A heatwave is a meteorological phenomenon characterized by abnormally high temperatures over a prolonged period. The combination of these two criteria - heat and duration - can constitute a health risk for the most vulnerable populations (elderly or disabled people, infants, etc.).
What is a heatwave?
A heatwave is an exceptionally intense and prolonged heat episode, which occurs mainly during the hot season, when the duration of sunshine is longer and the nights shorter. During heatwaves, temperatures are very high during the day and the nights are hot, with temperatures that exceed seasonal norms for a given region, for several days or even weeks.
It should be noted that global warming, which is the cause of a regular increase in temperatures, also affects the frequency, intensity and duration of these heat waves.
What are the alert and vigilance thresholds for heatwaves?
Heatwave periods are therefore characterized by abnormally high temperatures, which exceed the seasonal averages for a given region, for several days or even several weeks. Temperature thresholds vary depending on the geographic area and whether it is urbanized or rural.
The indicator used is called IBM: according to Public Health France, it is the combination of the three-day rolling average of minimum temperatures and the three-day rolling average of maximum temperatures.
Thus, the alert thresholds for heatwaves are, in Calvados, 31°C during the day and 18°C at night, while in Gard, they are 36°C during the day and 23°C at night. night. In Paris, the chosen torque is 31°C - 21°C.
Heatwave periods are accompanied by drought conditions, meaning a lack of precipitation. This drought can worsen the effects of heat because evaporation of soil moisture is reduced. They are generally associated with light wind conditions. The absence of wind can accentuate the feeling of heat.
Météo France has developed a classification into four levels, symbolized by four meteorological vigilance colors:
● Level 1, green vigilance: seasonal monitoring
● Level 2, yellow vigilance: heat warning
● Level 3, orange vigilance: heatwave alert
● Level 4, red vigilance: maximum mobilization
These levels enable effective communication of the degree of precaution required based on weather conditions, ensuring appropriate management of periods of intense heat.
What are the causes of heatwaves?
The presence of a high atmospheric pressure system
(anticyclone), which blocks hot air and acts as a barrier to winds, is the cause of heat phenomena. This leads to a significant rise in temperatures. Persistent anticyclones, during which the mobility of hot air masses is restricted, give rise to heatwave periods.
Human activities, in particular greenhouse gas emissions, are a major player in global warming, and contribute to the overall increase in the frequency and intensity of these heat waves on a global scale.
Across France, these heat waves have continued to increase, as reported by the resource center for adaptation to climate change:
46 heat waves in France since 1947.
7 times more episodes of heat waves in the last 35 years (1988-2023) than in the previous 35 years (1953-1987).
12 days per year of heatwave on average over the last decade (2013-2022) compared to 3 days between 1980-1989.
The topography of a region is also crucially important, with steep valleys, for example, being able to trap warm air and cause local temperatures to rise. Urban areas, due to the urban heat island effect, also have higher temperatures, attributable to the absorption and retention of heat by concrete and asphalt surfaces.
What are the impacts of the heatwave on health?
Heatwaves have significant impacts on health due to prolonged high temperatures. Health effects may vary depending on the duration and intensity of the heatwave, as well as the adaptation capacity of populations. Heatwaves can lead to heatstroke, characterized by a rapid rise in body temperature. Symptoms include hot, dry skin, confusion, dizziness, or even loss of consciousness. Heatstroke is an emergency and must be treated quickly. They can be fatal in certain cases or cause after-effects to persist (cramps, neurological disorders).
Heatwaves can sometimes aggravate existing health problems, such as cardiovascular and respiratory diseases. People with these conditions are more vulnerable to the effects of heat. High temperatures lead to breathing problems, especially in people with asthma and other respiratory disorders. Heat can lead to increased dehydration, especially if people don't drink enough water. Dehydration can cause headaches, dizziness, confusion and other health problems.
Hot nights during heatwaves often disrupt sleep, leading to problems such as insomnia. Lack of sleep can have consequences on mental and physical health. Prolonged heatwaves also have psychological effects, such as heat stress, anxiety and irritability, especially in people who cannot find refuge in cooler environments.
What are the consequences of heatwaves?
Heat waves worsen pre-existing health conditions. Heatwaves can cause serious health problems, such as heat exhaustion, heat stroke and other heat-related illnesses. The elderly, young children, sick people and individuals without adequate access to shade and water are particularly vulnerable. Heatwaves are associated with increased mortality, particularly in regions where populations are unprepared or do not have the means to cope with extreme temperatures.
High temperatures put pressure on infrastructure, particularly electricity and cooling systems. Increased energy demands for air conditioning can lead to power outages, while high temperatures compromise equipment operation. Prolonged heatwaves can harm crops and livestock. Extreme temperatures, combined with possible drought, lead to reduced agricultural yields, crop losses and difficulties for farmers. Heatwaves promote conditions conducive to forest fires. Dry vegetation, combined with high temperatures and strong winds, contribute to the rapid spread of fires, causing significant environmental damage.
Heatwaves disrupt ecosystems, affecting biodiversity and natural cycles. Animal and plant species that cannot quickly adapt to temperature changes are at risk. In certain regions of the world, heatwaves lead to social problems such as conflicts linked to access to water, population migrations due to uninhabitable conditions, and social tensions linked to
thermal stress.
How to protect yourself from heat waves?
To protect yourself from heatwaves and minimize health risks, you should drink plenty of water throughout the day, even if you don't necessarily need it. Drinks containing caffeine or alcohol should be avoided as they can contribute to dehydration. Stay in the shade as much as possible, especially during the hottest hours of the day, usually between 10 a.m. and 4 p.m. People who will be outdoors should wear a wide-brimmed hat and light, colored clothing clear.
Apply sunscreen with a high protection factor to protect your skin against UV rays. Reapply regularly, especially after sweating or entering water. If possible, stay in air-conditioned or well-ventilated areas, and use fans to create a draft. Avoid using appliances that generate heat during the hottest periods. Take cool showers or baths to help lower your body temperature. Choose light, loose-fitting, light-colored clothing to promote air circulation and reflect heat.
Watch out for symptoms of overheating such as confusion, headache, dizziness, nausea and excessive sweating. Be particularly vigilant towards the most vulnerable people, such as children, the elderly and those with pre-existing health conditions. Make sure they stay hydrated and fresh. Limit strenuous physical activities during the hottest hours of the day. If possible, do these activities early in the morning or late in the evening, when the weather is milder.
In France, a “Canicule” telephone information platform, accessible at 0 800 06 66 66 (free call from a landline in France, from 9 a.m. to 7 p.m.), can be activated in the summer from the first episode of heatwave, by the Directorate General of Health, to answer questions and provide information on the health recommendations to follow during periods of extreme heat.