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.
Global warming, which is the cause of a regular increase in temperatures, also affects the frequency, intensity, and duration of these heat waves. Temperature thresholds vary depending on the geographic area and whether it is urbanized or rural. In Paris, the chosen torque is 31°C - 21°C. In Calvados, the alert thresholds for heatwaves are, in Calvado, 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. 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: yellow vigilance. Level 3, red vigilance: red vigilance. 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 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. 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 use 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. Heat can lead to increased dehydration, especially if people don't drink enough water.