Storm Ciaran: how do we calculate the severity of such a weather phenomenon?

2023-11-02 07:13:24

A “major” storm on a national scale, undoubtedly “exceptional” for the three Breton and Normandy departments placed on red alert for violent winds. Here is how Météo France presented, on Wednesday, the Ciaran depression which affected the north-west of the country on Wednesday evening and especially during the night from Wednesday to Thursday.

At 4 a.m., gusts of more than 170 km/h or even nearly 200 km/h have already been measured in different places on the Breton coast, especially in Finistère. Inland, 150 km/h was exceeded locally. These values ​​are somewhat beyond the forecasts issued several hours earlier.

It will remain to be precisely established the severity of this “weather bomb”, in the sense that the pressure at its center decreased by more than 24 hPa in 24 hours. To then be able to compare it with previous storms.

For each of them, Météo France calculates a severity index with a mathematical formula established in 1980. This depends on the surfaces (as a % of French territory) of the zones affected by gusts greater than 100, 120, 140 or 160 km/h. Rainfall and possible marine submersion are not taken into account, because wind is the primary characteristic and consequence of a storm.

In Brittany, the “hurricane” of 1987 is the worst storm

On this basis, the two depressions of late December 1999, Lothar and Martin, appear to be the two most severe in more than 40 years. Each time, more than half of the country was affected. Only one storm among those in the Top 10 dates from the 2000s: Xynthia, at the end of February 2010.

Ciaran will remain very far from the two meteorological “monsters” of the end of 1999 on a national scale, given that only the north-west of the country (i.e. around 20% of the territory) will have experienced wind gusts greater than 100 km/h , the minimum threshold to be reached. On the other hand, it promises to be among the most severe storms in Brittany, where the wind blew at more than 110 km/h including inland.

First place in the ranking is this time occupied by the “hurricane” of mid-October 1987, ahead of Lothar. This term “hurricane” is used in quotation marks because in theory it only applies to tropical cyclones, whereas here we are talking about a large-scale depression. But its damage “was equivalent to that of a hurricane of force 3 on the Saffir-Simpson scale”, indicates Météo France.