A heatwave is a very hot spell that occurs in summer and lasts for several days and nights.
The thermal characterisation of a heat wave is subjective, because it depends on the population's sensitivity and therefore on the place it occurs. In Western Europe, a heatwave is considered to correspond to a night-time temperature above 18 - 20°C and a daytime temperature above 30 - 35°C.
In France, meteorological services issue heatwave risk warnings when minimum temperatures are above 20°C (especially at night) and maximum temperatures are above 33°C for at least three days.
In the event of a heatwave, people are advised to take special measures to stay hydrated and cool, especially the most vulnerable people (e.g. children, the elderly, the sick). For more information on measures to take, the French Ministry of Health has made the Heatwave and extreme heat file available to the general public on the internet.
Thermal anomalies in Europe during the deadly heatwave of 2003. © Nasa, Wikimedia public domain