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Hurricane Laura seen off the coast of the Texas-Louisiana border, photographed from the International Space Station. Photo...

Why some hurricanes grow into massive storms — and why they are hard to predict

Hurricane Laura has left a mighty toll — at least 19 deaths have been tied to the storm, which caused an estimated $8 to $12 billion worth of damage. But by some measures, the storm was not as fearsome as predicted — for instance, the “unsurvivable” storm surge was massive but the worst of it hit away from populated areas.

What did catch many off guard, however, was the speed at which Laura gained strength. Before it ground ashore in Louisiana and Texas, Laura rapidly intensified into the year’s first major hurricane of the year — from a Category 1 to Category 4 storm within a day — and left officials and residents scrambling to coordinate evacuations. Some research has shown that climate change makes certain storm conditions more likely, including the chances that storms could intensify quickly. How does this happen, and what can we expect in the future?

How hurricanes work

In the Atlantic Ocean, hurricanes usually form in the summer and autumn when the water is warm and strong winds shoot off the African continent, rushing west over the ocean. These winds help grow thunderclouds out of water evaporating from the ocean’s surface — the very beginnings of a potential hurricane. Marching across the Atlantic, the accumulating mass can turn into a massive storm spinning around a central eye.

“The hurricane is a giant engine for extracting heat energy from the ocean,” said Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology.

Once the temperature of ocean surface waters gets to 79.7 degrees Fahrenheit and above, the evaporation sometimes becomes a monstrous feedback loop, growing the hurricane by pulling more energy out of the ocean.

Animation by Megan McGrew/PBS Newshour

But there are also conditions that put the brakes on a hurricane. One of the most important is called wind shear, which represents changing speed or direction of wind over a short distance in the atmosphere. When it comes to making sure a hurricane is less powerful, more wind shear is better; if there is enough wind shear in a vertical direction — think from the top to the bottom of a hurricane — it can tilt over the whole system, weakening its ability to grow. Without wind shear, a hurricane can find power in a strong, straight vertical core.

“That’s like pouring water on the fire. It’s going to dampen the fire or even put it out. So hurricanes don’t like wind shear,” Emanuel said.

Colder ocean water churned up from below the hotter surface layers can also disrupt the growth of a hurricane, and when a storm finally hits land, it loses its energy source entirely.

Why rapid intensification is dangerous

But left unencumbered, a hurricane can rapidly intensify, especially if it hits a particularly warm patch of ocean – such as the often-warm Gulf of Mexico. Rapid intensification of a hurricane refers to an increase in wind speed, and is defined as “an increase in the maximum sustained winds of a tropical cyclone” of at least 34.5 miles per hour in a 24-hour period, according to the National Oceanic and Atmospheric Administration. In the case of Hurricane Laura, winds rose almost 64 mph in 24 hours.

“There is a reason with Laura that people thought there was a potential [for rapid intensification], and that’s because sea surface temperatures were ripe and the wind shear was low,” said Kieran Bhatia, discussing research he did as a former postdoctoral associate at Princeton University. He is now the climate science program lead with BP. “It was a situation where all the stars were kind of aligning.”

Since rapid intensification events involve many different factors interacting, they are tougher to predict than the track of a hurricane. And they can coalesce right before landfall, making them especially dangerous.

“The worst forecaster nightmare is having a storm bearing down on a populated coastline. You go to bed and it’s a tropical storm and you wake up with a Cat[egory] 4,” Emanuel said.

To get a well-rounded picture of a developing hurricane, meteorologists rely on satellites and (at least in the Atlantic) fly planes through and above the storm, dropping measurement instruments called dropsondes. Data like wind speed and sea surface temperature are fed into computer models to see how the storm may evolve. When it came to Hurricane Laura, its path was not uncertain – in fact, the National Hurricane Center managed to forecast the track with remarkable accuracy.

While hurricane forecasts have improved with better technology and models over the decades, rapid intensification is still an elusive circumstance to pin down. Emanuel believes more direct measurements of storms are needed to improve forecasts, and he is involved in building a drone that flies in the lower stratosphere, high above the hurricanes.

“It’s powered by sunlight, so you can leave it there for a very long time and drop these dropsondes,” Emanuel said. This could allow for more dropsondes to be launched over more time and area than using planes piloted by humans, improving the data feeding the forecast models.

Rapid intensification in the future?

Much of the warming happening on our planet is actually in the ocean, with average global ocean surface temperatures rising almost 1.5 degrees Fahrenheit since 1900. The Gulf of Mexico often harbors very warm waters – Hurricane Laura happened when the waters were in the mid to high 80s.

Considering hurricanes feed off of warm waters, will a warming world due to human-driven climate change make rapid intensification events more common?

In 2019, Bhatia and Princeton colleagues published a paper in Nature Communications that used sophisticated modeling to tease out this relationship. They found that climate change increases the likelihood of a rapid intensification event occurring in the Atlantic, at least in their climate model.

This and other studies of extreme weather have linked climate change with the probability of such storm conditions, not with individual storms. “You’re increasing the probability of this event happening, you’re not making this event happen,” Bhatia said.

Since we only have a few decades of high quality data on hurricanes and because of the multitude of factors involved in hurricane formation and growth, showing the impact of climate change on hurricanes has been tricky.

However, scientists are starting to see some trends. We may get fewer storms overall but those that do hit are more likely to be more intense and bring more rain. And, of course, rapid intensification also has huge implications for getting people out of danger.

Editor’s note: The impact of Hurricane Laura was clarified to note that the highest storm surge hit in a less populated area.