallest, and it limits also radiation diffusing. When it is cloudy, a part of radiation reaches the Earth after diffusion on clouds, but a large portion deflects from them and goes into the outer space.

Insolation is an important issue, especially as far as biometeorology is concerned. This is the time, in which direct solar radiation reaches the Earth surface during the day. The quantity is dependent on the length of the day, and therefore on the season (potential insolation) and cloudiness (actual insolation). They are measured by heliograph.

Elaborated by K. Wałaszek