DECRYPTION – This phenomenon appears when, under very specific meteorological conditions, water molecules in a cloud encounter impurities emitted by human activities.
They are flakes like any other, or almost. This Saturday in Loiret, Seine-et-Marne, Essonne and Oise, residents were surprised to wake up to snow… but so-called “industrial” snow, because it was linked to the encounter between very specific anticyclonic conditions and atmospheric pollutants. Indeed, indicates on its website La Chaîne Météo*, « this snow can form in a context of dry weather in France, unlike “natural” snow whose formation within a cloud is made possible thanks to the presence of disturbances ».
« Weather-wise, these snowfalls occur in calm, cold weather in the presence of a powerful winter anticyclone with a strong temperature inversion »indicates Météo France on its website. Cold, moist air gets stuck near the ground, where the temperature is colder than at altitude. « This inversion acts as a lid » et prevents “ dispersion of humidity and pollution ». Pollution adds the element that will allow snowflakes to form. « It is not a “fake snow”, but a snow whose formation conditions are made possible thanks to the industrial activity of man »specifies La Chaîne Météo. The phenomenon is generally very localized.
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Their formation follows the same rules as “natural” snow. For snow to form, two elements are needed: a cloud containing water molecules that remain liquid despite temperatures well below zero (they are said to be “supercooled”), and an impurity (dust, grain of sand…). When the two intersect, the water molecule freezes, encountering other droplets which freeze on contact, gradually forming aggregates of microcrystals which, under their own weight, obey gravity and fall to the ground, in the form snowflakes or, if they cross warmer layers of air, raindrops.
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In industrial snow, the impurity that forms the first “core” of the snowflakes is a pollutant microparticle. For chemical reasons, « this happens at less cold temperaturesChristelle Robert, forecaster at Météo-France, explained to us in 2017. With any dust, water can remain liquid down to -10 or -12°C, but with certain pollutants, it will become solid more easily, for example at only -3°C. » This often forms very fine snow.
The phenomenon is difficult to predict. On the one hand, this sometimes plays out to within a degree, and in very specific weather conditions with calm and dry weather. Furthermore, weather models do not take into account pollutant emissions. For the phenomenon to take place, there must also be an absence of wind (which would disperse them). Industrial snow is not dangerous for health, and neither more nor less “dirty” than natural snow.
*The Weather Channel is a group company Figaro.
