Gauthier Delomez 4:57 p.m., June 15, 2022, modified at 4:58 p.m., June 15, 2022
Mercury continues to panic in France.
Temperatures will approach 40 degrees this week, or even exceed them according to Météo France forecasts, while summer has not yet officially started.
Several specific terms are used to describe heat episodes.
Europe 1 takes stock of meteorological jargon.
An increasingly early summer for the French.
France is experiencing record temperatures this week for the season, with peaks over 40 degrees in some parts of the country.
This episode of heat can increase the worrying dryness of the soil and the risk of fires.
But how should we qualify this rise in temperature?
Is it a heat spike, a feather or a wave?
When do we speak of a "heat wave"?
Europe 1 takes stock with Jérôme Lecou, forecaster for Météo France.
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The peak of heat, a sudden and brief rise in temperatures
The "heat peak" is usually "a sharp rise in maximum temperatures", says the forecaster, "but we can also have a nighttime heat peak".
This rise in temperature "is rapid, but it is not lasting", specifies Jérôme Lecou, "so we go down very quickly after having reached remarkable values".
On a curve, this is therefore materialized by a peak.
There is not really a temperature threshold to qualify a heat peak: the level of mercury must be remarkable in relation to the season.
"Typically, in June-July, heat peaks are temperatures that will be beyond 35 degrees, towards 40", he adds for Europe 1.
The heat feather, a frequent phenomenon
The term "heat feather" has been relayed lately on social networks, but it is an Anglo-Saxon expression.
"This term means nothing in the French language. We should use the term 'panache' of heat", explains the forecaster from Météo France, who illustrates this precise term: "It is hot air which is transported and which will spread at a given moment on the territory coming from another. It's a bit like a blowtorch, where you suddenly have a hot plume which will come out and move geographically."
This phenomenon is quite classic, underlines the meteorologist.
At the start of the week, it resulted in warm air over the Iberian Peninsula which headed towards the south-west of the Rhône Valley.
It must then reach the Loire then the Paris basin.
The heat dome, where warm air is trapped in the lower layers
The "heat dome" differs from the plume or the plume by its duration.
"We are going to talk about a dome when there are anticyclonic conditions with hot air which remains trapped in the lower layers. The air mass is compressed", explains Jérôme Lecou.
"It's like a bicycle pump: if you put your finger at the air outlet, and you compress it, the tip of your finger will heat up," he explains.
This heating of the air mass occurs most frequently in the summer.
"When we have a heat dome, we can have a duration that is quite long, with temperatures that will rise gradually over time", he specifies.
This heat dome can also be associated "with pollution phenomena, since all the particles emitted by human activity will be trapped in the lower layers", adds the forecaster.
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The heat wave, a long-lasting episode of hot weather
The expression "heat wave" is more technical.
It is currently used by Météo France experts to qualify the climatic phenomenon affecting France.
“We calculate a thermal indicator at the level of France, made up of the average temperatures over a whole sample of meteorological stations. It is a national average temperature. For there to be a heat wave, this indicator must be higher at 25.3 degrees", indicates the meteorologist.
"There is also a notion of duration", continues Jérôme Lecou.
"This indicator must be above 23.4 degrees for three days, and at least always above 22.4 degrees", he specifies, summarizing that this phenomenon "corresponds to an episode of hot weather, sustainable and which meets these precise criteria for Météo France".
The heat wave, with high temperatures day and night
To qualify an episode of heat, there is finally the term "heat wave" which stands out from the heat wave.
“It is also an episode of high temperatures, but both day and night. We have a criterion of hot nights too”, informs Jérôme Lecou.
In France, there is no single threshold to determine whether it is a heat wave or not: each department has one, with specific criteria.
“We have developed biometeorological indicators which depend on criteria which refer to past episodes, and crossed with excess mortality thresholds in emergencies in collaboration with Public Health France. In each department, we have defined minimum temperature thresholds and maximum beyond which we expect to have excess mortality. When they are exceeded, we will be able to speak of heat wave vigilance", explains the forecaster from Météo France.
For example, for the Paris region, the thermometer must rise to more than 31 degrees during the day, and more than 21 at night, for it to be called a heat wave.
Thresholds that change with global warming
Each heatwave episode therefore includes a sample of previous episodes.
"We can move the thresholds for a department", underlines Jérôme Lecou.
If these episodes do not repeat each year, he recalls, the thresholds can be impacted by global warming.
“We can clearly see that the 40 degrees for example, which were quite exceptional at the beginning of the 2000s, are beginning to be observed more frequently nowadays”, affirms the meteorologist.
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As a result, "the intensities of heat waves are more marked. And the other effects of climate change, it is the frequency of these episodes which is accelerating and the temporality with episodes which arrive earlier", lists the forecaster from Meteo France.
Who adds: "This week, the heat wave should be the earliest ever observed since a thermal indicator was put in place in 1947.
If these episodes occur earlier in the season, they can also extend further, evokes Jérôme Lecou.
"We can have episodes of very strong heat or heat waves in September. Finally, we can encounter this kind of hot episodes over a much wider period in today's climate, and even more so in tomorrow's climate. “, concludes the forecaster.