Once exceptional, periods of high heat are now multiplying and becoming more intense under the effect of global warming. Heat is one of the deadliest weather events: in the summer of 2022, high temperatures killed more than 60,000 people in Europe alone, according to a recent study.
Globally, June 2023 was the hottest month ever measured, according to the European Copernicus and American NASA and NOAA agencies. The first full week of July was in turn the hottest on record, according to preliminary data from the World Meteorological Organization (WMO).
Different phenomena are at work. So 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 glossary to understand these meteorological terms.
What is a heat wave?
A “heat wave” is when abnormally high temperatures are observed for several successive 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, insists the meteorological service: “Of the forty-six heat waves detected since 1947, nine occurred before 1989, compared to thirty-seven between 1989 and 2022. So there have been three times as many heat waves in the past thirty-five years as in the previous thirty-five. »
Since 2010, there have been twenty-two heat waves (only the years 2014 and 2021 have not suffered), more than 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”, warns Météo-France, which estimates that these climatic phenomena “are projected to double by 2050”.
When can we speak of a heat wave?
When temperatures rise, the term “heat wave” 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.
What is a Heat Feather?
Among various weather phenomena, it was a “heat plume” that hit Europe in mid-June 2022. The maximum temperature was recorded in Montoro, in the province of Cordoba, in southern Spain , with 42.9°C.
A “heat plume” is a geographically narrow upwelling of hot air that can look like a feather or tongue of hot air on maps. It is not a heat dome, since the phenomenon is dynamic and not blocked above an area.
The “feather” of early summer 2022 resulted from warm air that had flowed up from North Africa over the Iberian Peninsula. Christophe Cassou, climatologist and research director at the National Center for Scientific Research (CNRS), had described on Twitter a “heat wave by advection (transport of hot and dry air by southerly winds)”.
This heat plume was associated with a cold drop located off the Iberian Peninsula. It was an isolated depression with cyclonic winds rotating counter-clockwise, pushing warm air up.
What is the difference with a heat dome?
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 high pressure zone 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.
This is the phenomenon that is affecting part of Europe today. The edge of the heat dome is over southeastern France: Météo France has placed seven departments on heatwave orange vigilance for July 18 and 19. In July 2021, a heat dome had already caused record temperatures in western Canada and the United States.
What are climate normals?
Climate normals are references used to represent the climate in a given place and for a given period. They concern all 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 have been in use since the end of June 2022. “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”, explains Meteo 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.
What is an urban heat island?
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, recalls 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 reach 10°C in summer, particularly in the event of a heat wave.
