The Linke turbidity factor (TL, for an air mass equal to 2) is a very convenient approximation to model the atmospheric absorption and scattering of the solar radiation under clear skies. It describes the optical thickness of the atmosphere due to both the absorption by the water vapor and the absorption and scattering by the aerosol particles relative to a dry and clean atmosphere. It summarizes the turbidity of the atmosphere, and hence the attenuation of the direct beam solar radiation (WMO, 1981; Kasten, 1996). The larger TL, the larger the attenuation of the radiation by the clear sky atmosphere.

The TL denotes the transparency of the cloudless atmosphere. If the sky were dry and clean, TL would be equal to 1. When the sky is deep blue, the TL is small. In summer, in Europe, the water vapour is often large and the blue sky is close to white. The TL is larger than 3. In turbid atmosphere, e.g. in polluted cities, the TL is close to 6 - 7.

A typical value of TL for Europe is 3. However, this value exhibits strong fluctuations in space and time. Maps of the TL were created to provide TL values for any site in the world on a climatological basis. As a consequence, the former service Helioserve has been replaced by the SoDa Web service (see note). The worldwide database is also available as files (see note).

References

Kasten F., 1996. The Linke turbidity factor based on improved values of the integral Rayleigh optical thickness. Solar Energy, 56 (3), 239-244.

World Meteorological Organization (WMO), 1981. Technical Note N° 172, WMO-No. 554, Geneva, Switzerland, pp. 121-123.