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John Wheeler, WDAY, Published February 26 2013

Weather Talk: Fog is essentially a cloud that forms at ground level

Trees, shrubs, and everything else exposed to the air has been covered in ice crystals almost every morning going back to late last week. Fog made of tiny ice crystals has been the reason for the hoarfrost. Sunday night was the exception. That night, the fog was made of liquid droplets supercooled below freezing, which caused windshields to become coated with smooth ice instead of crystals.

But why the fog? Fog forms when the humidity is hovering around 100 percent in an area of stagnant high pressure that yields very weak wind. As the temperature cools at night, the moisture in the air in vapor form begins to turn to tiny ice crystals or water droplets. The result is essentially a cloud that forms at ground level.

Many people hold on to a myth that fog is a signal for heavy rainfall at some point (60 days, 90 days, six months, etc.), but any relationship between fog and future rain is purely coincidental.


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