Freeze

See freezing.

1. The condition that exists when, over a widespread area, the surface temperature of the air remains below freezing (0°C) for a sufficient time to constitute the characteristic feature of the weather.
   This is a general term, and the time period necessary is usually considered to be two or more days; only the hardiest herbaceous crops survive. It differs from a dry freeze or black frost, for these terms are usually used to describe purely local freezing due to chilling of the surface air by rapid radiation from a restricted portion of the earth.
   Compare light freeze, hard freeze, killing freeze.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.