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Why do temperatures sometimes become warmer on Carpathian mountaintops than in nearby valleys during winter?

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Carpathian Mountains; cross section of the Western Carpathians — Carpathian Mountains Regional division of the Carpathian Mountains (top) and a geologic cross section of the Western Carpathians (bottom). The location of the cross section is shown by the line N–S on the map.

In the Carpathian Mountains, it is common during winter for temperatures to be warmer on mountaintops than in nearby valleys due to a phenomenon known as a temperature inversion.

Under normal atmospheric conditions, air temperature decreases with altitude. However, during a temperature inversion, this gradient is reversed, and a layer of warmer air sits above a layer of cooler air near the surface. In the Carpathians, this occurs because the region's deep intermontane depressions and valleys act as basins that retain cold, dense air, while the higher elevations remain above this cold layer, experiencing relatively milder temperatures.123

This process is often exacerbated by topography; cold air formed on higher slopes tends to drain downward into hollows and valleys, thickening the layer of cold air trapped at lower elevations. These inversions are a characteristic feature of the Carpathian climate during the winter months.13