Fog forms when water vapor in the air condenses into tiny liquid water droplets near the ground, creating a cloud at surface level. This happens when the air cools to its dew point—the temperature at which air becomes saturated and water vapor condenses—or when moisture is added to the air

. Key processes that cause fog include:

• Cooling of air near the ground: After rain or during clear, calm nights, the ground loses heat and cools the air above it. Cooler air holds less moisture, so water vapor condenses into droplets, forming fog. This is called radiation fog and is common in valleys and low areas, especially in autumn and winter

• Mixing of warm, moist air with cooler air: When warm, moist air moves over a cooler surface (such as cold ocean water or snow-covered land), the air cools to its dew point and fog forms. This is known as advection fog and is typical along coastlines or where air masses meet

• Evaporation adding moisture: When water evaporates into already moist air, such as over lakes or wet land, it can increase humidity to saturation, causing fog to form. This is called evaporation or steam fog

• Topographical lifting: Warm air rising up a mountain slope cools as it gains altitude, leading to fog formation called upslope fog

Fog usually forms when relative humidity approaches 100%, meaning the air is saturated with moisture. Tiny particles like dust, salt, or sea spray act as nuclei for condensation. Different types of fog—radiation, advection, sea fog, ground fog, and evaporation fog—occur depending on the specific cooling and moisture conditions

. In summary, fog is essentially a cloud at ground level caused by the cooling of moist air to its dew point or by adding moisture to already saturated air, leading to condensation of water vapor into tiny droplets suspended in the air