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Fog doesn’t 'burn off’, so what really happens?

Posted at 5:56 PM, Feb 14, 2020
and last updated 2020-02-19 07:58:10-05

Southwest Florida woke up to a foggy start for their Valentine’s Day, and it didn’t make things easy for drivers as they headed out the door for their morning commute.

It’s not uncommon to hear people say things like the fog will “burn off” when the sun starts to come up, but it’s not correct. So what really happens to all of that fog?

As the sun starts to rise, the air temperature increases and the fog will dissipate through the process of evaporation.

Instead of saying "burning off", you can use terms like evaporating, dissipating, or mixing out.

What causes fog to form?

Fog is considered to be a cloud that hangs at ground level.

There are many different types of fog, but there were two specific types that we saw during the morning commute Friday-- advection fog and radiation fog.

Advection fog forms when warm moist air runs over colder surfaces and cools to the dew point. Radiation fog occurs when the air near the ground cools and the water vapor condenses.

The air temperature just before 7 a.m. on Friday was on the warm side around 70 degrees, and the dew point was also around that same number.

2020 WFTX Current Temperatures DMA.png

It’s the perfect mixture for dense fog when those two numbers are almost equal. But remember—the dew point can never be greater than the air temperature.

2020 WFTX Current Dewpoints DMA.png

We saw some of that this morning when the National Weather Service issued a dense fog advisory for all of Southwest Florida with some areas down to zero visibility.

2020 WFTX Current Visibility DMA.png