A SPECTRAL MODEL OF LINKE TURBIDITY FACTOR AND ITS EXPERIMENTAL IMPLICATIONS

A model of Linke's turbidity factor, T(L), is developed by means of updated spectral extraterrestrial irradiances and extinction coefficients of gaseous absorbers. It is shown that the new values of T(L) are clearly different from those obtained by Kasten's formula which parameterizes the optical thickness of the clean dry atmosphere. The model is used to investigate the dependence of T(L) on the relative optical air mass and to elucidate the relationships linking T(L) to Angstrom's turbidity coefficient and to the water vapor content. For any T(L), the corresponding value related to the air mass 2.0 can be determined. Such a standardized value is independent of solar elevation and is therefore strictly representative of the atmospheric turbidity. It can be linked to Angstrom's turbidity coefficient. Practical procedures and algorithms for computing the standard Linke's turbidity factor and determining Angstrom's turbidity coefficient are described. A relationship for converting the T(L) values obtained by Kasten's formula into the new values is proposed.