Fog is a cloud that is on the ground. Just like a cloud, it is composed on tiny numerous water droplets. Fog varies in visibility from only reducing the visibility slightly to producing low visibility even a few feet away. Fog is often thicker in warm air since more moisture can be evaporated into warm air. The moisture sources for fog are moisture evaporating off the ground, moisture evaporating in the air from rain and then recondensing on small cloud droplets, and moisture being transported in from a moisture source such as a warm ocean. Dynamic uplift, saturated soils, persistent rain, being night, and plenty of condensation nuclei from smoke and emissions are other factors that can contribute to a thicker fog. This writing looks at freakishly low visibility fog.

Extremely low visibility fog is very dangerous when traveling. It can be so dense that the tail lights of a car are faint even when a few feet away. Major pileup accidents occur as a car slows down and then gets rear ended by a car behind. A chain reaction of dozens of cars can crash and pile up. It is important to be extremely cautious when driving through dense fog. Often the densest fog occurs near a river valley, near a lake/ocean, in a highly vegetation area, or in a lower elevation region that is surrounded by higher elevations. Roadways can experience denser fog when driving near these locations. It can be so foggy that only faint lights can be seen in the distance. These conditions can bring roadways and airports to a standstill. It is best to not travel during freakishly low visibility fog.