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Incoming shortwave radiation

In this section the incoming shortwave radiation, , is considered, which is the radiation received directly or indirectly from the sun by an horizontal plane at the earth surface per unit time and per unit area, integrated over all wavelengths in the shortwave interval. In good approximation the sun can be considered a black body with a surface temperature of about 5700 K. In some textbooks is denoted also as solar radiation or global radiation.
It consists of two components, notably the direct (S) and the diffuse radiation (D). The direct component is the contribution of the direct solar beam. The diffuse component is solar light that reaches the earth surface after being scattered firstly by clouds, particles or air molecules in the clear sky. The direct component depends on the solar zenith angle (, = 900 - the solar elevation angle). can be written as:

(2)

I is the solar radiation at the surface through a surface perpendicular to the solar beam.

It will be obvious that the turbidity ('clearness') of the atmosphere and clouds will effect I and D. The ratio D/S depends on solar zenith angle (), i.e. at small solar angles the path length through the atmosphere is long and D/S is about 1 (cloudless conditions). At large solar angles and clear clean sky conditions D/S is typically 1/5.

In meteorological models various radiation transfer models are used. It is outside the scope of this module to dwell on the complicated models. In order to give the reader an impression of the magnitude for different solar zenith angles and cloud cover conditions, in this module a simple parameterisation is considered that applies to mean mid-latitudes conditions. It does not describe the dependency of on turbidity and is inaccurate for low solar angles.

(3)

in which I0 is the solar constant (1375 W m-2), C is the cloud cover (C =0 means no clouds; C=1 overcast), r the distance Earth-Sun and r0 the mean of r.