In summer, we hear about heat peaks, heat waves and heat waves. The difference is not obvious, but it does come into play for weather alerts or health vigilance plans.
A peak of heat is characterized by brief episodes (24 to 48 hours) where temperatures are well above seasonal norms. It can occur locally or at the level of an extended territory. When these high temperatures continue for several consecutive days, it is called a heat wave.
There is no universal definition of a heat wave: it can vary according to the regions of the world and the climate specific to each country. For Weather report France, a wave of heat is detected when the temperature reaches or exceeds 25.3°C at the national level and remains high for at least 3 days. This threshold corresponds to the average of daily temperature measurements in 30 meteorological stations spread over the metropolitan territory. France has thus experienced more than 40 heat waves since 1947. We also identify heat waves departments based on differentiated thresholds.
Heat wave: not just a matter of weather
A heat wave is defined as an episode of high daytime and nighttime temperatures over an extended period. There is no national threshold here: the heat wave is determined at departmental level with temperature thresholds and durations variables. For Toulouse, for example, we speak of a heat wave as soon as the temperatures do not drop below 21°C at night and exceed 36°C during the day, for at least 3 days and 3 nights.
Contrary to popular belief, the heat wave threshold does not depend solely on meteorological criteria: it is jointly determined by Météo France, Public Health France and the Ministry of Health. Local environment and population vulnerability criteria are therefore taken into account. In particular, we calculate from which temperature threshold a excess mortality in each department. When the forecast exceeds this threshold for a period of more than 3 days and 3 nights, the department is placed on heat wave vigilance.
The waves of heat occur mainly between the beginning of July and mid-August, but it happens that the phenomenon occurs later, as was the case in 2016 between August 23 and 28, or earlier, as in 2017 between August 18 and 22 June. Temperatures had then reached 26.4 ° C on national average in France on the day of June 21.
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