Why is fog close to the ground?

Fog sits near the ground because that is where the air often cools enough for invisible water vapor to become tiny liquid droplets. After a clear night, the land loses heat, chills the air touching it, and makes a low cloud right where people walk.

Those droplets are so small that they do not simply plop down like rain. Light breezes can hold them in place, making fields, sidewalks, and trees look softened. Once sunlight warms the surface, the droplets turn back into vapor and the fog thins away.