Forest fires: why are dry storms so dangerous?

Every summer, fires break out in our forests. Unfortunately, they are often the work of arsonists and sometimes the result of carelessness or accidents. They can also have a natural cause that can wreak havoc: a dry storm.

In June 2021, a forest fire in central Portugal killed more than 60 people! According to the authorities, neither carelessness nor malice were to blame this time. The cause of this devastating and deadly fire is said to be completely natural (about 10% of forest fires have a natural cause): a dry storm. Thus, it would have taken only one tree struck by lightning for the entire forest to catch fire.

The Larousse defines a dry storm simply as “a stormy phenomenon without rain”. The reality is different. For dry thunderstorms — which form in the middle tier, between 2,500 and 3,000 metres above sea level — do produce rain. But it remains imperceptible from the ground. The cumulonimbus clouds that cause them develop, for example, from unstable altocumulus clouds with a high level of condensation. The precipitation then has all the more difficulty in reaching the ground as the air mass in the lower layer is dry.

Severe thunderstorms without rain

If the rain generally evaporates before reaching us — what the meteorologists call a virga — the electrical activity of the dry thunderstorms remains important. Even violent. The discharges, especially inter-storm discharges, are numerous. A particular electrical activity which produces, for the greatest happiness of the photographers, magnificent branched lightnings.

This type of storm is more likely to occur in the middle of summer. When temperatures are high and humidity is low. There is a risk that lightning will strike a tree, which is also extremely dry. This causes an immediate flashover. The fire then spreads all the more rapidly as there is no rain to slow it down. And the wind can also get involved.