A model for the parameterization of lake temperatures and lake ice thicknesses in atmospheric models is presented. The model is verified independently, and it is also tested within the framework of the High Resolution Limited Area Model(HIRLAM), applied operationally for short range weather forecasting at the Swedish Meteorological and Hydrological Institute (SMHI). The lake model is a slab model based upon energy conservation and treats the lakes as well mixed boxes with depths represented by the mean depths. The model is forced by near surface fluxes calculated from total cloudiness, air temperature, air humidity and low-level winds. A data base, describing 92000 Swedish lakes. provides the model with lake mean depths, areal sizes and locations. When the model is used for parameterization of lake effects in the atmospheric model, all the smaller lakes and the fractions of larger lakes within each horizontal grid square of the atmospheric model are parameterized by four model lakes, representing the lake size distribution. The verification of the lake model is done by comparing it with a more advanced, vertically resolved model, including parameterization of turbulent mixing processes, as well as by comparison with observations. A sensitivity test shows great interannual variations of the ice-covered season, which implies that lake models should be used instead of climate data. The results from an experiment with two-way coupling of the lake model to the atmospheric model are verified by comparing forecasted weather parameters with routine meteorological observations. These results show that the impact of lake effects can reach several degrees C in air temperatures close to the surface.