Last night the temperature in Bütchenbach dropped to -0.4°C. Remarkable, because a national heat wave is currently taking place. Why is there night frost in the middle of summer? And is this unusual?
In the night of 14 to 15 June, the temperature in Bütchenbach, Belgian Ardennes, reached a minimum of -0.4°C. This light night frost is striking given the current national heat wave. Mürringen (0.4°C) and Braunlauf (1.8°C) also experienced significant cooling. And that while the temperature did not fall below 10.5 ° C at night at the highest point in Belgium, Signal de Botrange. It is only 13 km from Bütchenbach. A remarkable contrast.
How is this possible?
This cooling can be attributed to several factors, namely a clear sky, a dry landscape, little wind and the presence of a gently sloping valley.
Bütchenbach is located in an area known as a cold hole, which means that the temperature here often drops more than in the surrounding region. The area is therefore sometimes referred to as the Belgian Siberia. A cold hole is an area where cold air accumulates during clear, windless nights. During such conditions, the ground cools down quickly at night due to the radiation of heat. Due to the higher density and greater weight of cold air relative to warm air, it is attracted by gravity and moves to lower positions. At the onset of temperature drops, the cold air flows into the valleys.
Bütchenbach is therefore located in such a high valley surrounded by the peaks of the High Fens. In this valley, the cold air accumulates around the mentioned villages, which has earned the region the nickname “Belgian Siberia”. On the other hand, the surrounding peaks, such as Signal de Botrange, remain relatively warmer. The dry weather of the past few weeks may also have played a role. A drier soil heats up faster and cools down faster.
Is this unusual?
Although frost in the Ardennes is more common during the summer months, it is not the case every year. For example, on June 26, 2013, there was a temperature of -0.1°C in Elsenborn (Bütchenbach). Based on data from the RMI, it freezes on average 0.2 days in June in Bütchenbach, which corresponds to a frequency of about 1 time per 5 years
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