By Mathieu Normand
Published on 4 Jan 21 at 15:58
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Some have been waiting for it or even hoping for it all month of december, and again Monday January 4, 2021, in vain: the snow did not fall to Rouen. A few small flakes have flown well in recent weeks, but they were quite incapable of whitening the ground unlike those which fell inland, in Seine-Maritime and elsewhere.
But why does snow never seem to fall in big cities again around Christmas time? Here are some answers …
It is hotter in town than in rural areas
A cold air mass is needed to allow the snow to hold. “It can come mainly from the north, and cause snow showers on the entire coast, or be of continental origin from the east,” said Patricia Moulin, Météo France. We can also have snowfall when a very cold air mass has settled on the territory and a softer mass arrives by the Atlantic. “
To know if the snow crystals that form in the clouds will hold, you have to observe the air masses that they cross vertically. They indeed influence the nature of the downpour on arrival. “It all depends on the level of iso zero”, indicates the specialist, ie the altitude at which the temperature reaches zero degrees.
Temperature is therefore the key element. “With urban warming, it only takes one degree to go from snow to rain,” explains Patricia Moulin. However, it is often hotter in cities than in rural areas, due to urbanization and greater human activity.
Very mild December months since 2011
In addition to the higher temperature in town, the months of December were very mild, particularly since 2011. The last was no exception, in Rouen. If the feeling may have seemed quite contrasted, with a rather cold end of the month, the average temperatures over the whole period are around 2 ° C above the seasonal norms, at 6.1 ° C.
“The second ten days were particularly mild, with minimum temperatures in the morning around what there should be maximum during the day, underlines Patricia Moulin. On the night of the 21st to the 22nd, we even broke records of softness. “
The showers were significant, with 143 mm of water falling in the month in the Normandy capital, including “six or seven days with about 10mm of rain”, but hardly any snowflakes, therefore!
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