Weather : What Is A Dry Storm ?
Contrary to popular belief, thunderstorms do not always bring rain. When the air is very dry, a rare weather phenomenon called a dry thunderstorm can occur. We propose to discover what these stormy episodes consist of, which are of particular concern to firefighters.
Thunderstorms without rain
In mid-August 2022, dry thunderstorms made the news in France as this weather phenomenon was likely to affect the Gironde department already plagued by fires.
But what exactly is this weather phenomenon? Its name may seem paradoxical since we generally think that thunderstorms bring rain.
In reality, it happens that stormy episodes are not accompanied by no rain on the ground. Therefore, a dry thunderstorm can be defined as a thunderstorm event with no precipitation on the ground and high electrical activity.
A rare phenomenon
If you had never heard of dry thunderstorms before this summer, it's probably because this weather phenomenon is rare in France.
For such a stormy episode to occur, specific conditions must be met. This type of thunderstorm erupts generally in the middle of summer and forms between 2500 and 3000 m in altitude. Like any other thunderstorm, it does produce rain, but it does not reach the ground.
Because the air mass in the lower layer is very dry, the rain evaporates before it reaches the ground. This is a phenomenon that meteorologists are familiar with and refer to as virga.
However, the absence of precipitation does not prevent this type of storm from being accompanied by significant electrical activity and many lightning strikes. Branching lightning bolts appear in the sky that, unlike raindrops, reach the ground.
A dreaded plague for the forests
For firefighters fighting summer forest fires, the announcement of thunderstorms is not necessarily good news... Indeed, it only takes lightning to fall on a tree for an entire forest to catch fire.
In 2004 and 2005, the western United States was devastated by fires caused by dry storms. Closer to home, the deadly June 2017 fire in Portugal also had the same natural cause.
A dry thunderstorm is all the more dangerous to forests because it occurs when temperatures are high and atmospheric humidity is low.
Because vegetation is so dry at this time, the fire risk is maximum. Vegetation ignites immediately when lightning strikes and the fire spreads very quickly since there is no rain to slow its progress.
After the drought, risks of flooding
As France experiences record Drought this summer, one would think that the arrival of thunderstorms and rain would be good news. Unfortunately, when the air is very warm, it is severe thunderstorms that form, due to air mass conflict (because of the meeting of cold and warm air).
These thunderstorms can result in intense rainfall, hail, and powerful wind gusts which are not the best weather conditions to remedy dry soils.
As during a Cevenol or Mediterranean episode, it can fall in 48 hours the equivalent of 3 weeks to a month of rainfall. This sudden and very localized rainfall does not manage to infiltrate the hardened soils.
The water does not penetrate the earth but runs off, which can cause flooding and 'flash floods' that are particularly dangerous for people living near waterways.
The dangerousness of these phenomena explains why Météo France often places departments under vigilance for risk of thunderstorms, shortly after the periods of vigilance for heat waves:
???? 5 dpts in #vigilanceOrange
— VigiMétéoFrance (@VigiMeteoFrance) August 16, 2022
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