During the night when the temperature drops, the air near the ground cools more quickly than the air higher up. As a result, the cool air sinks down into any low-lying areas. This process of air flowing downhill is called "katabatic flow".

As colder, denser air sinks into valleys, it will displace the warmer air that is already there. This warmer air will start to float above the cold air, forming a layer of warmer, moist air. This layer of warm air acts like a blanket, preventing the cold air from dispersing and allowing the fog to collect.

In addition, when the relative humidity is high, water vapour in the air will start to condense, forming tiny droplets of water. These droplets will become visible as fog when they are illuminated by the sun or other light sources.

The combination of katabatic flow and high relative humidity creates the perfect conditions for fog formation in valleys. This is why you often see fog in valleys during cold winter evenings and mornings, while the surrounding hills and mountains may be clear.