While most of the country experienced scorching temperatures in recent weeks, one anomalous place in Belgium dipped below freezing last week.
When most Belgians were desperately trying to stay cool in the summer heat, in Bütgenbach, a high valley some 600 metres above sea level in the Ardennes, there would have been no such problem.
The area, dubbed the "cold hole of Belgium" and "Belgium's Siberia", experienced a storm of weather variables which caused the temperature to drop to -0.4°C during the night of 14 June, when the overnight temperature was in the mid-20s elsewhere in the country.
Freezing in a heatwave
"There are a number of factors that contribute to these lower temperatures," Alexandre Dewalque from the Royal Meteorological Institute of Belgium told The Brussels Times.
"Clear skies is one factor. When the sky is cloudy, heat is trapped in the lower atmosphere. When the sky is clear, infrared radiation is emitted from the surface to higher levels and space so the heat can escape, leading to a loss of energy at the surface and rapid cooling of the ground and the air directly above."
"Low wind speed is another factor," Dewalque explains, as wind "mixes the cooler air near the ground with the warmer air". This reduces the nocturnal cooling near the surface, but this mixing does not occur with low wind speed.
Dry atmosphere also contributes to low temperatures. "Air vapour is a source of infrared radiation. Consequently, if the air is dry, with little water vapour, less infrared radiation is emitted from the atmosphere to the ground, resulting in greater cooling."
"The topography also plays a role," he added. "In the hills and valleys, the density of cold air is greater than that of warm air. As a result, the coldest and densest air accumulates at the bottom of valleys."
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On the night in question, all these factors came into play in the municipality of Bütgenbach. However, it was not the only place in Belgium to feel a chill in the heatwave last week.
It cooled down significantly in Mürringen in the province of Liège, where temperatures also dropped below freezing, and in Braunlauf in the German-speaking community which shivered at 1.8°C.
Both areas – low valley points in high regions – experienced the same culmination of factors which made the temperatures drop so dramatically.
