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There has been a lot of fog this summer; here's why

The water temperature of Lake Superior is the coolest in nearly 25 years.

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The James R. Barker, a 1000-foot laker, emerges from thick fog while departing Duluth under the Aerial Lift Bridge in Canal Park during a recent shipping season.
Clint Austin / File / Duluth News Tribune
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DULUTH — For people who live near Lake Superior, fog is a not-too-uncommon sight.

In fact, thick fog reducing visibility to less than a quarter of a mile happens on average around three to four times a month. There are two basic kinds of fog: advection fog and radiation fog.

Advection fog is often produced over the cold water of the lake during winter, spring and early summer. Radiation fog is more prevalent during late summer and autumn.

Simply put, fog is a cloud that develops on the ground. Fog forms when a layer of air is cooled to its saturation point. Water vapor will begin to condense onto small nuclei (such as dust), and over time these droplets grow. As the air mass cools more and more, these little water droplets will grow until they are large enough to be seen, reducing visibility.

Because Lake Superior is cold, especially in late spring and early summer, warm air that arrives over Lake Superior becomes cooled. Fog forms over the lake and then is blown onto land by a gentle breeze.

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Radiation fog, also known as "ground fog," is more common in the late summer and in autumn. It forms when air temperatures near the ground or the lake surface cool more than the air higher up, creating a temperature inversion. This happens when the wind is light. When air near the ground cools to the saturation point, fog is likely.

This spring and summer, Lake Superior has been colder than average. In fact, the average temperature of the lake during August is the coolest in nearly 25 years, which has made fog a little more common this year.

Editor's note: John Wheeler and Tim Albertson are chief meteorologist and meteorologist, respectively, for Forum Communications' StormTracker team.

Related Topics: WEATHER
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