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Where Does Fog Come From?

Have you ever looked out the window on an otherwise clear day and wondered where the fog suddenly came from? Or only been able to see the school bus heading down the street because of the flashing light on top?

(Photo via Shutterstock)

Fog sometimes seems as though it comes out of nowhere, suddenly limiting how far we can see. When it does, it's because of many factors, including humidity and temperature. Fog is basically a cloud, but while clouds can form at any altitude, fog only forms near the ground.

Fog develops when water vapor — the gas form of water — condenses. As it does, the water vapor molecules combine, which then form water droplets that hang in the air. The droplets are what make fog visible, because water vapor, like all gases, is invisible.

Fog develops in humid conditions, because there has to be a lot of water vapor in the air for it to form. Water vapor isn't the only thing that must be present, however. Some type of air pollution or dust particles must also be present, because the water vapor condenses around the particles to form the fog.

For example, sea fog, which forms over oceans, forms when water vapor condenses around salt in the air. And one of the most dangerous types of fog, super fog, forms when water vapor combines with smoke from wildfires, causing extremely dense fog. This type of fog is sometimes so thick you can't see your hand in front of your face.

Sea fog and super fog do not occur in Will County. Instead, we experience other kinds of fog, usually radiation fog, which is also called ground fog. Radiation fog usually develops in the evening, when the heat absorbed by the ground during the day is radiated back into the air. This is what causes the fog to develop.

In colder temperatures, freezing fog or ice fog can develop. These two types of fog are different, but both occur in cold weather. Ice fog forms only at extremely cold temperatures. It develops when the air at ground level is cold enough to turn the water in fog into ice crystals. Freezing fog is similar to radiation fog, but it occurs when the water vapor forms ice crystals instead of water droplets.

In weather reports, we often hear that the fog will "burn off" by a certain time. That terminology is misleading. Fog doesn't burn off. Instead, as solar energy — heat from the sun — heats up the ground near the edge of the fog, drier air enters the foggy area, causing it to evaporate. Ground fog usually first begins to evaporate at the edges, where it is thinnest. Then the evaporation works toward the center of the fog, where it is thickest.

Some places, like San Francisco, are known for fog. The California city is noted for its foggy summers, but why is it so foggy there? Essentially, it's because air is moving from a colder place over the ocean to a warmer place over land. As the air moves, fog forms and then moves over land. This type of fog is called advection fog.

As foggy as San Francisco is, it's not as foggy as it used to be. Summer fog in the city has decreased by about 33 percent over the past 100 years because of climate change and warming ocean temperatures. If those trends continue, the city will probably continue to experience less fog than in the past.

The foggiest spot on Earth is the Grand Banks of Newfoundland, off Canada's Atlantic coast. It is foggy for the same reason as San Francisco, cold air from the north combines with warm air to create thick fog nearly every day.


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