Heat waves and peaks, omega blockage… The words of the heatwave

The thermometer will reach 38°C this weekend, particularly in the west of the country, Météo France reported on Wednesday, June 18. This heatwave is due to an omega blockage. Sixteen departments will be placed on orange heatwave alert starting Friday.

Many phrases are used to describe heatwaves, each with its own unique characteristics. They can be divided into two categories: those that describe the intensity of the heat; and those that explain its cause.

The difference between a heatwave and a heat spike lies in the intensity of the weather event. This refers not to the temperatures, but to the duration of the period when they are higher than seasonal norms.

“A heat peak designates a brief episode (24 to 48 hours) during which temperatures are significantly higher than seasonal norms,” explains Météo France. A peak can be observed on a local as well as national scale.

When a heatwave lasts for several days and affects a large part of the country, it can be called a heatwave. This depends on the national heat indicator. This is the "average temperature measured across the country" at 30 weather stations spread across the country. The heatwave ends when the national indicator falls below 22.4°C or when it is below 23.4°C for two consecutive days.

When a heatwave lasts three days or more, and temperatures remain high at night, the term heatwave is used. To determine whether or not it is a heatwave, Météo France meteorologists refer to temperature and duration thresholds that vary by department.

Thus, in Haute-Garonne, recalls the weather'n'co website, it must be at least 36°C during the day and 21°C at night for the heatwave alert to be triggered. In the Manche region, where the climate is more temperate, the thresholds are set at 31°C during the day and 18°C ​​at night. For the weekend , Météo France plans to place around thirty departments on yellow alert and sixteen others on orange alert.

Météo France establishes different alert levels when the temperature rises. Yellow alert corresponds to a heat peak or heat wave. Orange alert is activated in the event of a heatwave. Red alert is used in the event of an "extreme heatwave, exceptional in its duration, intensity, and geographic extent, and (which) presents a significant health impact for the entire population and societal impacts."

This week's heat is due to an omega blockage . A low pressure system located over the Iberian Peninsula and another over the Balkans are bringing up warm air, which remains stuck over France. This phenomenon takes the form of the Greek letter omega (Ω), and notably caused the 2023 heatwave.

Rising temperatures can also be the result of a heat dome, which develops when there is no wind, acting as a kind of cover. They can also come from a " heat plume " caused by the rise of warm air from southern Europe and the Maghreb.

These phenomena are becoming more frequent and will worsen in the coming years due to global warming. Apart from the summer of 1947, the most significant heat waves observed in nearly eighty years all occurred in the 21st century (August 2003, July 2019, and September 2023 in particular ). By 2100, Météo France warns, heat waves could even last up to two months continuously.