What is super fog?

The National Weather Service called it “super fog” — a combination of thick smoke from fires in marshy wetlands of south Louisiana and the fog that often hangs thick in the air on cool, windless mornings.

It was a deadly combination this week. Smoke spreading through the region from a marsh fire east of New Orleans combined with thick fog and reduced visibility on highways to near nothing. It caused a series of horrific crashes that turned a section of Interstate 55 near New Orleans into a virtual junkyard of mangled and charred vehicles.

Officials said 158 vehicles were involved. There were 25 reported injuries, and the death toll from Monday's accident stood at seven as of Tuesday afternoon.

Super fog or smog?

Super fog and smog are both types of fog, according to the National Weather Service. But smog — often a problem in cities with heavy car traffic or industry — is formed when fog combines with smoke from pollutants. It can reduce visibility and pose a health hazard.

The term super fog describes fog enhanced by smoke from damp, smouldering organic material, according to the weather service. It can lower visibility to less than 10 feet (3 meters).

How often does super fog occur?

Meteorologist Tyler Stanfield with the weather service's New Orleans office said super fog events “are not overly rare, but they do take a perfect storm of conditions to materialize” and can happen a few times per year. Fireworks can contribute to super fog conditions, and Stanfield said super fog has occurred during Fourth of July and New Year's celebrations.

Stanfield said very light winds and high pressure over the region Monday trapped moisture near the ground, which he described as “a pretty typical fog setup.” The moist leaves and brush that were burning resulted in very low visibility.

Are fires common in Louisiana marshes?

They may be considered wetlands, but marshes can dry out, said Louisiana Agriculture Commissioner Mike Strain, particularly when the state is suffering through a drought that has contributed to wildfires in marshes and timberland. When the dry grass ignites, “it burns not only the grass on top, it burns through the crust and the peat layer underneath,” Strain said.

Such fires occur every year in south Louisiana. They can be caused by lightning strikes or people — hunters seeking to flush game, wildlife officials doing controlled burns to promote new growth, or sometimes arsonists. The fires are usually in remote, hard-to-access areas, making it difficult to fight them when they get out of control.

“They’re not necessarily like big out-of-control wildfires, but they kind of smoulder and produce smoke,” Stanfield said, adding that they can last for weeks or months at a time.

“It’s one of those phenomena that, with climate change, we might see it more often,” said Stephen Murphy, director of the disaster management program at Tulane’s School of Public Health and Tropical Medicine. “The drought conditions down here certainly fueled, no pun intended, all that marsh fire.”

Have conditions improved?

The super fog cleared by Monday evening. Increasing wind and high pressure were moving towards Georgia, eliminating any imminent threat of a repeat super fog event. Meanwhile, Strain said, efforts continue to extinguish the marsh fire near New Orleans that has pumped out much of the smoke that contributed to Monday's accidents.

“We have two high-volume pumps that are flooding the whole area with water,” Strain said. “It can still smoulder, even as it vents up. We hope to markedly reduce the amount of smoke.”