A cold wave (known in some regions as a cold snap or cold spell) is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24-hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criteria for a cold wave are the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year.In the United States, a cold spell is defined as the national average high temperature dropping below 20 °F (−7 °C). A cold wave of sufficient magnitude and duration may be classified as a cold air outbreak (CAO).