Fog

A cloud at the surface that reduces visibility, often to less than one mile. Common types include radiation fog (forms on clear, calm nights as the ground cools), advection fog (warm moist air moves over a cooler surface), and upslope fog (air is forced up terrain and cools to its dew point). Fog is one of the most common weather hazards for VFR pilots because it can form rapidly and reduce visibility to near zero.