What causes fog to form in southern Louisiana? | Weather

Dense fog has returned in full force this month, causing repeated weather reports and problems for morning commuters, including a series of multi-vehicle crashes on the Lake Pontchartrain Causeway bridge.

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

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

How is fog formed?

There are a handful of different types of fog with varying properties, according to the weather service.

“Radiation haze” forms overnight as heat absorbed by the Earth’s surface during the day radiates 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 remember “superfog,” a particularly dangerous weather condition that occurs when fog and smoke combine. It was superfog created by a wildfire that made it nearly impossible for drivers on Interstate 55 to see on the morning of October 23, 2023, leading to one of the deadliest pileups in Louisiana history.

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

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

“We basically have all the right conditions in place right now for that fog to happen,” he said.

Why now?

The peak fog season in southern Louisiana generally runs from December to March, Grigsby said, when bodies of water like Lake Pontchartrain are cooler from low nighttime temperatures and cold spells.

“The biggest reason for the fog right now is that we have unusually warm and unusually humid conditions in place,” Grisgby said.

Temperatures this week are expected to trend above average, averaging highs of 63 and lows of 47 degrees for this time of year, he said. This means that there are more opportunities for the warm air to move over cooler water.

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

More fog on Friday

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

Forecasters are urging morning commuters to slow down and allow extra time to reach their destinations.

See tips for driving in fog here.