While it is drier and sunnier at the sea in spring and summer than inland, it rains more often in the west of our country in autumn. This time of year we can expect more rain in West Flanders, although the amounts of precipitation that the west has received this month are exceptionally high. In West Flanders, up to 10 times as much precipitation fell in some places than in the center of the country. Why does it rain more in the west in the fall and why is it exceptionally wet this fall?
In October and November it rains on average about 20 to 30% more in the coastal region and the polders than elsewhere in the country. We can expect more rain every autumn in West Flanders, but over the past 10 days the differences in precipitation amounts have been particularly large. At the RMI in Uccle about 37 mm of rain fell, while in West Flanders they had to process more than 100 mm in many places. In Poperinge the counter even stands at 140 mm of rain, which is exceptionally high.
Everything has to do with the temperature of the sea water, the cold upper air and the southwest wind
In the autumn it rains more often at the sea and in the west due to the so-called coastal convergence. Convergence means that air flows together from different directions and is forced to rise. At sea this happens due to the difference in friction between the water surface and the land. Above water the wind has complete freedom, but once above the rough surface of the earth it is strongly slowed down by dunes, buildings and trees. This causes the air to accumulate and can only go in one direction and that is upwards. Rising air cools and condenses, creating clouds and raindrops. The more friction, the more the direction of the wind will also change. Once the wind comes on land, it will shrink (turn counterclockwise) and that also makes it easier for showers to form.
Of course, the wind will experience a difference in friction between land and sea in every season, but these showers are still a typical autumn phenomenon. So there must be another important ingredient: the temperature of the sea water. In the autumn the sea water is still quite warm, but colder air gradually flows to our regions from the north. This creates a large temperature contrast between the sea water and the upper air, making it much easier for showers to form. When the temperature difference between the Earth’s surface and the air at an altitude of 5 km is more than 40 °C, thunderstorms can even occur.
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We see the most intense coastal convergence when the wind blows more or less parallel to the coast. Due to the orientation of our coast and the Channel, this means a south-westerly wind for us. The showers above West Flanders are then fed by the warm seawater of the canal. New shower clouds are constantly forming that pass by as elongated and relatively narrow cloud bands parallel to the coast. They can pass over the same region for hours, which is why we speak of rain trains that can cause enormous amounts of precipitation. In the coastal region or the polders, 20 to 30 mm can fall in a few hours, while it remains completely dry a few kilometers further on. When the showers move a little further inland, they lose their energy due to a lack of supply of moist air. The showers usually die out once they reach East Flanders, which means that much less precipitation falls inland in the autumn.
Every year we can expect more rain in the west, but this year the differences with the center of the country are exceptionally large. This is mainly due to the high temperature of the sea water in combination with depressions that are rapidly moving across our country. The temperature of the sea water in the Channel and the North Sea is about 15 degrees, which is a few degrees more than average at this time of year.
Such a rain train can discharge tens of liters of water in just a few hours.
Moreover, in recent weeks we have often had to deal with depressions that move over our country, they bring with them cold upper air, which further increases the temperature difference between the sea water and the upper air. At an altitude of about 5 km, the temperature today is -30 degrees, which means that showers can form much more easily than usual and we have also often had to deal with a south-westerly wind, which could create trains of showers that passed over the same region for hours. . Such a rain train can discharge tens of liters of water in just a few hours.
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