A one-dimensional thermodynamic model of sea ice is presented that includes the effects of snow cover, ice salinity, and internal heating due to penetration of solar radiation. Surface-energy balances determine rates of ablation and accretion; diffusion equations govern heat transport within the ice and snow. The incoming radiative and turbulent fluxes, oceanic heat flux, ice salinity, snow accumulation, and surface albedo are specified as functions of time. Starting from an arbitrary initial condition, the model is integrated numerically until annual equilibrium patterns of temperature and thickness are achieved. The model is applied to the central Arctic. Input values for the initial test of the model are based on observational data. Values predicted by the model for the average ice thickness (288 cm), amount of surface ablation (40 cm), and the temperature field all agree closely with field observations. Other results from the model indicate that, under present conditions, the ocean must supply 1 to 2 kcal/cm2 year to the ice; an additional 4 kcal/cm2 year would cause the ice to vanish. Annual snow depths less than 70 cm are shown to have little effect on equilibrium thickness; snow depths greater than 70 cm would result in much thicker ice. Comparison of observed and calculated temperature profiles suggest that about 2.0 to 2.5 kcal/cm2 year of the incoming short-wave radiation penetrates the ice and contributes to internal heating. Average ice albedos under 0.50 would cause the ice to vanish in a few years.