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A freezing degree-day accumulates air temperature below a declared reference freezing temperature, with time in days and temperature difference in degrees Celsius or kelvins. The reference temperature and averaging convention must match the empirical sea ice thermodynamics formula using it. Warming by a constant $\Delta T$ subtracts $\Delta T$ times the number of days only if those days remain below the reference threshold.

The Maykut-Untersteiner model is a seasonally forced one-dimensional sea ice thermodynamics model with snow and saline ice layers. It couples radiative and turbulent surface fluxes, heat conduction, interior shortwave absorption, brine-dependent effective heat capacity and conductivity, snowfall, and a basal Stefan condition driven by prescribed ocean heat flux. Its annual sea-ice thermodynamic equilibrium is a periodic cycle with zero net yearly thickness change. The snow insulation and summer ice protection mechanisms compete, so additional snow need not reduce annual equilibrium thickness. The transient temperature profile need not be linear.