fogforming

Fog is a cloud that forms at or near the Earth's surface when air becomes saturated with water vapor and condenses into tiny droplets. Fog droplets are typically 1 to 50 micrometers in diameter, and the presence of many such droplets lowers visibility, often to less than 1 kilometer.

Fog forms when the air cools to its dew point or when additional moisture makes the air supersaturated. Condensation requires condensation nuclei, such as dust, salt, or other particles, on which droplets can form. Conditions that favor fog include high relative humidity, light winds, and surface cooling or moisture advection.

Common forms of fog arise from different cooling or moisture processes. Radiative fog forms on clear nights when the ground loses heat by longwave radiation, cooling the air near the surface. Advection fog occurs when warm, moist air moves horizontally over a cooler surface and cools to its dew point. Upslope fog develops as moist air is lifted up terrain and cools adiabatically. Steam fog (or sea smoke) appears when cold air moves over warm water and water vapor condenses as the air cools. Freezing fog contains droplets that freeze in subfreezing air, producing ice crystals and frost-like conditions.

Fog intensity and duration depend on atmospheric stability, wind, humidity, and surface conditions. It is a significant factor in transport, agriculture, and outdoor activity. Meteorologists describe fog using visibility thresholds and often monitor dew point, temperature, and humidity to predict its formation and dissipation.