cold wave

1. As used in the U.S. National Weather Service, a rapid fall in temperature within 24 hours to temperatures requiring substantially increased protection to agriculture, industry, commerce, and social activities.

Therefore, the criterion for a cold wave is twofold: the rate of temperature fall, and the minimum to which it falls. The latter depends upon region and time of year.

1. Popularly, a period of very cold weather.

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.