Weather. Four questions on atmospheric pressure records in Brittany and Normandy

The barometer needle goes up ! Exceptional high atmospheric pressures were noted in the north-west of France, the south of Great Britain and up to Belgium, Sunday January 19 or Monday January 20.

1. What records have been observed?

In Cherbourg, the 1,049 hectopascal (hPa) threshold was crossed at midnight on January 19. It is an absolute record pressure in post-1950 France, exceeding 1,048.9 hPa of March 3, 1990 at Pointe de Chemoulin [Loire-Atlantique], underlines Étienne Kapikian, on Twitter. This forecaster of Meteo France, very active on the social network, raises other records on old stations, in Normandy or in Brittany: 1,048.7 hPa at Pointe de la Hague at midnight, beating the 1,048.2 hPa of February 15, 1934, or 1,048.6 hPa in Brest at midnight, beating the 1,048.2 hPa of March 3, 1990.

Other data: 1,048.4 hPa in Caen, 1,048.2 hPa in Quimper or 1,048.4 in Le Havre.

According to provisional data, an absolute record was also reached in Belgium with 1,049.9 hPa recorded at Chièvres on January 20 at 7 a.m. The previous record dated from 1932, in Uccle, with 1048 hPa.

While in Wales, pressure was measured at 1,050.5 hPa at Mumbles Head on the evening of January 19, according to the Met Office. Not far from the 1,050.9 hPa recorded in Benbecula, Scotland, in… 1957. The British record was 1,053.6 hPa, in Aberdeen, Scotland, in 1902.

2. Why these high pressures?

They are linked to the high pressure, which means that the western regions have found the sun and the blue sky. In recent days, the high pressure has been positioned from Germany to the British Isles. It blocks the progression of depressions and shifts the disturbed flows further north, towards Iceland, Scotland, Norway, describes Thierry Maingault, forecaster at Météo France at the Rennes regional center. While in the south, the Iberian Peninsula is facing a big depression, with the storm Gloria.

Without being extremely rare, this positioning of the high pressure is not classic. Let’s say that it’s not the idea that we usually have winter here, notes Thierry Maingault. Generally, in this situation, we are around 1040 hPa. Here, we reach exceptional values ​​but it is not abnormal with powerful highs. The statements flirt with absolute records, but we often only have about fifty years of measurements, moderates the forecaster.

This anticyclone shifts slowly, like a big soap bubble. Because anticyclones move more slowly than depressions. And by the end of the week, western France should be back to the clouds.

3. What is atmospheric pressure?

It’s the weight of the air column above ussums up Thierry Maingault. This weight is measured in hectopascals (hPa), in reference to the 17th century scientist, Blaise Pascal, who greatly contributed to revealing to the world the existence of vacuum and the gravity of air. To measure atmospheric pressure, a barometer is used.

In our latitudes, the atmospheric pressure measured at sea level is on average around 1015 hPa. The higher we climb, the less air there is above our heads and therefore the lower the pressure.

Atmospheric pressure also varies depending on weather phenomena. Generally, the weather is rainy when the pressure is low. When it falls, it is a sign of bad weather and wind. To simplify, depression, when the weight of the air column is lower, promotes upward movements: that is to say that the air rises, condenses and causes clouds, describes Thierry Maingault. When the air column is heavier, the residual movements [NDLR. le mouvement lent lors duquel l’air est tassé vers le sol] cause dryness of the air mass and stable periods of time. This high air pressure indicates a calm, but not always good weather. In winter, for example, fog and low clouds can go hand in hand with these high pressures.

4. What are the consequences of these high pressures?

High pressures can lead to pollution risks, because the air stagnates, descends towards the earth’s crust and is evacuated less quickly.

Under certain conditions, they can also sometimes disrupt the movement of waves, and television reception by a rake antenna. In fact, reception becomes possible well beyond the distances for which the frequencies were initially planned, which generates interference, a pixelated image and a jerky sound. The reason : Sudden changes in weather conditions: rise in atmospheric pressure, clear skies, rapid rise in temperatures, or on the contrary a sudden drop in the barometer, precise the public site Receive TNT.

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Weather. Four questions on atmospheric pressure records in Brittany and NormandyOuest-France.fr

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