Every summer, fires break out in our forests. Unfortunately, they are often the work of arsonists and sometimes the result of carelessness or accidents. It can also have a natural cause that can wreak havoc: dry thunderstorms.
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[EN VIDÉO] Kézako: where do lightning come from, from the ground or from the sky? Storms are usually accompanied by rain, lightning and thunder, but do you know where exactly lightning originates? Unisciel and the University of Lille 1 explain to us, with the Kézako program, the mysteries of this phenomenon.
In June 2017, a Forest fire triggered in the center of Portugal killed more than 60 people! According to the authorities, neither recklessness nor malevolence would be to blame this time. The cause of this devastating and deadly fire would be completely natural (about 10% of forest fires have a natural cause): a dry storm. Thus, it would have sufficed with a single tree hit by lightning so that the whole forest is set ablaze.
The Larousse defines a dry storm simply as a “storm phenomenon not involving rain”. The reality is different. Because dry storms – which form at the middle level, between 2,500 and 3,000 meters above sea level – do produce rain. But it remains imperceptible from the ground. The cumulonimbus clouds that cause it develop, for example, from altocumulus unstable with a level of condensation raised. The precipitation have all the more difficulty in reaching the ground as the air mass in the lower layer is dry.
Violent thunderstorms without rain
If the rain therefore generally evaporates before it reaches us — which themeteorologists qualify as virga –, the electrical activity dry thunderstorms, meanwhile, remains significant. Even violent. Landfills, especially intercloudy ones, are numerous. A particular electrical activity that produces, to the delight of photographers, magnificent branched lightning.
This type of storm breaks out more readily in the middle of summer. When temperatures are high and humidity low. Lightning then risks falling on a tree, which is also extremely dry. It is the immediate conflagration. the fire then spreads all the more quickly as there is no rain to slow it down. And that thewind can also get involved.
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