A widespread heatwave continues across France on Wednesday, August 12, with 78 departments placed under orange heat vigilance as exceptionally high temperatures spread across much of the country.
Temperatures around 35°C (95°F) are affecting a broad area, while the hottest locations are approaching 38–40°C (100–104°F).
Météo-France expects the heat to intensify further on Thursday before cooler conditions begin making progress from western France.
Talking about a stubborn atmospheric ridge..
The heat is being maintained by persistent high pressure and exceptionally warm air over western Europe.
An upper-level ridge suppresses the development of widespread clouds and rainfall while encouraging sinking air through the atmosphere.
As air descends, it compresses and warms.
Strong August sunshine then provides additional heating near the surface.
The resulting pattern allows heat to accumulate over successive days rather than being regularly interrupted by passing Atlantic weather systems.
Warm continental and subtropical air has further reinforced temperatures.
Nights are becoming increasingly important
Maximum temperatures receive most attention during a heatwave, but overnight conditions are crucial for determining its health impact.
When nighttime temperatures remain unusually high, buildings and urban surfaces cannot release the heat accumulated during the day.
The human body also has less opportunity to recover.
That cumulative effect becomes increasingly important as a heatwave continues for several consecutive days.
Urban areas can be particularly vulnerable because concrete, asphalt and buildings absorb solar energy during daylight hours and release it slowly overnight.
The peak is expected on Thursday, August 13th
Météo-France expects another intensification of the heat on Thursday.
The geographical extent of orange vigilance illustrates how widespread the episode has become, rather than being confined to traditionally hotter southern regions.
Credit: La Chaine Meteo
A gradual change is then expected from the west as Atlantic influence begins eroding the hottest air mass.
That process is unlikely to occur everywhere simultaneously.
Eastern and southeastern France may therefore remain hot after temperatures have begun declining closer to the Atlantic.
The boundary between the retreating heat and incoming cooler air will also require monitoring because strong thermal contrasts can sometimes support thunderstorms.
Until the hotter air is displaced, however, prolonged daytime heat and unusually warm nights remain the dominant weather hazards.
