Valley fog formation

Breathtaking images of fog were recently captured by user Boris Srdos in Berikon, Switzerland, so we thought we'd take this opportunity to explain how it forms.

Fog, by definition, is a thick, low-level cloud that causes a reduction in visibility of less than 1000m, and though beautiful from above, can in fact be dangerous when travelling within such dense pockets.

We tend to see more fog in the spring and autumn months, however, as we head towards summer, it becomes less frequent.

Several different types of fog exist, but the fog pictured above is known as valley fog, or, radiation fog. This type of fog can only develop in clear, calm conditions overnight and is exclusive to mountainous areas.

In the general profile of the atmosphere, temperature decreases with height, by around 1C every 100m. So as a rough rule of thumb, the top of a mountain, standing at 1000m, would be 10C lower than the temperature at the valley bottom.

After sunset, in clear, calm conditions, the ground will begin losing heat via radiation to the atmosphere.

However, cold air is heavier and more dense than warm air, so under clear skies and light winds, will drain down the valley sides and eventually pool at the valley bottom. This downslope wind that develops, is called a katabatic wind.

This in turn produces a temperature inversion, with the coldest air gathering at the bottom of the valley, and temperature increasing with altitude.

The temperature at the valley bottom then cools to the dew point temperature, allowing the air to become saturated and condense into water droplets, consequently enabling the formation of fog.

If the wind were to pick up at any point however, this would cause the air to mix and the fog to dissipate.

Fog tends to become most dense around, or just after sunrise, when ground temperatures are at their lowest, so the early risers amongst us may be more familiar to such a sight!

Have you got any pictures you’d like to share? We’d love to see them, send them via our uploader!