Why valley fog forms, and how to be there when it does

The single most photographed condition in valley landscape work is also the most predictable, and almost nobody treats it that way.

What is actually happening

Radiation fog forms when the ground loses heat to a clear sky overnight, cooling the air immediately above it to its dew point. The water vapour condenses, and because cold air is dense it drains downhill and pools in the valley floor.

That is why it sits where it sits. Fog in a valley is not weather that happened to arrive; it is cold air that ran downhill and stopped.

The three numbers

Every forecast publishes them.

Cloud cover overnight. It has to be near zero. Cloud acts as a blanket and stops the ground radiating heat away. A clear night is the precondition and nothing else substitutes for it.

Wind speed. Between roughly two and eight kilometres per hour. Still air lets a shallow layer form and stay put. Anything above about ten mixes the layer and destroys it before dawn.

The gap between the overnight minimum and the dew point. This is the one people never look at. If the forecast low is close to the dew point, the air will reach saturation. If there is a gap of several degrees, it will not, however clear and still the night is.

Clear, calm, and a low that meets the dew point. All three, or nothing.

Timing

The fog is deepest just before sunrise and burns off from the edges inward as the sun reaches the valley sides. The window where the light is on the fog but the fog is still intact is frequently under twenty minutes.

Be in position before civil twilight, not at sunrise. By sunrise the interesting part is already half over.

Height matters more than position

The difference between standing in fog and photographing fog is a hundred metres of elevation. Get above the layer. From below it is grey; from above it is a white sea with ridgelines coming out of it, which is the picture everyone is actually after.

Scout the height in daylight. Finding it in the dark, on the morning it works, is how mornings get wasted.

After autumn

Radiation fog needs long nights, so it is a late-autumn and winter phenomenon in most temperate places. In summer the nights are too short for the ground to cool enough, however clear the sky.

That is worth knowing when you are searching a catalogue for a fog frame in June and finding nothing shot that month.