The Linke turbidity factor is an important parameter in solar radiation studies. Longterm monthly average values, TL m , are sufficient for most applications. Estimates of TL m may be obtained by way of a linear regression established for several European sites between the sum of the Angström regression coefficients on a monthly basis (a m + b m ) and the measured TL m . During the European Solar Radiation Atlas (ESRA) project, values of a m and b m have been obtained for 710 sites in Europe

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... countries surrounding the Mediterranean sea, from the analysis of measurements made during the period 19811990. Using the abovementioned formula, these values are turned into climatological values of TL for each month of the year. This data set of TL is unique in the world and attention has been paid to the means of disseminating the resulting information. A World Wide Web site "Helioserve" has been established, which offers an access to the database through a standard HTML interface. The customers of the Helioserve site are professionals, in research or industry. The database outputs are used as inputs into their own models for atmospheric optics. The site Helioserve comprises also a numerical simulation aspect, which provides to the customer, estimates of the clear sky radiation by running the clearsky model of the ESRA. The TL values requested by the model can be supplied by either the database using an internal query, or the customer himself, or can be set to default values.