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Heavy floods have hit Germany, Belgium, the Netherlands and Luxembourg for a few days, killing at least 126 people, including 103 only in Germany, the heaviest toll of a natural disaster in the country since the post -war. At the origin of this tragedy, the meteorological phenomenon known as “the cold drop”, and climate change. François Gourand, meteorologist at Météo-France, returns to 20 minutes on the situation and its causes.
What are these floods due to?
The phenomenon observed in Germany, Belgium, the Netherlands and Luxembourg is what is called a cold drop, that is to say a pocket of cold altitude air, generating humidity and rain. The cold drop is in itself a very classic phenomenon, mainly observed in spring, although it is also found in summer. It is this typical phenomenon of rainy spring days, quite common among us in France.
The cold air comes from the North Atlantic and the polar regions, it glides towards the south and comes off being surrounded by warmer air. Once in contact with warmer and humid conditions, this creates a circulation loaded with humidity and abundant rain. In spring, the air is still very cold after winter, especially at altitude, while the sun heats the lower layers of the atmosphere, and this hot-cold contrast explains the regular number of cold drops, but there is nothing shocking about it in summer.
What is less classic is the intensity and immobility of this phenomenon. Normally, a cold drop moves quickly, this is not the case. The rains poured down for hours on the same places and with a rare intensity.
How to explain such an immobility of the cold drop?
There is no scientific consensus, but we can try to explain it by observing that it has been hot and dry for several weeks in Scandinavia, but the hot and dry air is stationed at high altitudes, therefore blocking the circulation of the cold drop. The weather is also hot, even very hot, in Central and Eastern Europe, which blocks progress towards the East, when heat waves in the South, towards the Mediterranean, block the passage there too. The cold drop would therefore be a little constrained on all sides, which would explain its immobility.
The German president called for action against climate change in the wake of this disaster. Are the current floods due to global warming?
We can answer in two parts. First, this cold drop phenomenon has always existed in weather, and a cold drop is not linked to climate change, it is a phenomenon that can be observed in summer and in this respect the current situation n nothing surprising or historical.
On the other hand, that there is so much rain is undoubtedly the result of climate change. Without the rise in temperatures, it would have rained less, the phenomenon would still have taken place but not in similar proportions. The hotter the air, the more water vapor in the atmosphere, and it is this same water vapor that turns into rain.
So there is a risk that there will be more and more phenomena of this type?
There have always been heavy floods and floods, and there will always be, but expect them to be more powerful and violent than before due to rising temperatures.
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