What causes fog to form in southern Louisiana? | Weather

Dense fog has returned with a vengeance this month, prompting repeated weather warnings and problems for morning commuters, including a series of multi-vehicle crashes on the Lake Pontchartrain Causeway bridge.

Fog is a common nuisance in south Louisiana this time of year, which the National Weather Service meteorologist calls “the heart of fog season.” But why?

Here’s what’s causing the recent bout of fog:

How does fog form?

According to the weather service, there are a handful of different types of fog with different characteristics.

“Radiation fog” forms overnight when heat absorbed from the Earth’s surface during the day is radiated into space.

“Freezing fog” occurs when water droplets remain in a liquid state until they come into contact with a surface.

And of course, many people remember “superfog,” a particularly dangerous weather condition that occurs when fog and smoke combine. On the morning of October 23, 2023, there was super fog caused by a swamp fire that made it nearly impossible for drivers on Interstate 55 to see, resulting in one of the deadliest pileups in Louisiana history.

But the most common way fog forms in south Louisiana, especially this time of year, is when warm, moist air moves over a cooler body of water, Grigsby said. The cool water surface causes the air temperature to drop to the dew point, which can cause fog to form.

Then, Grigsby said, you need just enough wind to push the fog inland without lifting it too far from the ground.

“Basically, all the right conditions are in place right now for this fog to form,” he said.

Why now?

Southern Louisiana’s peak fog season generally runs from December to March, Grigsby said, when bodies of water like Lake Pontchartrain are cooler due to low nighttime temperatures and cold snaps.

“The main reason for the fog right now is that we have unusually warm and unusually wet conditions locally,” Grisgby said.

Temperatures this week are expected to be above average, with an average high of 63 and a low of 47 degrees for this time of year, he said. This means the warm air has more opportunity to move over cooler water.

Coastal areas and places near large bodies of water are most vulnerable to dense fog this time of year, he said.

More fog on Friday

Much of southeast Louisiana was under a fog warning Thursday morning, and similar conditions are expected across the region Friday. Patchy fog is expected in New Orleans and Baton Rouge from about 5 a.m. to 10 a.m. Friday.

Forecasters are encouraging morning commuters to slow down and allow extra time to reach their destination.

Tips for driving in fog can be found here.