Heat balance models of ice, leads, and the underlying water column were used to investigate the role of shortwave radiation in the summer decay of ice cover. Because of the potential importance of positive feedback between decreasing ice concentration and increasing solar input to the ocean, particular emphasis was given to the treatment of heat transfer and lateral melting in leads. The rate of heat loss to floe edges was calculated from lead temperatures using an empirical boundary layer parameterization. Coefficients for this parameterization were obtained from field measurements in the dynamically active ice cover of the Greenland Sea during 1984 Marginal Ice Zone Experiment and in the static ice of Mould Bay, Northwest Territories. Field coefficients were found to be considerably larger than those previously obtained in the laboratory, suggesting that heat transfer rates near floe edges were controlled more by winds and currents than by local density differences. Model studies of individual leads indicate that heat losses to the atmosphere cause lateral melt rates to asymptotically approach a constant value as lead size increases. Thus although very wide leads transmit considerable energy to the underlying ocean, they have relatively little effect on lateral melting. Other results provide information on the relative importance of horizontal heat transfer rates, ice thickness, floe perimeter, atmospheric forcing, and oceanic heat flux. In general, the calculations indicate that much of the shortwave radiation entering the ocean is absorbed below the ice where it contributes to bottom ablation rather than lateral melting. This suggests that previous work has seriously overestimated the magnitude of the feedback between ice concentration and incident shortwave radiation. ¿ American Geophysical Union 1987