Depending on the situation in which you encounter the phenomenon known as fog, it can be at the same time a picturesque spectacle and a nuisance. When having to travel through foggy areas, whether it is by car, boat, or plane, fog can prove to be hazardous, even deadly. In fact, according to the Federal Highway Administration, nearly 500 people (on average) are killed in fog-related crashes each year. But what exactly is fog and how does it form?
Regardless of how it forms, fog is defined as water droplets suspended in the air at the Earth’s surface, or even more basically, a cloud on the ground. These water droplets are formed through condensation. The manner in which this condensation occurs, determines the type of fog. Examples of the varieties of fog include radiation (ground) fog, valley, advection, upslope, sea smoke, and precipitation fog.
The most common type of fog, radiation or “ground” fog, forms on clear/calm nights. As the ground radiates its heat to the atmosphere, the ground and the air immediately above it become cooler. If this shallow layer of air near the ground cools to its dew point, the moisture in the air condenses into tiny droplets in the air, which is fog. This fog will usually dissipate when the sun rises and warms the fog layer back above the dew point, or when the winds increase, thus allowing warmer air to mix down from above.
A special type of radiation fog, called valley fog, occurs in valley regions when radiatively-cooled air sinks into these low-lying areas. Valleys, as a consequence of being surrounded by high hills or mountains, are shielded from the prevailing winds. Thus, on days with weak warming from the sun, as in winter, valley fog can persist for days and become quite thick.
The other major type of fog is advection fog. Advection is the transport of something by the atmosphere. In the case of advection fog, warmer air is the “something” being advected. As this warmer air moves over a colder ground surface it cools to the dew point and forms fog. Many times this colder ground surface results from it being snow-covered, which is why advection fog is often seen in winter. Other types of advection fog include upslope and sea smoke. Upslope fog results from humid air advecting up hills or mountains. The air cools as it rises, and if it cools to its dew point, fog will form. Sea smoke, or steam fog, occurs when colder air blows over a warmer body of water. When water near the surface evaporates into the colder air above, it increases the humidity of the air. When the air reaches its dewpoint the vapor condenses into steam fog over fresh water (sea smoke over bodies of salt water).
Cross Section of a Warm Front - Source: Ravedave
Fog can also form when rain falls. As the raindrops fall, if they encounter a layer of cooler/drier air, such as that found at a warm front and evaporate into this layer of cooler/drier air, the dew point of the air surrounding the evaporating drops will be raised. If it is raised until it is the same as the air’s temperature the vapor can condense into smaller fog droplets. This type of fog is called frontal or precipitation fog.
Regardless of the way which fog forms, if encountering fog while driving on the road, make sure to use your fog lights or low-beam headlights to prevent glare. If necessary, reduce speed to allow for a safe stopping distance according to the current visibility. Finally, if fog becomes so think that driving is unsafe, either pull off into a rest stop/parking lot, or if none is available, pull off as far onto the shoulder of the road as possible, stop your car in park, and after taking your foot off the brake put on your hazard lights so that other cars will see you. Proceed again on your way when conditions improve.
- Fog Formation Pics Courtesy of Knox College