What does a heat spike and wave mean?

“A peak of heat designates a brief episode, lasting 24 to 48 hours during which temperatures are higher than normal for the season”, explains Météo-France on its website.

“A heat wave designates an episode of temperatures significantly higher than normal for the season for several consecutive days”, continues the public service of meteorology and climatology in France.

The definition of a heat wave varies by region, depending on the temperatures and its duration, as populations do not have the same adaptive capacity.

“A heat wave episode is detected as soon as the national thermal indicator reaches or exceeds 25.3°C and remains high for at least 3 days”, adds Météo-France.

And a heat wave?

The heat wave designates “an episode of high temperatures, day and night, over an extended period (at least three days)”, explains Météo-France.

Meteorologists “have defined thresholds which vary according to the departments. For example, in Toulouse, Météo-France will speak of a heat wave when, during the three days and three nights, the maximum temperatures will be higher than 36°C and the minimum temperatures higher than 21°C”, gives the meteorological institute as an example.

Temperatures may be lower in northern France.

Heatwave vigilance can be yellow, orange or red, the latter threshold corresponding both to an exceptional meteorological event and to a health alert justifying maximum mobilization, with risks of excess mortality.

Multiplication of episodes

These heat episodes usually affect mainland France between “early July and mid-August, but they can also occur outside of this period, such as in 2016, 2019 or 2020.

Since 1947, 45 heat waves have been recorded in France, the last dating back to July 2022.

These waves “have been significantly more numerous in recent decades. Over the past 35 years, they have been three times more numerous than over the previous 35 years. The number of days of heat waves has increased ninefold”, indicates Meteo France.