It is a striking photo, captured on Saturday April 2 by a resident of Grenoble as he descended from the Bastille. Grenoble, cut in half. On one side, the west of the agglomeration is covered with a white coat of up to 6 cm around the CEA. On the other, to the east, which is nevertheless at the same altitude as the west, no snow.
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The image did not fail to react on the networks but the question that everyone continues to ask is: “But what happened?” Serge Taboulot, meteorological engineer and former head of the Météo France center in the Northern Alps answers.
➤ How could this phenomenon have occurred?
“The explanation is simple and geographic: it’s a classic effect due to the Voreppe cluse. And for this to happen, there must necessarily be fairly strong north/north-westerly winds in the very low layers of the atmosphere, cold, humid and disturbed air. So, we have two distinct phenomena, one which concerns the west and the other the east, where the beginning of Grésivaudan is. »
➤ Why was there snow to the west of the Grenoble conurbation?
“To the north of this western part of the city, there is the cluse de Voreppe which is at the same altitude as Grenoble. When the wind – the moist, disturbed air – entered the cluse, it was accelerated and compressed a bit. Then, arrived in Grenoble, it opened up again since at the exit of the cluse, there is a wide opening on the agglomeration. The compressed air has moved away, it’s called expansion. This compression followed by expansion, without changing altitude, induces a cooling of the air. Thus, the air to the west was colder, the snow too, which explains why the precipitation was stronger and the snow cover was greater. You could call it a snow cannon effect! »
➤ And why weren’t there any to the east, near La Tronche and Meylan?
“Conversely, in the east of the department, the air coming from the north/north-west was forced to pass the Chartreuse barrier and therefore to rise in altitude before descending on the agglomeration. This rise in altitude then this descent to the level of the Isère is what is called the fœhn effect, which induces a milder air. Admittedly, on the Chartreuse, the precipitation was more abundant but arrived at the bottom, on the Grésivaudan and Grenoble, due to a less cold air, the falls were lighter or even non-existent. In some places there were not even snowflakes but rain. »
➤ Does this situation happen often?
“This happens every 4-5 years in Grenoble. Sometimes, in a very strong way like the episode of October 28, 2012 which had paralyzed the city with exceptional snowfalls. Other times, in a more dubious way like this Saturday, April 2, but in any case, we have this impression of “two distinct cities”. »
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