Fog, mist, haze

050-04-02Pablo Asensio Martínez2026-04-013 min

The formation of fog, mist, or clouds occurs when invisible water vapor condenses into visible liquid water. This happens when the air becomes saturated (Relative Humidity = 100%), either by cooling to the dew point or by the addition of moisture.

Fog Types

Definitions and Differences

Phenomenon METAR Code Visibility Obscuring Agent Relative Humidity
Fog FG < 1,000 m Water droplets ~100%
Mist BR 1,000 m - 5,000 m Water droplets > 95%
Haze HZ < 5,000 m Solid particles (dust, smoke) Low (dry air)

Types of Fog

1. Radiation Fog

Formed by the cooling of the earth's surface at night due to terrestrial radiation, cooling the air in contact with the ground by conduction.

  • Ideal Conditions:
    • Clear sky: Maximizes nocturnal cooling.
    • High relative humidity: Requires little cooling to reach saturation.
    • Calm or very light wind (2-8 kt): Allows some mixing to thicken the layer, but if too strong, it lifts it forming Stratus.
  • Characteristics: Occurs only over land, typically in autumn/winter, at night or at dawn. Usual thickness up to 500 ft.
  • Dissipation: Insolation (solar heating) or increased wind.

2. Advection Fog

Formed when a warm, moist air mass moves horizontally (advection) over a colder surface. The air is cooled from below.

  • Ideal Conditions:
    • Moderate wind (up to 15 kt): To move the air and allow vertical development.
    • Cold surface: Cold land or cold sea (cold currents).
    • High humidity.
  • Characteristics: Can occur over land or sea, day or night. Common in coastal areas (e.g., sea air over cold land in winter) or at sea (e.g., Grand Banks). It is persistent.
  • Dissipation: Change of air mass (passage of a cold front) or change in wind direction.

3. Steaming Fog / Arctic Sea Smoke

Formed when very cold air (from land) moves over a comparatively warmer water surface.

  • Process: Rapid evaporation from the "warm" water saturates the cold air immediately above.
  • Conditions: Large temperature difference (>10°C) between the air (very cold) and the water. Light wind.
  • Characteristics: Common in polar regions. Appearance of "smoke" rising from the water. Unstable in very low layers but often limited by an inversion aloft.

4. Frontal Fog

Associated mainly with warm fronts.

  • Cause: Warm rain falls through a layer of cold air (cold sector) beneath the front. Evaporation of raindrops saturates the cold air, and additionally, the cloud base lowers to the ground.
  • Dissipation: Upon passage of the warm front (cold air is replaced by warm air).

5. Hill Fog (Orographic Fog)

It is simply a cloud (Stratus or Stratocumulus) covering the top of a hill or mountain.

  • Cause: Forced ascent of air up the slope (windward), adiabatic cooling to condensation.

6. Freezing Fog

Fog composed of supercooled water droplets at temperatures below freezing.

  • Hazard: Upon impact with a surface (aircraft, ground), the droplets freeze instantly forming rime ice.

Comparative Summary

Type Main Location Wind Main Cause Dissipation
Radiation Land Calm/Light Ground cooling (Night) Sun / Strong wind
Advection Sea / Coast Moderate (15kt) Warm air over cold surf. Air mass change
Steaming Sea (Polar) Light Very cold air over warm water Strong wind / Dir. change
Frontal Warm Front Variable Warm rain in cold air Front passage