A year ago: exceptional early heat wave, 3 heat days in Paris

TO Regis CREPE, meteorologist

March 2021 ended with an exceptional period of early heat. If the national heat record of 31 °C in Pau in 1955 was not broken, hundreds of others were, especially in Brittany, the Île-de-France and in the east. Here are the highlights of this historic episode.

Although March 2021 generally remained cooler than the statistical averages until the last weekend, a heatwave began over France from Sunday March 28. From Monday, March 29, records began to fall, especially in the regions where these records were the lowest: initially Brittany and the Pays de la Loire. The heat then intensified until Thursday 1 April with midsummer-worthy values: three-quarters of France then had temperatures between 23° and 27°C. Thus, Paris experienced 3 days of heat in a row. at temperatures above 25°C.

Weather worthy of summer

The trigger for this early heatwave was the formation of a depression in the Canary Islands off the coast of Morocco during the weekend of March 27th. In its rotational motion, this depression acted as a “heat pump”, delivering air from the Sahara first to Portugal. In this regard, Morocco also experienced record-breaking early heatwaves (up to 35°C in Agadir). This air mass then rose to France, dragging sand from the Sahara with it, the fourth episode of its kind to reach our country since February.

Heat records in unused regions

Thus, the period from March 29 to April 1 is an unusual series of hot days for our country (i.e. with temperatures reaching and exceeding 25 ° C) from the southwest to the east, but especially in the northwest of the country, where such a situation has become exceptional, for example, in Brittany. Such heat in Brittany is explained by the origin of the current: weak winds coming from the south to the southeast sector directed hot and dry air coming from land to the northern shores of Brittany, where records were broken up to, for example, the beaches of Dinara (35) and Saint -Caast-le-Guildo (22). Thursday 1 April was the hottest day in the series with temperatures in Brest reaching 27.6°C (29), representing the hottest day on 1 April ever recorded, and especially the second hottest afternoon ever observed in April in 77 years of measurements in Brest, after an absolute record that is held on April 15, 2015 at 28.2°C.

Hundreds of heat records broken

Thus, the last two days of March 2021 have become historic for two-thirds of the country, in particular for the northwest, the Paris Basin and the Far East. Nearly 200 monthly heat records had been broken as of March 30, often broken again on March 31. We have selected the most representative of them on our map, but you will find all details here.

As a result, the National Thermal High Temperature Index rose to 24.1°C on March 31, making it the hottest noon since 1947 in our country. It was also the “hottest” March 31 since the March 31, 2006 record, when the mean thermal index (minimum and maximum temperatures) was 15.64°C. These temperatures continued into the first day of April, and in some places even slightly higher, especially in Brittany, where the hot air reached the tip of Finistère with a temperature of 27°C in Brest. But in April, the record levels are higher, and only the city of Pludalmeso (29) broke the absolute April record with a temperature of 26.6°C.

After all, this early heatwave is now being recorded as the most intense at the end of March since 1947 in France. However, there are precedents: the figures of 1955 and 2017 were comparable, but still slightly lower and less durable. The 2012 figure was more resilient during the last ten days of March, but with lower readings than this 2021.

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