In the terminology of the United States Weather Bureau, an unusual fall in temperature, to or below the freezing point, exceeding 16° in twenty-four hours or 20° in thirty-six hours, independent of the diurnal range.
Criteria for describing departure from normal .(i) When normal minimum temperature of the area is 10 0C or more Nomenclature Departure from normal Normal 1 to -1 0C Below normal -2 0C Appreciably below normal -3 to -4 0C Moderate cold wave -5 to -6 0C Severe cold wave -7 0C or less (ii) When normal minimum temperature of the area is less than 10 degree. Nomenclature Departure from normal Normal 1 to -1 0C Below normal -2 0C cold wave -3 to -4 0C Severe cold wave -5 0C or less
A rapid fall in temperature within twenty-four hours to temperatures requiring substantially increased protection to agriculture, industry, commerce, and social activities. National Weather Service criteria includes the rate of temperature fall and the minimum to which it falls, depending on the region of the country and time of the in year. The Weather Channel uses the following criteria for a cold wave: a cold spell of two days or more with below normal temperatures in at least fifteen states, with at least five of them more than fifteen degrees below normal.
A cold wave is a weather phenomenon that is distinguished by marked cooling of the air, or the invasion of very cold air, over a large area. It can also be a prolonged period of excessively cold weather, which may be accompanied by high winds that cause excessive wind chills, leading to weather that seems even colder than it is. Cold waves can be preceded or accompanied by significant winter weather events, such as blizzards or ice storms.