Through Cyrille DUCHESNEmeteorologist
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The first part of the meteorological summer (June 1 to July 15) has just ended. If the sun and heat have returned since this weekend, most of you still have the feeling of a gloomy and chilly start to summer. Let’s see how this first half of summer 2021 is compared to previous years with regard to the climatological data available.
After the first two decades of June with very excess temperatures, cooler air affected France at the end of June as well as during the first half of July. The drop in temperatures took place at the start of summer on the calendar with a temperature deficit of around -0.7 ° C between June 21 and July 15. Such a deficit had not occurred since the beginning of summer 2007. If we stick to the period June 1 to July 15, we keep a temperature surplus of around + 0.9 ° C. on average in France, which can be explained by the first two decades of very hot June.
High temperatures in mid-June: up to 38 ° C in Roussillon
A yellow alert for the significant risk of heatwave affected Languedoc-Roussillon and the Rhône valley for a few days in mid-June. This wave of strong heat affected more particularly the Pyrénées-Orientales and the Aude with several consecutive days where temperatures exceeded 35 ° C in the shade during the day and did not drop below 20 ° C. night. We observed up to 37.2 ° C in Durban-Corbières (11), 37.3 ° C in Perpignan (66), 37.9 ° C in Narbonne (11) and 38.2 ° C in Boulou (66 ).
The day of June 16 was the second hottest day with an average of 24.2 ° C over France, just behind June 14, 2003 with 24.6 ° C.
Great freshness for July 14 in the east: 14.5 ° C in Langres
The first half of July lacked heat since we observed a temperature deficit of around -1.3 ° C on average over France. Only regions close to the Mediterranean have retained lasting warm weather. The coolest period was that of July 13-14, when the cold drop passed which brought bad weather to the northeast. Thus in the afternoon of July 14, the temperature did not exceed 14.5 ° C in Langres (Haute-Marne), 16.8 ° C in Dijon (Côte d’Or) and 17.5 ° C in Lyon (Rhône), values of around 8 ° C below seasonal norms.
Record rainfall for early summer 2021
The highlight of this early summer 2021 comes from the very abundant rainfall in many regions. While the Azores high pressure generally swells towards western Europe in early summer, this has not been the case this year. It remained with absent subscribers, leaving the field open to depressions and the famous “cold drops”. Very unstable weather therefore prevailed between the beginning of June and mid-July. Very active rain and storm damage brought significant accumulations of rain, especially between Auvergne-Rhône-Alpes, the north and the north-east of the country.
June 2021 ranks 4th among the wettest months of June since 1959, after 1997, 1992 and 1987. Monthly rainfall records have been broken in some cities such as Paray-le-Monial (Saône-et-Loire ) with 189 mm, Tours (Indre-et-Loire) with 154 mm, Gonneville (Manche) with 151 mm, Vélizy-villacoublay (Yvelines) with 137 mm and Rennes (Ille-et-Vilaine) with 134 mm. Conversely, the Mediterranean regions remained aloof from heavy rainfall. The rainfall deficit is particularly large between the PACA region and Corsica: 1 mm in Six-Fours (Var) and Istres (Bouches-du-Rhône), 2 mm in Ajaccio (Corse-du-Sud) and Nice (Alpes-Maritimes ), 3 mm in Bastia (Haute-Corse).
The weather remained very unstable and disturbed at the beginning of July with a particularly active depression from July 12 to 15 bringing copious rainfall to the north of the Seine. In some cities such as Besançon (Doubs), Saint-Dizier (Haute-Marne) and Reims (Marne), more than a month of rain fell in 48 hours, resulting in floods and flooding observed between the Grand-Est and Bourgogne-Franche-Comté.
From June 16 to July 15, it fell on average over France 144 millimeters, more than twice the normal, a record for this period of the year, far ahead of 1997 with 123 millimeters.
With these record rains across the country, the soil moisture index rose to a record level for July 15 nationwide with a value of 0.70 very close to the absolute record in July, 0.73 on July 1, 1977.
A marked drought near the Mediterranean
While most of the country observed excess rainfall during the first part of summer 2021, this is not the case for the Mediterranean regions which have mostly remained away from rainfall and storm damage. . The French Riviera and Corsica were the least watered regions with only 3 millimeters in Bastia (Haute-Corse) for an average of 48 millimeters. In Nice (Alpes-Maritimes), it fell only 7 millimeters in a month and a half for an average of 40 millimeters. Languedoc-Roussillon is affected to a lesser extent by durably dry weather. It fell 17 millimeters in Perpignan (Pyrénées-Orientales) since June 1 for an average of 31 mm or half of the usual rainfall.
Significant thunderstorm activity in June
The month of June saw several times severe meteorological phenomena with the passage of very active thunderstorms. The confrontation between the warm air coming up from Spain and North Africa, and the arrivals of cooler air from the Atlantic caused severe rainfall and storm damage. Thus, according to Météorage, this month of June 2021 comes in second place among the most lightning months of June for 30 years with 169,764 lightning bolts, after the month of June 1993. The first half of July was not very stormy. Météorage records a little more than 12,000 flashes for an average of 62,750 flashes at this time of the year.
A very poor amount of sunshine during the first half of July
If the month of June saw close to normal sunshine, due to the first part of June being correctly sunny, the first half of July was not very sunny due to the frequently disturbed weather. Brittany and the regions located between the north-east and the center-east were the least sunny with a sunshine deficit of 40 to 60%. It even reaches 63% in Brest (Finistère) with only 31 hours of sunshine from July 1 to 15. It is in the south-east that the sun showed itself the most durably with 170 hours in Nice (Alpes-Maritimes), that is to say a sunshine close to normal.
This beginning of summer really contrasts with the last very hot and dry beginnings of summer that we have had. In 2019, we broke absolute heat records at the end of June in the south with 46 ° C locally in the Hérault. A month later, absolute heat records fell in the north of France with 41.5 ° C in Lille (North) and 42.6 ° C in Paris, values never reached since 1900.
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