Why can Alpine valleys be foggy and colder than higher slopes in winter?

In the Alps, valleys can be colder and foggier than higher slopes during winter due to a phenomenon known as a temperature inversion.12

Under normal atmospheric conditions, air temperature decreases with altitude. However, during an inversion, a layer of cooler air becomes trapped at the surface, overlain by a layer of warmer air. This occurs frequently in the Alps during autumn and winter because cold air is denser than warm air; consequently, cold air formed on higher slopes drains downward and settles into the valley bottoms.1

This process leads to several conditions:
* Stagnant Air and Fog: Because the inversion acts as a cap, it limits vertical air movement and convection. This traps moisture and pollutants in the valley, often resulting in persistent fog and stagnant air for days at a time.12
* Temperature Disparity: While the valley floors remain cold and foggy, the higher elevations—often above 3,300 feet—can be significantly sunnier and warmer.2

Topography plays a critical role in this, as the hollows of the valleys collect the cold air, creating a thicker inversion layer than what is found at higher elevations.1