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.
The METAR code for shallow fog is "MIFG" - MI (descriptor) = shallow, FG (weather phenomena) = 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:
- Clear skies;
- Moist air; and,
- Light wind.
Initially it may be mist that forms and then thickens into fog as the temperature drops and more water vapour condenses into water droplets in the air. Air does not conduct heat very well so in still air conditions fog may not form at all and a layer of dew or frost will form on the surface instead. However, if there is a light wind of around 5 kts, then this will mix the air in contact with the surface and the layer of fog will be thicker. With stronger winds, the fog may lift to form layers of Stratus. Thicker instances of radiation fog tend to form in valleys or over calm bodies of water.
As the sun rises, and the surface temperature increases, the air in contact with the surface will warm and the fog will gradually disperse or may rise to form a low layer of stratus.
If the fog is particularly thick, then it may prevent the sun from heating the surface and the fog will not clear. This situation is common in the autumn in northern Europe when some airfields may be affected by fog for many days. Radiation fog also is very common throughout the United States during the fall and winter months.
For information on flight safety considerations, see the separate article Shallow fog: Guidance for flight crews.