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Living in a Cloud: Learn All About Fog

Updated: Nov 1, 2023

Most clouds are high overhead and way out of reach unless you are in an airplane. But did you know fog is also a type of cloud?

A white-tailed deer standing in trees shrouded by fog.
A white-tailed deer standing in trees shrouded by fog. (Photo courtesy of Amy Miller)

Fog is sometimes called a ground cloud and can be quite common in Will County, especially in the fall. Whether seen as an addition to a magical morning or an eerie evening, there are some cool things to learn about fog.

How fog forms

There is always water vapor in the air. Water vapor is water in its gas form, and it is invisible to us. When there is a lot of water vapor in the air, we call it humid. If humidity reaches 100%, the gas condenses into a liquid and forms a cloud that we can see. This process is part of the water cycle.


Words to know

Dissipate: To disappear or cause to disappear.

Evaporate: To turn from liquid to vapor.

Precipitation: Rain, snow, sleet or hail that falls to the ground.

Vapor: A substance diffused or suspended in air.


In some clouds, the water gets so heavy that precipitation, like rain or snow, falls from them. In other clouds, the droplets of water are so tiny that they just hang in the air defying gravity. Fog is a stratus cloud that forms close to the Earth’s surface. There are many other kinds of clouds.

How fog fades away

Fog can linger awhile or it can disappear as fast as it formed. When fog fades, the tiny water droplets evaporate back into invisible water vapor. Sunlight is most often the cause of evaporating fog. The heat from the sun turns the water back into a gas. Dry air mixing with the foggy air can also cause fog to dissipate, which means spread out and disappear. A strong wind can swoop in and literally blow the fog away.

Varying visibility

Fog can be short or tall. There might be just a short layer appearing to float above the surface of the earth. You might clearly be able to see where it starts and stops. Or you might be in a fog that is so tall that you cannot see where it ends.

Fog can also be thick or thin. You might be able to see through the fog, or it can be so thick and dense that you cannot see what is right in front of you. When fog is extra thick, it may be best to stay indoors. The weather service will put out a dense fog advisory if fog is expected to be so thick as to make travel dangerous.

There are many different types of fog. They are named for the different ways water vapor can condense into visible water droplets. Learn more about the two types of fog commonly seen in Illinois.

Evaporation fog

Evaporation fog over water.
Evaporation fog over water. (Photo via Shutterstock)

Evaporation fog is most common in the fall because it takes water longer to cool down than the air. As cold air passes over a river, lake or pond, the warm water evaporates. But then, because the air is so cold, the water vapor immediately condenses back into a liquid. We can see this liquid water as hazy fog just above the surface of the water. Sometimes it is spiraling up in small columns.

Radiation fog

Radiation fog in a forest.
Radiation fog in a forest. (Photo via Shutterstock)

The word radiation often brings to mind either something very serious and harmful or something causing supernatural mutations into heroes or villains. But radiation simply means energy coming from a source. The sun radiates light, and plants turn solar radiation into the food they need.

Radiation fog forms on clear, still nights, especially in the fall and early winter. The heat absorbed by Earth’s surface during the day is radiated into the air. This causes the ground to cool and water droplets to form. This type of fog first forms near the surface and, if conditions are right, it can grow to a thick, tall layer of fog. Radiation fog can last into the morning before it begins to fade away.

Fog is always a fun site to behold. Watch for it swirling up from the surface of water or adding mystery to a cool fall evening. Take time this fall to appreciate the fog.

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