A period of marked unusual hot weather (maximum, minimum and daily average temperature) over a region persisting at least three consecutive days during the warm period of the year based on local (station-based) climatological conditions, with thermal conditions recorded above given thresholds. Heat waves differ from warm spells. Similar to heat waves, warm spells are defined as a persistent period of abnormal warm weather. A warm spell occurs at any time of the year, whereas heat waves can only occur in the warm season (WMO).
A period of marked and unusual cold weather characterized by a sharp and significant drop of air temperatures near the surface (maximum, minimum and daily average) over a large area and persisting below certain thresholds for at least two consecutive days during the cold season. “Cool spell” refers to persistently below-average temperature conditions occurring during the warm season (WMO).