Radiation fog

Radiation fog is fog that forms when the ground loses heat by radiation, usually at night under clear skies and light wind. The air close to the surface cools to its dew point, so water vapour condenses into tiny droplets. It is common over land in the late night and early morning, especially in low areas where cold air collects. For a pilot it can reduce visibility quickly near the surface and may delay departure or landing until it lifts after sunrise.