A new heat wave has arrived in France and should spread quickly. Orange heatwave vigilance is currently active in Gard, Vaucluse, Lot-et-Garonne and Gironde, but other departments could quickly be affected.

These classifications are mainly the result of the work of meteorologists and are far from being left to chance.

“When we detect the arrival of a heat wave in France, we try to identify the minimum and maximum temperatures that will arrive in the next five or six days or even beyond. Then, depending on the regions, we place the orange heat wave vigilance thresholds”, explains Patrick Marlière, meteorologist for Agate Météo.

For a department to be in orange, it is necessary that “for three consecutive days, we have temperatures that do not fall below a certain threshold at night and that during the day the maximum exceeds another threshold. These thresholds are different depending on whether you are in the south or in the north of the country. In Lille or Paris for example, the thresholds are 21°C at night and 31°C or 32°C during the day, whereas in Marseille, it’s 24°C at night and 34°C during the day,” says the expert.

Thus, if all these conditions are not met, then we do not speak of a heat wave but no longer of a heat wave. However, yellow vigilance is a preventive measure and therefore precedes the heat wave phenomenon which only becomes official with the orange alert.

“There is a higher level represented by red vigilance. This concerns heat waves of very long intensity, both in duration and in terms of temperatures”, develops Patrick Marlière.

The meteorologist also explains that he and his colleagues work jointly with the government. “Orange vigilance thresholds are generally triggered in collaboration with the prefecture as this entails a number of measures. Red vigilance, on the other hand, is normally triggered directly in agreement with the Prime Minister. It entails the implementation of exceptional measures: the organization of working time, the organization of public services.”