How the ground turns damp air into fog
Air can hold water as an invisible gas called water vapor. But cooler air cannot hold as much vapor as warmer air. When air near the surface cools to the point where it is saturated, some vapor condenses—gathers into tiny liquid droplets suspended in the air. That collection of droplets is fog.
Cooling can happen in several ways. On a clear, calm night, the ground loses heat, chilling the air just above it; this often produces fog in low-lying places where cool air settles. Fog can also form when moist air moves over a colder surface, or when rain evaporates into cooler air below the clouds.
Why a little water makes the view disappear
Each droplet is tiny, but a stretch of fog contains countless droplets. Light scatters as it hits them instead of traveling cleanly from a distant object to your eyes. The farther away the object is, the more its light is scattered along the way, so it fades into a gray blur. Nearby objects remain easier to see because their light crosses less fog.
Fog is therefore not water vapor you can see. It is liquid water hanging in the air—the same basic kind of droplet cloud found aloft, but close enough to wrap around the landscape. When the air warms or dries, the droplets evaporate back into invisible vapor, and the view clears.
