A condensed, succinct, local overview of climate extremes

They characterize the landscape with their short and destructive corridor: extreme weather events will be more frequent and intense according to Part Two of the Sixth Report of the Intergovernmental Panel on Climate Change. Despite the somewhat conciliatory climate in mainland France, the increase in average temperatures reveals exceptional events that some regions are not accustomed to. Between the intense heat wave and the downright wind, a short review of the brief, sudden and devastating accidents that can affect our country.

front shock wave

The phenomenon of a storm front, nicknamed Galerne, Embata or Borouillarta, is commonly known in southwestern France where it results in sudden uplift of winds, from 0 to 70 km/h on average, and temperatures dropping of the order of 10°C in a few tens of minutes. This powerful storm causes significant damage to the Earth, and is associated with the leading edge of a cumulonimbus cloud formed by differences in pressure and temperature between the ocean and the coast. This same cloud, as if announcing its arrival, allows you to make a strong and cold cloud. Violent storms generate a wall-shaped wave front of giant clouds, advancing quickly and visually impressive. Often confused with small tornadoes, these storms cause damage that can be just as important, but the process of their formation is completely different.

Therefore, this phenomenon is known in the southwest of France because the geography of the Bay of Biscay allows the concentration of storm cells far from the shore: then they experience the intense heat of the Basque Coast. The gusts of wind that occurred in Calvados in the early evening of June 18, 2022, were caused by the same phenomenon, as winds gusts at a speed of 70 km / h and a decrease in temperature of about ten degrees in less than an hour were recorded. This very rare episode in this region was made possible by the difference in temperature between the hot coast, caused by the previous heat wave, and the cold ocean currents.

(Accelerated video of the storm front, which took place on June 18, 2022 in Le Havre)

Wind speed and wind velocity curves of the Deauville station from 16 to 20 June 2022 (Source: infoclimat.fr)

Hair dryer effect of “heat blast”

This particularly rapid phenomenon, also called “sunstroke” in French, leads to gusts of wind. It also results in a rapid rise in temperature and decrease in humidity within minutes. This singularity is observed almost everywhere on the globe, and it comes from the pressure of hot air on the ground at the end of a storm. During such a loop, warm surface air is absorbed and stored aloft. When this storm begins to weaken, the air stops rising and quickly descends. This flow then pushes air masses from the cloud to violently compress and heat them toward the ground, creating strong, hot, dry, topical winds.

The last recorded heat stroke in France occurred in Port Vendres, in the Pyrenees-Orientales, on June 15, 2022. The city experienced winds of more than 150 km/h, which suddenly raised the temperature from 22°C to 37°C in a few Dozens of minutes. This very hot dry air replaced the more humid surface air, also lowering the humidity from 80% to 16%. The effect of this real “hair dryer” is so violent that plants lose all their water within a few minutes with the massive increase in evaporation and transpiration. Agricultural climatologist Serge Zaka explains in a tweet.

Whole dried vines on Donadil Farm (agricultural school in Nim Rohelan). The photo was taken on Saturday, June 29, 2019 (Source: Lucas Schneider)

The bigger the storm, the bigger the hailstones

Although the incidence of hail in France is higher than that of other phenomena, hail attacks are short and devastating. Formed in the heart of storm clouds with strong vertical currents, hailstone embryos form from freezing water droplets in very hot and humid air masses that are pushed to very cold heights by the rising current. As they travel, hailstones grow by breaking up nearby supercooled water droplets, forming concentric circles around their core. Thus, the more a hailstone passes through a mass of moist air, the greater its volume. So the clouds with the strongest currents have the potential to generate large hail that can clump together until they are heavy enough to overcome the force of the currents and fall. Hail storms are often short and limited, concentrating force in the same area, thus increasing damage to the land.

Large hailstorms that followed the warming in June 2022 caused extensive damage to infrastructure. Almost everywhere in France, buildings, cars, and crops were affected by extraordinary sizes of hailstones. Usually no more than 2 or 3 cm in diameter, strong storms from high temperatures caused hailstones to fall from 5 to 7 centimeters. Punctured roofs, broken windshields and devastated fields, southwest France suffered extensive damage. The world record for the volume of cold was set on February 8, 2018 in Villa Carlos Paz, central Argentina, with blocks of ice measuring 23 centimeters long.

Extreme cold occurred at Villa Carlos Paz in the central Argentine province of Cordoba on February 8, 2018 (Source: University of Pennsylvania)

Debris flow: unstoppable, brutal and sudden

More than just a short, powerful event, lava, as its name suggests, spreads into torrents (steep corridors, buried in the ground with an erratic flow). So it’s very (very) local. Mainly in areas with high terrain such as the Alps or the Pyrenees. Even if this phenomenon concerns only a small part of the area with a low density of habitation, it indicates its speed and destructive power. This wave of mud, block of stone, water and plant debris of all kinds is caused by precipitation concentrated on a height. Drop by drop, stone by stone, more and more items begin to roll down the slope, carrying everything in their path with extreme speed and brutality. The accumulated energy is such that blocks of stone weighing several tons can be carried and transported at a very high speed. The sound of the crash of this unstoppable wave could be perceived for several kilometers.

Debris flow from the St-Andre Ciel in Chamoson, Switzerland, on August 7, 2018. 20,000 square meters3 Lava flowed. (Source: YouTube: Francois Fauvray).

The damage can be likened to the simple uprooting of trees or dropping mud on roads and homes, but it can amount to destroying protective infrastructures and transporting victims on its way. Many systems have been put in place to avoid human casualties, particularly with alerts and appreciable slowdown thresholds.

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