Foggy Formations | weatherology°
By: Meteorologist Michael Karow
Updated: Oct 27th 2020

Foggy Formations

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, it 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.

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 dewpoint, the moisture in the air condenses into tiny droplets, forming fog. This fog will usually dissipate when the sun rises and warms the fog layer back above the dewpoint, or when the winds increase, thus allowing warmer air to mix down from above.

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 dewpoint 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 dewpoint, fog will form. Sea smoke, or steam fog, occurs when colder air blows over a warmer body of water.

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.

fog Sweden Lenz
Ground fog in Sweden - [Leonhard Lenz]
advection fog golden gate bridge ohmer
Advection fog near the Golden Gate Bridge in San Francisco, CA; “Biased Fog” - [David Ohmer, 2008]
pileup fog Tennessee 1990
Aftermath of a 99 vehicle pileup caused by fog, which killed 12, near Calhoun, TN on Dec. 11, 1990 - [carthrottle.com]