Fog is a cloud on the ground. Fog can range from very light to very dense. Dense fog can cause serious travel problems. The visibility can get so low that it is difficult to see a car even a short distance ahead. When an accident occurs, a chain reaction of accidents one right after the other can occur since there is not enough time to brake before hitting the car in front.

Conditions that typically cause dense fog include high moisture, relatively light wind, and some lifting. Moisture can be supplied by wet soils, precipitation falling or moisture advection. Light wind reduces the mixing of saturated air with drier air. Stronger wind helps promote evaporation which will tend to reduce fog density unless conditions are very saturated. Some lifting can be supplied for example by upslope flow, warm air advection, or surface convergence. This lift helps sustain the saturation of the air. Sinking air will decrease the relative humidity and cause fog to dissipate.

Dense fog can occur anywhere but is most common near oceans, in valleys, near mountains, near rivers, and bodies of water due to moisture or lifting that can be present at these locations. Dense fog can change a good day into a day with severe travel headaches. This makes dense fog an important weather changer.