Why is France’s fourth heatwave set to be different from the others?

Today (July 29) has seen the arrival of a record fourth heatwave this year in France, with temperatures of up to 42C in the south-west and well over 35C in central and eastern areas.

State forecaster Météo France has placed 16 departments on tier three orange heatwave alerts, covering the east of France and Gironde and Landes, the site of two recent major wildfires. 

Meteorologists however have been quick to point out that unlike the two major heatwaves experienced so far this summer in June and July (as well as the exceptional early heatwave in May), this heatwave will be considerably different.

This is despite Météo France forecasting maximum temperatures today ranging from 38C to 41C in Gironde, with possible inland temperatures in the region and in Landes reaching as high as 42C, and with the spike in heat increasing the wildfire risk in the south-west. 

What is going to be different? 

While conditions today seem like they point to another prolonged heatwave that will cover much of the country, this will not be the case. 

The national average temperature today is set to reach around 28C, about 6C above average for the season but below readings during the peak of the heatwaves in June and earlier in July.

These heatwaves saw a ‘heat dome’ effect take hold, where a mass of warm air covered France for several days and led to an ongoing drought that has resulted in tap water restrictions across much of the country.  

This dome prevents cooler air from entering the atmosphere as well as trapping radiation and heat attempting to escape the atmosphere at night. 

Heat domes lead to gruelling conditions where both day- and night-time temperatures climb for several days, and often only end with a violent rush of new cool air, typically resulting in storms. 

Conditions today however are a result of a surge of hot air moving in from the south, but this will not lodge over France.

A heat dome effect is expected, but the epicentre will be in central Europe, meaning the hot air is only moving across France today and not settling above the country.

Eastern parts of the country may see a weakened version of the typical heat-dome effects over the coming days, but unlike the earlier heatwaves this will not be a prolonged issue for most of the country. 

Indeed, as early as Thursday (July 30) cooler Atlantic air will arrive over France, bringing temperatures down in several departments and shifting remaining warm air eastwards, with a second burst of cooler air on Friday (July 31).

The freer movement of cooler air means that temperatures across France will vary over the coming days. 

Areas in the west and along the Loire Valley, which faced the brunt of previous heatwaves, will instead see temperatures that are above average but not forecast to reach red-level warnings. 

Coastal areas in Normandy and Brittany are almost certain to avoid heatwave conditions, remaining cooler due to the English Channel winds.

Temperatures in the east and south-east – where cool air is unlikely to reach – may reach 40C over the coming days, as well as see high night-time temperatures, but these should still be less intense than conditions seen in other areas during the earlier heatwaves. 

Forecast models are unable to give exact predictions about the temperature levels across France for the coming days, as well as the effects of other conditions such as localised rain, but are certain that eastern areas will see the most intense heatwave conditions. 

While Météo France only gives official heatwave warnings one day in advance, it remains uncertain whether red-level alerts will be reached during the heatwave.

Even if they are, the number of departments facing these extreme alerts will be considerably lower than in June and earlier in July.

How long will conditions last?

Another difference is the relatively short length of this heatwave, or at least a ‘pause’ period. 

While the previous summer heatwaves lingered for up to two weeks, current conditions are set to be shorter. 

The lack of a heat dome across most of the country means other weather conditions will have more of an impact, including rain and cooler air. 

A drop in temperatures over the weekend is expected on Saturday (August 1), with highs of 27C in the north and 30C in the south and south-west – considerably cooler than in the middle of the week.

A renewed spike in temperatures from Sunday (August 2) onwards is expected, covering the first part of next week.

Again, models cannot give accurate predictions, but there is the possibility of temperatures reaching 43C and conditions lasting until potentially next Wednesday (August 5) or Friday (August 7).

The early weekend respite should allow homes and people to cool down and prevent the difficulties of a prolonged heatwave.