What is radiation fog and why does it form in plains and valleys?

Arab Weather - Fog often forms in the fall and winter, especially during the dawn and early morning hours, but there are several types of fog. Some of it forms on clear days without weather conditions and is called radiation fog, and some of it forms with weather conditions due to clouds touching the Earth's surface.

In this report, we will focus mainly on radiation fog due to its formation on Sunday morning, 12-1-2025, until the time of writing this report.

What is radiation fog?

It is a form of fog or in other words a form of water vapor condensation.

Radiation fog occurs at night (or occurs late at night and early in the morning hours) above the Earth's surface when the Earth loses heat through radiation and cools, and thus the air in contact with it cools. If water vapor is available and the air temperature reaches the dew point or below, radiation fog is formed, provided that the wind speed is light and not still, so as to allow the warmer air to pass over the cold surface and touch it, and thus cool to the dew point or below.

Why is it formed in plains and valleys?

Since cold air is heavier than warm air, the cold air that has been cooled by radiation descends from the sides of hills into valleys, and therefore fog can easily form in such places, especially if they are humid. In such places, radiation fog is often called valley fog. This type of fog occurs most frequently in the fall and winter, and reaches its highest density in the early morning when the sun rises above the horizon.

Ideal conditions for the formation of radiation fog:

• Clear sky (no clouds to obstruct thermal radiation).
• Light winds (to avoid mixing cold air with warmer air).
• High humidity.
• Air temperature reaches dew point

This type of fog is common in the fall and winter, when the nights are longer and there is a greater chance of heat loss.

How does radiation fog disappear and fade?

• Solar heating.
• Increase wind speed.
• Heating by descending air.
• The wind changed direction and started blowing from a new source.