Fog

Fog is simply cloud at ground level.

It forms when moist air near the surface is cooled to its dew point, causing the air to become saturated and condensation to occur.

The closer the air temperature is to the dew point, the greater the chance of fog forming.

For example:

OAT 8°C / Dew Point 6°C

The temperature and dew point are only 2°C apart, so the air is close to saturation and fog may form if further cooling occurs.

Radiation Fog

Radiation fog forms mainly at night over land when the ground loses heat by terrestrial radiation and cools the air immediately above it.

For radiation fog to form, the air should generally be:

• moist;

• stable; and

• accompanied by a light wind, typically around 2–8 knots.

With no wind, cooling may mainly produce dew at the surface.

A light wind helps mix the cooled air through a deeper layer, allowing fog to develop.

Radiation fog is most likely:

• during clear or nearly clear nights;