2023-05-14 13:06:01
It is a term well known to meteorologists and widely used in this month of May with changing weather, such as this Saturday when a radiant sun in the north of the country gave way to sometimes stormy showers. After a month of March and April wetter than average in some French regions, a phenomenon of “cold drop” must still postpone the arrival of fine weather.
In its forecast bulletin for the next few days, Météo France indeed anticipates “two cold drops at altitude which will successively descend on the country, maintaining an unstable trend in most regions”, in particular over a large southern and eastern part of the country this weekend. They should even make the weather more than uncertain for the next 10 days.
According to the definition of the public establishment, a cold drop is “a pocket of very cold air located at an altitude of more than 5,000 m. When the polar jet stream deforms, it happens that a pocket detaches from the circulation associated with the polar jet stream to descend to our latitudes, which is nicknamed a cold drop. Its positioning can fluctuate, depending on the surrounding warmer air masses.”
She can be the size of a country like France
It is, more concretely, cold air at altitude (-20°C to -36°C at 5,000 m) which comes into conflict with warmer flows and bursts into rain. “These areas of low pressure are associated with an unstable air mass. They often cause disturbed weather, with showers, sometimes consisting of sleet to the ground, associated with strong gusts of wind. They can also generate milder and more humid air rising from the Mediterranean,” specifies Météo France.
“A cold drop can make the size of a country like France. They generally come from the Atlantic. When we manage to identify it, it is a strong signal that the air mass is unstable,” explains Jérôme Lecouforecasting engineer at Météo France.
This phenomenon, which is difficult to predict, is usually associated with spring but can also occur in summer, disrupting the sun and the heat expected at this time of year. A cold drop had thus made July 2021 one of the rainiest in the last six decades.
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But even with these spring rains, which are always welcome, regions around the Mediterranean that will not benefit from them risk running out of water this summer, due to a lack of sufficient recharge this winter.
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