Fog forms when air near the ground cools to its dew point, causing water vapour to condense into tiny suspended droplets. Calm air, moisture and a surface that cools quickly make fog more likely.
Air reaches its dew point
Warm air can hold more water vapour than cool air. When air near the surface loses heat during the night or moves over a colder surface, its relative humidity rises. At the dew point, the air is saturated and excess vapour begins to condense.
The droplets are so small that they remain suspended in the air, just as droplets in a cloud do. The difference is location: fog touches or forms close to the ground.
Tiny particles help droplets begin
Water vapour often condenses on aerosols such as dust, sea salt or smoke. These particles provide surfaces where liquid water can form. Cleaner air can still produce fog, but the number and type of condensation nuclei influence droplet size and visibility.
Wind can mix the moist layer away or lift it, while calm conditions let a shallow fog pool in hollows and valleys.
Several kinds of fog share the physics
Radiation fog forms when the ground cools under clear skies. Advection fog appears when moist air moves over a colder surface, including cold ocean water. Valley fog is shaped by terrain that traps cooled air. The names describe the route to saturation, not different types of water.
Sunlight, wind and mixing usually reduce fog by warming or dispersing the droplets. A forecast therefore depends on both moisture and the local landscape.
Sources and evidence
- National Weather Service — Fog ↗
Dew point, condensation and common fog types.
- NOAA JetStream — Fog formation ↗
Cooling, saturation and visibility.
Sources consulted: 9 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Added a surface-level explanation of dew point, aerosols and fog types.