From summer to winter in three days. This is the phenomenon experienced by Colorado in the United States. While it was nearly 32°C on Thursday, May 19, 2022, temperatures fell to -1°C in less than 36 hours, even causing snowfall.
All things considered, significant differences have also been observed in France in recent days. The resort of Saint-Côme-d’Olt (Aveyron) thus holds the prize for the most brutal drop in maximum temperature between May 22 and 23: -16 ° C, from 36.3 ° C to 20.3 ° C, as forecaster Gaétan Heymes notes on Twitter.
How to interpret these phenomena at a time when climate change is on many minds? Christelle Robert, forecaster at Météo-France, helps us to see things more clearly.
How do you explain the phenomenon that affected Colorado?
This is called a thalweg, a barometric trough. A low-pressure axis at altitude jostled a mass of warm air at the surface on a meridian flow (from north to south). It’s a bit like throwing a punch in the atmosphere. This encounter between the mass of cold air at altitude and layers of warm air at the surface in a warm and humid atmosphere caused a rapid drop in temperature and snowfall. It is the same mechanism that is at the origin of thunderstorms.
Is it comparable with the less spectacular temperature drops in France?
Yes, the same phenomenon is at work. It also caused hailstorms which affected the Châteauroux region in particular, overnight from Sunday to Monday. With cumulonimbus clouds, normally short-lived clouds, which feed themselves and create a thunderstorm supercell. Here we also have a changeover between zonal flow (from east to west) and meridian flow.
Can we envisage one day a drop of 30°C in such a short time in France?
Drops of 20°C over a short period of time have already happened in France. 30°C, I don’t think I’ve ever seen that. It is more common in the United States, especially in what is called the Tornado Alley (area where tornadoes frequently occur). Moreover, after this sudden drop in temperatures, the cold current continued on its way towards Michigan where it caused a tornado.
Is there a seasonality to these phenomena?
They are more frequent in spring, a season of greater contrast, with more marked flows of warm air and cold air. In the fall, the phenomenon is less: the warmer seas are there to soften the masses of cold air.
Should we see in these sudden drops in temperature a consequence of climate change?
It is being studied without us being able to really determine whether or not climate change comes into play here. What we do know, however, is that it is the source of heat waves that will make this type of phenomenon likely to repeat itself and be exacerbated. Climate change does not create a new dynamic on meridian and zonal flows but it amplifies their effects.
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