France will be hit by a particularly early heat wave from the middle of the week. At the height of the episode, between Thursday June 16 and Saturday June 18, the thermometer could reach between 35°C and 39°C in the South – from South-West to South-East going up to Lyonnais – with peaks of heat to 40°C.

The heat wave should also affect the northern half of the country: Météo-France expects maximums between 33°C and 35°C from the banks of the Loire to Franche-Comté via the Paris basin, and between 25°C and 31°C in Brittany, Normandy, Hauts-de-France and the North-East.

What is the difference between a “heat wave” and a “heat wave”? What is a “heat feather” or a “heat dome”? What are climate normals and how are they calculated? Le Monde offers you a lexicon to understand these meteorological terms.

A “heat wave” is when abnormally high temperatures are observed for several consecutive days. Météo-France explains, however, that “there is no universal definition of the phenomenon”.

Two criteria are used to characterize a heat wave: the temperature levels and the duration of the episode. However, these criteria vary according to the regions of the world and the fields considered (characterization from a climatological point of view, research activity, meteorological vigilance system).

The census of heat waves since the middle of the 20th century shows an increase in these climatic events, points out the meteorological service:

“Of the forty-one heat waves detected since 1947, nine occurred before 1989, compared to thirty-two between 1989 and 2019. So there have been three times as many heat waves in the last thirty years as during the forty- two previous years. »

Since 2010, there have been seventeen heat waves (only 2014 did not experience any), i.e. as many as over the period 1947-2000.

“Whatever the greenhouse gas emission scenario envisaged, global warming will continue for at least several decades and will be accompanied by increasingly frequent and intense heat waves”, alert Météo-France, which estimates that these climatic phenomena “are expected to double by 2050”.

When temperatures rise, the term “heatwave” is often used. However, the definition of this phenomenon is very precise and varies according to the regions.

“A heat wave is an episode of high temperatures, day and night, over an extended period,” explains Météo-France. For its part, the government speaks of “a level of very high heat day and night for at least three consecutive days”.

Météo-France has determined alert thresholds based on thirty years of daily mortality data and various meteorological indicators. The different regions of the country being more or less used to and therefore adapted to heat, these alert thresholds are not the same everywhere. Thus, Météo-France estimates that the heat wave will be proven in Deux-Sèvres when minimum temperatures of 35°C during the day and 20°C at night are observed. Haute-Loire will be considered a heat wave when temperatures exceed 32°C during the day and 18°C ​​at night. In Paris, these thresholds are 31°C during the day and 21°C at night, and in Marseille 35°C during the day and 24°C at night.

The current “heat wave” is the result of warm air moving up from North Africa over the Iberian Peninsula. On Monday, Spain was already suffocating. The maximum temperature was recorded in Montoro, in the province of Cordoba, in the south of the country, with 42.9°C. This plume of hot air is called a “heat plume”. “It’s a heat wave by advection (transport of hot, dry air by the southerly winds), at least for the start of the episode”, explains on Twitter Christophe Cassou, climatologist and research director at Scientific Research National Center.

This heat plume is associated with a cold drop located off the Iberian Peninsula. It is an isolated depression with cyclonic winds that spin counter-clockwise, pushing warm air up. “Unlike a rather static heat dome, everything here is very dynamic,” adds Christophe Cassou.

The “heat dome” is a meteorological phenomenon linked to the presence of a blocking anticyclone which persists in one place, causing the stagnation of warm air masses. This zone of high pressure also promotes the descent of air, which, under the effect of compression, warms the atmosphere. As the trapped heat continues to heat up, the system acts like a lid on a pan.

In July 2021, a heat dome caused record temperatures in western Canada and the United States.

Climate normals are references used to represent the climate in a given place and for a given period. They concern all the climate variables (temperature, precipitation, wind, sunshine, etc.). These indicators are calculated by Météo-France over thirty years and updated every decade.

The meteorological organization therefore undertook, in 2021, the calculation of new normals for the period 1991-2020. They will be used from June 28. “The 1981-2010 normals are representative of the average climate over a period around the 1990s and no longer represented the current climate in the context of climate change, particularly in terms of temperature”, points out Météo-France.

Because of this change, “these new ‘climate normals’ fall far short of our normal climate of just a few decades ago,” the weather service insists. Since 1900, the average temperature in France has risen by 1.7°C. Over the period 2011-2020, the increase reached 0.6°C compared to the previous ten years, marking the strongest increase observed between two decades since the beginning of the 20th century.

The city and the countryside do not store heat in the same way. In rural areas, vegetation and permeable soils evacuate, thanks to solar energy, the water they draw or contain. This phenomenon of “evapotranspiration” has a virtue: plants and soils do not accumulate solar energy that they could restore at night.

Conversely, in the city, this solar energy is absorbed and stored by buildings and asphalt floors. When night falls, the temperature drops, but the impermeable mineral surfaces release the heat accumulated during the day into the air, preventing the urban atmosphere from cooling as much as in the countryside. It is this temperature differential that corresponds to the “urban heat island”, an “essentially nocturnal” phenomenon, underlines Météo-France.

In Paris, for example, the differences in night temperatures with the surrounding rural areas are on average 2.5°C over the year. They can even reach 10°C in summer, particularly in the event of a heat wave.