Definition

In weather-observing terminology, a shallow fog is a low-lying fog that does not obstruct horizontal visibility at a level 2 m (6 ft) or more above the surface of the earth.

METAR Code

The METAR code for shallow fog is "MIFG" - MI (descriptor) = shallow, FG (weather phenomena) = fog.

Radiation Fog

Shallow fog is almost always a form of radiation fog.

On a cloudless night, especially within a high pressure system, the land surface loses heat to the atmosphere by radiation and cools. Moist air in contact with cooling surface also cools and when the temperature falls below the dew point for that air, fog forms. This type of fog is known as radiation fog.

The three conditions required for radiation fog are: