Heatwave in Marseille: what to expect and how to stay safe

Marseille has a quirk few Mediterranean cities share: everything depends on the mistral.

This north-westerly wind normally ventilates the city and cuts the felt temperature by several degrees. The problem arrives when it stops. Without the mistral, hot air stagnates in the amphitheatre of hills surrounding the city, and the sea — at 26 °C or more in late summer — stops being a cooling factor and becomes a source of humidity instead. Heat plus humidity pushes the felt temperature well above the number on the thermometer.

Fire risk follows immediately. After several days of heatwave, the garrigue and pine forests of the Calanques reach a critical dryness threshold. The prefecture then closes access to the massifs — Calanques, Sainte-Baume, Étoile — sometimes overnight. Check before setting out on a hike: access can be banned under a perfectly clear sky.

The northern districts, with less greenery and denser building, take the urban heat island considerably harder than the south of the city.

Every day of a heatwave strips moisture from vegetation. After four or five days, scrub and pine reach a point where a single spark — a cigarette, a shard of glass, farm machinery — is enough to start a fire that advances faster than a person can run.

The map below shows thermal hotspots detected by satellite over the last 48 hours. Each dot is a temperature anomaly seen from orbit, not necessarily a forest fire: it may be agricultural burning or an industrial facility. What matters is the pattern — dots in a line usually mean a front advancing with the wind.

Around Marseille, four massifs concentrate the risk: the Calanques to the south, the Étoile massif to the north, the Garlaban to the east and the Sainte-Baume further out. The Bouches-du-Rhône prefecture publishes an access map every day at 6 PM for the following day, with four levels from open access to total closure. At red or black level, entering the massif is banned and fined — including under a perfectly clear sky.

This is the part most people overlook. A heatwave stores an enormous amount of energy in the atmosphere. When cooler air finally pushes in, that energy releases all at once — and the result is not gentle rain but violent thunderstorms, hail and damaging wind gusts.

The pattern repeats every summer: five days of unbearable heat, and on the sixth a storm that brings down trees and blacks out whole neighbourhoods. The end of a heatwave is, statistically, the most dangerous moment of the entire episode.

Telling heat exhaustion apart from heatstroke is not an academic distinction: the first is treated at home, the second is a life-threatening emergency whose mortality rises sharply with every minute of delay.

Heat exhaustion — pale, clammy skin, heavy sweating, weakness, nausea, headache, muscle cramps. The person stays conscious and oriented. Act: shade, fluids, loosen clothing, damp cloths. It usually improves within half an hour.

Heatstroke — and here is the decisive sign: the skin stops sweating and turns hot and dry, core temperature passes 40 °C, and confusion, incoherent speech, aggression or loss of consciousness set in. The body's cooling mechanism has failed completely.

For suspected heatstroke call 112 immediately and start cooling while help is on the way: shade, cold water on the neck, armpits and groin, remove clothing. Do not give fluids to anyone with altered consciousness — choking risk.

Météo-France issues colour-coded warnings based not only on daytime highs but on overnight minima and the length of the episode. Those warnings trigger the National Heatwave Plan.

• Drink before you feel thirsty. Thirst appears once dehydration has already begun. Avoid alcohol and very sugary drinks.
• Stay indoors between noon and 6 PM. If you are visiting historic sites, go at opening time or late afternoon — shade there is close to nonexistent.
• Shutters closed by day, ventilate at night. It sounds obvious and a great many people do exactly the opposite.
• Check on elderly people living alone. Most heat deaths happen indoors, not on the street. One phone call a day is a genuine prevention measure.
• Never leave anyone — or any animal — in a parked car. The interior passes 50 °C in under fifteen minutes.

In an emergency call 112. For non-urgent medical advice, 15.

• Check the storm for your town