Fog Formation Factors

Key Fog Formation Factors

Fog develops when air is cooled to its dew point or when moisture is added to already cool air, resulting in saturation. The main fog formation factors include:

• High Relative Humidity: Air close to saturation is more likely to form fog, especially when the temperature-dew point spread is less than 2°C.
• Cooling Mechanisms: This can occur through radiational cooling (clear, calm nights over land), advection of warm moist air over a colder surface (land or sea), or upslope movement of air.
• Moisture Addition: Precipitation, evaporation from wet surfaces, or advection of moist air can increase humidity, promoting fog formation.
• Wind Speed: Light winds (typically 2–8 knots for radiation fog) are ideal. Too little wind leads to dew, while too much wind causes mixing and may form low cloud instead of fog.

Types of Fog and Their Causes

• Radiation Fog: Forms over land at night when the ground cools rapidly, chilling the air above. Requires clear skies, light winds, and high humidity.
• Advection Fog: Occurs when warm, moist air moves over a colder surface (land or sea), cooling the air to its dew point. Needs moderate wind (up to 15 knots) to transport the air.
• Frontal Fog: Develops ahead of warm fronts where rain evaporates into cooler air, saturating it.
• Arctic Smoke (Sea Smoke): Forms when very cold air passes over warmer water, causing rapid evaporation and condensation.
• Hill/Orographic Fog: Created when moist air is forced up slopes, cooling and condensing as it rises.
• Thaw Fog: Appears over melting snow as warm air increases evaporation from the surface.

Dissipation Factors

Fog dissipates when air is warmed (by sunlight, advection of warmer air, or subsidence) or when drier air mixes in, lowering relative humidity.