Gusts of more than 100 km / h, hailstorms and floods affected several places in France on Tuesday, where 17 departments were placed on orange storm vigilance in the South West, after the south of the country was plagued by daily thunderstorms for almost a month.
This weekend, the north and west of France were also affected by heavy rainfall. A tornado thus reached Normandy, around 11,000 homes were deprived of electricity in the Val-d’Oise and train traffic was severely disrupted on Sunday evening, while the Eiffel Tower was struck down in Paris.
3:30 this morning. Nice thunderbolt on the Eiffel Tower. Photos taken with my 2 boxes since the 15th.
Françoise Vimeux, climatologist, research director at the Institut de recherche pour le développement (IRD), where she studies thunderstorms, tells us more.
Viewpoint: Are thunderstorms getting stronger and more frequent?
Françoise Vimeux: We don’t have enough long and fine-grained observation in spatial terms to see a trend in the evolution of thunderstorms over the last decades. Thunderstorms are very local phenomena: they can escape our observations and remain difficult to predict. We can identify the conditions conducive to their triggering in a region or department, but we cannot locally predict the place they will affect.
These phenomena are too complex and too small compared to the mesh of meteorological observations. In addition, their propagation, location and maintenance depend on a lot of parameters, such as the state of the surface or the relief. It is however possible to locate them quite precisely one hour in advance, thanks to radar images.
However, we see that thunderstorms occur earlier in the season: as it is warmer, they burst in June, when they would normally be expected rather in the heart of summer.
And, in the future, will global warming have an effect on thunderstorms?
If we do not observe robust trends in recent years, climate projections tell us about the future. In a warmer climate, the atmosphere can hold more moisture: the evaporation demand of the atmosphere is said to increase. Every time the atmosphere warms by 1°C, it can hold 7% more water vapour.
More moisture in a warmer surface atmosphere increases convective instability. Basically, moist, warm air will be carried aloft by updrafts. As it rises, it cools and the water vapor generates droplets, which form clouds. If these updrafts are very strong, with very humid air, these clouds rise even higher in the atmosphere. Theoretically, we can therefore see that a warmer climate, and therefore more humid on the surface, offers more favorable conditions for the formation of storms.
We also know that the rains associated with severe thunderstorms will intensify because the atmosphere will be able to hold more water. In 2050, so-called violent thunderstorms, those that start suddenly, or those that rise very high and form hail will therefore potentially have a greater accumulation of rain than currently. Indeed, we know that the rains will increase by 7% per additional degree, even, in certain stormy conditions, by 15%. Imagine: in a scenario with a 4°C increase in France, that makes the rains 28% heavier. Cyclones in overseas territories will also be wetter.
Finally, what also changes is the way it will rain: in summer, we are going towards longer periods without rain, and heavier and more abrupt rains.
What are the risks and how do you guard against them?
Rainier storms increase the risk of flooding: water that falls on dry ground in summer cannot infiltrate it and runs off more to join watercourses, which then risk overflowing. In addition, these summer rains do not benefit groundwater, since they are mostly captured by vegetation.
In cities with impermeable ground, rainwater drainage networks are no longer suitable. They were designed for 20th century rains, and already overflow in Paris during severe thunderstorms.
The flood risk is also one of the four key risks identified in Europe in the context of global warming.
To protect yourself as well as possible, you must de-impermeabilize the soil, review the evacuation of rainwater and not build on flood-prone areas. But more thunderstorms also means more lightning, and therefore more risk of a fire starting.
It also means more high winds, like we had this weekend in Paris: and, against a soaring roof, there’s not much to do. It is not possible to protect against everything, so mitigating global warming is very important to limit the degree of warming.
