Charleston was blanketed in two types of dense fog beginning Friday, which continued into Saturday and ended Monday.

The city experienced Advection fog during the day and then Radiation fog in the evenings.

Advection fog is when warm and moist air moves over a cool surface, which allows the air to be cooled by saturation from the ground cooling the air.

Radiation fog, however, is more common in the United States during fall and winter.

This type of fog forms overnight as the air near the ground cools and stabilizes, according to the National Weather Surface website.

Cameron Craig, Eastern’s climatologist, said fog is a complex situation.

He said the sun usually burns off the fog, but because of the high pressure those three days last week, the cold air was rising to make stratiform clouds.

Craig said the city experienced this because of all the moisture from the rain and snow lately and the warm air from the South.

He said it was “bizarre” because there was a colder air front that moved it, which was hard to forecast.

“We were having a huge change of the atmosphere that was very moist,” he said.

Craig said this type of weather is not out of the norm for December.

He said the city’s fog lasted an entire day Friday, but Saturday almost experienced a freezing fog, which is when moisture would have stuck to everything creating a blanket of ice.

He said the conditions for the two fogs had enough moisture to keep happening each day, but because of the advectional fog, instead of the sun heating up the air to remove the chances of fog, it created more clouds.

“Essentially, we did have two types of fog,” said Craig.

Roberto Hodge can be reached at 581-2812 or rlhodge@eiu.edu