Fog — types and formation

Fog is a cloud at the ground — tiny water droplets or ice crystals reducing visibility below 1000 m.

Source: WMO International Cloud Atlas (fog section); AMS Glossary; ICAO Annex 3.

Definition

Fog is very small water droplets or ice particles floating in the atmosphere causing a reduction of visibility below 1000 m:

Visibility < 1000 m → fog.
Visibility 1000-5000 m → mist.
Visibility > 5000 m due to dry aerosols → haze.

Vertical visibility

The term "vertical visibility" on the ground means visibility measured vertically upwards or estimated using buildings or masts in fog or snowfall.

Main fog types

The main types of fog are radiation fog, advection fog, mixing fog. Plus evaporation fog and valley fog.

1. Radiation fog

Radiation fog is the most common type of fog encountered over land masses in Central Europe.

Formation:

Clear, calm night with little cloud cover.
Small spread (T and Td close together).
Light wind (0-3 kt, some mixing).
Ground cools by IR radiation.
Near-ground air cools with it to the dew point → condensation.

High-risk conditions (DWD):

Few clouds, weakening wind, valleys and wells with moist ground promote the formation of radiation fog.
Clearing of the sky with a small temperature-dew point spread, especially in autumn and winter during night and morning hours with calm winds, strongly suggests the formation of radiation fog due to rapid radiative cooling.

Basins and wells: Basins and wells with moist ground are beneficial for mist and fog development as they trap cold air drainage and provide sufficient humidity for condensation during calm, clear nights with radiative cooling.

Formation via radiation: Cooling and condensation caused by radiation in layers of air close to the ground on clear, calm nights leads to radiation fog formation when the temperature reaches the dew point.

Dispersal: Increasing movement of air and heating are beneficial to the dissipation of radiation fog — morning sun.

2. Advection fog

Advection fog develops when warm, moist air flows over cold surfaces.

Formation: Advection fog occurs when warm, moist air is transported (advected) over a colder surface, such as coastal waters or land, leading to cooling below the dew point and condensation into fog.

Typical: coastal regions, snow-covered surfaces in winter.

3. Mixing fog

Mixing of warm, moist air with cold, moist air are weather conditions favourable for the development of mixing fog.

4. Evaporation fog ("sea smoke")

Evaporation fog is independent from the temperature of the surface. Flow of very cold air over warm bodies of water with a high rate of evaporation causes the development of evaporation fog:

Cold air (winter) over open water (lake, river).
Water evaporates quickly → vapour condenses in the cold air.

Fog → high fog transition

Increase of wind with mixing of layers near the ground and decrease of dew point just above the surface is the reason for fog often transitioning to low stratus (high fog before noon):

Wind mixes warm upper air into the fog.
Fog lifts → becomes stratus.

Fog in winter high

Fog or high fog can develop below a surface inversion, the type of weather to be expected in the centre of a dynamic high in winter:

Subsidence inversion in a winter high.
Surface air trapped, cools → persistent fog/high fog.

Fog conditions can prevail over a longer period of time in winter, when a strong low-level inversion exists in low-gradient weather.

Visibility reduction in precipitation

Moderate snow: < 1-2 km visibility.
Moderate drizzle: < 3 km.
Moderate snow causes the most significant reduction of visibility.

International Cloud Atlas* (fog/mist); AMS Glossary; ICAO Annex 3; DWD Klima-Atlas.*