Freezing degree-day
= Freezing degree-day
{title2=$N=\int\max(T_f-T_a,0)\,dt$}
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.