A meteorological preprocessor is described. It computes key physical parameters on an hourly operational basis for use in dispersion models. Input to the preprocessor is hourly surface measurements and twice-daily vertical profiles from, for example, radio soundings. Sensible heat flux from the surface, friction velocity and Monin-Obukhov lenght are computed. Two alternative methods used for this purpose are described: a method applying resistances at the surface, and one applying similarity profiles in the surface layer. The convective mixing height is estimated for daytime hours by numerical integration. An operational mixing height is chosen for all hours from the convective mixing height and other available candidates. A stability parameter is computed for layers above the mixing heights. (Au).