Temperature extrema

The image shows the heat emanating from Death Valley on 30 June 2013. The measurement is surface temperature as measured by the Thermal Infrared Sensor on the Landsat 8 satellite. The accompanying natural color view from the Operationa

Definition

A general term for temperature variations above (extreme heat) or below (extreme cold) normal conditions (IRDR Glossary).

Since climate varies regionally, the definition of an extreme temperature and its threshold will differ from location to location. In other words, an extreme value in one location may be within the normal range in a different location (WMO).

A simple method is to establish a specific threshold for temperature and extreme precipitation events and evaluate the extremes that occur over (or under) that given threshold. Another common mean of ascertaining thresholds is based on selecting the tail of distributions for temperature and precipitation.Statistical partitions such as by quartiles or percentiles of the distribution have provided a means for evaluating extremes (WMO).

Facts and figures

Heat waves

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).

Cold waves

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).