Fog and mist are both created by water droplets, differing only in their overall locations and density. Fog is a cloud that reaches ground level, even if that "ground" is a hill or mountaintop. Mist forms wherever water droplets are suspended in the air by temperature inversion, volcanic activity, or changes in humidity. Fog is denser than mist and tends to last longer. In terms of visibility, fog reduces it to less than one kilometer (0.6 miles), while mist can reduce visibility to between 1 and 2 kilometers (0.6 - 1.2 miles).
|Effect on visibility||Reduces visibility to less than 1 km (1,094 yards)||Reduces visibility to between 1 and 2 km|
Mist vs. Fog Causes
Fog is formed when any cloud type makes contact with the ground. In low-lying areas, such as valleys and plains, the fog bank (a mass of fog) is essentially a cloud formation subject to the same wind and temperature reactions clouds experience in the upper atmosphere.
Clouds form when water droplets condense and merge, but fail to achieve a size large enough to precipitate as rain. Clouds will form or drift closer to the ground when humidity rises or changes abruptly, or when wind speeds drop or acutely change direction.
Mist is also formed by water droplets, but with less merging or coalescing. This means mist is less dense and quicker to dissipate when wind, temperature, or relative humidity changes. Mists can form due to abrupt temperature changes (such as when exhaling in cold air), high levels of humidity (in a sauna, for example), or from evaporation or condensation, such as when rain hits sun-warmed rocks and street surfaces or evening allows dew to form.
Effects on Visibility
Fog is much denser than mist and thus has a greater effect on visibility. A person can still see out to about 2 kilometers (1.2 miles) in mist, but a fog will reduce visibility to under one kilometer (0.6 miles). Dense fogs, usually caused by rapid changes in humidity or combined with smoke, can reduce visibility to under 50 meters (60 yards). London's infamous "pea-soup fogs" of the 19th century were said to reduce visibility to less than 20 feet.
How to Drive Safely in Fog and Mist
When driving in misty conditions, it is important for drivers to use wipers with care. The water droplets in mist are often not dense enough to require the constant use of wipers, so intermittent patterns will probably do a better job of keeping the windshield clear. In foggy conditions, wipers might play a smaller role to that of fog lights or driving lights. In some dense fogs, headlights or "long" lights will actually reduce visibility as the light is reflected by the fog itself. To check for best visibility, a driver should flick between headlights and fog lights to gauge visibility with each. This also serves as a warning signal to oncoming drivers.
Driving "past your lights," means that the limit of visibility is constantly exceeded (the speed is too fast to react to what appears out of the fog or mist) and one is "driving blind." It is best for drivers to slow down in that situation. If the driving speed in the fog drops to half of the posted maximum speed, it is a good idea for drivers to find a safe spot to pull well off the road and wait for the fog to clear. Many accidents are caused in fog banks by cars going too slow and being hit from behind. When pulling off the road, drivers should keep their car's hazard lights flashing for additional safety.
In the following video, driving safety instructor Zundrea Baldwin gives further tips on how to drive safely in foggy conditions.