Does fog really ‘burn off’? Dispelling the myth of combustible clouds
People commonly refer to the dissipation of fog as ‘burning off’ but the reality is much less exciting
Watch: Time-lapse of fog in northwestern Washington from San Juan Islands
Time-lapse of fog in northwestern Washington from San Juan Islands at Samish Overlook. (Video: Amie Wheeler/ Tandem Wheels Photography)
The morning sometimes starts out as foggy darkness, but when the sun comes up that soup suddenly disappears.
The usual dissipation of fog at the appearance of that bright ball of heat has led to most people saying that murk was "burned off" by the sun. The reality of how fog dissipates is much less dramatic than the imagined combustion.
What is fog?
Most clouds have their bases above the surface of the earth, but fog is basically a cloud that has a base located on the surface.
The more technical definition is water droplets that are suspended in the air at the Earth’s surface, according to the National Weather Service’s glossary.
If visibility, or the distance that you can see in front of yourself, is reduced to a quarter-mile or less, the fog is considered dense. This makes it very difficult to safely navigate roads or waterways.
The Golden Gate Bridge is seen surrounded by a layer of fog in Sausalito, California, on Wednesday, March 2, 2022.
(Stephen Lam/San Francisco Chronicle via Getty Images)
How does fog form?
For fog to form, the air at the surface must be cooled to the dew point – that’s the temperature at which the water vapor in the air condenses into droplets. It’s the same process that happens above the surface for the clouds floating above our heads.
Another factor that helps with the formation of fog is light winds. Breezes mix the air at the surface with air above it and usually prevent the formation of fog. In general, the more stagnant the air, the more likely it is for fog to form.
Watch: Dense fog makes for very poor visibility in Bay Area
Timelapse video captures dense fog rolling through the Bay Area Wednesday morning. (Video: @absolutepepper/Twitter)
How does fog dissipate?
Basically, there are two ways that fog dissipates – warming and mixing. Here’s an explanation of these processes.
1. Warming
In general, the coldest time of the day is just before sunrise. Coincidentally, that is usually when fog will be the thickest. That’s because the air has been cooled as much as it can be at that point.
Once the rays of the sun hit the ground, the air just above the surface that contains the fog begins to warm. Eventually, the air warms above its dew point and leads to the evaporation of the murky droplets of water. Suddenly, the fog disappears.
How long this evaporative process takes really depends on how effectively the sun can heat the ground. If there are cloudy skies above the fog, it will take much longer for the gloomy conditions to improve.
2. Mixing
As noted above, fog likes light winds the best. Once winds begin to pick up, the moisture-rich air at the surface mixes with the drier air above it. This leads to the evaporation of those droplets.
Mixing is sometimes a function of warming, as well. When the sun begins to warm the ground, small currents of air called thermals begin to rise. Thermals take the foggy air with it and mix it with the drier air above the surface.