Essentially, fog is a cloud that occurs at ground level instead of in the sky. Fog is made up of tiny droplets of water that float suspended in the air, but it also may consist of ice particles during very cold conditions. Under most circumstances, air contains water vapor. The warmer the air, the more water it can hold. As the air cools, it loses its ability to hold that water. When the air cools to the so-called dew point, it becomes fully saturated with water vapor. If the air cools any further, it loses the ability to hold all of that water vapor and forces some of the vapor to condense around microscopic particles, like dust, forming droplets. When enough of these droplets form, fog is made.
Answered by: Discovery Channel