What is fog composed of water droplets in the air

Fog is visible condensation at or near the ground, basically a ground-based or low lying cloud. It is composed of tiny water droplets, or in colder weather, ice crystals. Fog is not precipitation, although in the right conditions precipitation (drizzle) can occur in fog. According to the international definition, fog reduces visibility to less than one kilometer (0.62 mile). However, in general use the term "fog" is visible condensation, even at higher visibilities. Heavy fog is defined as fog that reduces visibility to 0.25 mile or less. Dense fog is most prevalent in the Midwest from October through March in the hours after midnight to mid-morning.