Why did Hurricane Ida stay so strong for so long?

Ida remained a hurricane for 16 hours after it made landfall on Sunday (Aug. 29), and was a major hurricane (defined as a storm of Category 3 or above) for six hours of that time. How did the storm have so much staying power?

It basically didn't know it was over land, meteorologists say.

Hurricanes draw their energy from warm ocean waters. But when they make landfall over a wet, marshy, or saturated spot, they can still power themselves with evaporating moisture.

"We always knew that places like the Everglades or the swampy wetlands of Louisiana could provide a fuel supply for storms that might linger over them, and I think that's what we saw with Ida," said Marshall Shepherd, a meteorologist and the director of the Atmospheric Sciences Program at the University of Georgia.

Related: The 20 costliest, most destructive hurricanes to hit the US 

Brown ocean

Shepherd and his colleagues have long studied the phenomenon of hurricanes and tropical storms that stay strong even as they travel inland — long enough that they've dubbed the phenomenon the "brown ocean effect."

Hurricanes draw their fuel from warm ocean waters. As the warm water evaporates and rises, it condenses, releasing heat and driving the rotation of the storm. As the hurricane's winds gather around its eye, they blow across the ocean surface, driving faster evaporation and feeding even more energy into the storm.

When the hurricane hits land, it typically loses this fuel source and begins to weaken and eventually fall apart. But when "land" is a swamp — as it is where Ida came ashore in southern Louisiana — there's still plenty of moisture to draw in. This can keep a storm organized and deadly for long periods over land.

"The storm had a very classic structure, it still had an eye, it still had a warm core," Shepherd told Live Science. "So if it's maintaining that structure, maintaining its integrity, you're still going to have very low pressure and that means you're going to continue to have stronger winds and intense rainfall."

Strengthening inland

Ida isn't the first storm to feed off marshy land. In 2016, an unnamed storm that caused flooding in Baton Rouge underwent a very similar process, according to a paper published in 2019 in the journal Scientific Reports and co-authored by Shepherd. That storm dumped 30 inches (780 millimeters) of rain on the region.

The effect can even occur farther inland, where rain-saturated soils can power tropical cyclones far from the sea, according to 2013 research by Shepherd and geographer Theresa Andersen, an assistant professor at Kennesaw State University in Georgia. One example was 2007's Tropical Storm Erin, which made landfall in Texas, subsequently weakened, but intensified again in Oklahoma. The state saw flooding, high winds and power losses, and several people drowned. Inland intensification has also been seen in Asia and in northern Australia. For instance, Tropical Cyclone Kelvin made landfall over northern Australia in 2018 and continued to intensify after coming ashore, possibly driven by warm, sandy soil that had experienced a recent rainfall.

Another factor that might have helped Ida stay strong was the unique topography of southern Louisiana just west of New Orleans. The area where the hurricane came ashore is extremely flat and low-lying, said Levi Cowan, a meteorologist and owner of tropicaltidbits.com. With little topography to stop it, storm surge can reach dozens of miles inland. That's important because one of the factors that slows hurricanes on land is friction.

"It's the drag of the circulation against the ground that slows it down a lot," Cowan told Live Science. With the storm surge flooding the coastline, it's possible the eye of Ida didn't contact land for some time even after it officially made landfall, he said. 

Originally published on Live Science.