The thermal discharges from various sources that occur during winter may prevent ice formation at the free surface. In order to predict the extent of the free surface without ice, numerical models of thermal discharges should take into account: (1) the particular heat transfer at the free surface that occurs during winter time and (2) the nonlinear relationship between density and temperature for water near 0 °C. This article indicates a method of modifying existing models and presents some results obtained from a modified buoyant surface jet model.