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Hurricane Irma: how the storm got so big, intense, and scary

Vox.com logo Vox.com 9/10/2017 Brian Resnick
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 “You need a perfect recipe to get a storm like Irma.”

Hurricane Irma is an absolute monster of a storm. It made landfall in the Florida Keys Sunday as a Category 4 storm with 130 mph winds, and threatens much of the Florida peninsula with dangerous storm surge, high winds, and drenching rains.

And it’s already a storm for the record books. At its peak, Irma sustained 185 mph winds. That’s one of the strongest wind speeds ever recorded for a hurricane (the record is 190 mph). And it kept up those 185 mph winds for 37 hours, a record duration for winds of that intensity. Those winds are “similar to a tornado, except this tornado is 80 miles wide,” says Jeff Weber, a meteorologist with the National Center for Atmospheric Research. Irma was a Category 5 storm for 3.25 days, which ties the previous record. (Though Irma has since downgraded, make no mistake: It’s still incredibly dangerous.)

We’re witnessing a truly extreme event. Irma, with its hurricane-force winds extending 80 miles out from its center “will bring life-threatening wind impacts to much of Florida regardless of the exact track of the center,” the National Hurricane Center reports.

But as Weber and Phil Klotzbach, an atmospheric scientist at Colorado State University, explain, this is the exact time of year you’d expect the most powerful storms to form. And it’s also not that surprising to see multiple tropical cyclones forming at once: As hurricane Jose is also in the Atlantic with Category 3 winds (and earlier in the week, hurricane Katia formed in the Gulf. It has since downgraded into a tropical depression.)

But for a monster like Irma, all the conditions — that are actually quite common for this time of year — have lined up. “You need a perfect recipe to get a storm like Irma,” Klotzbach says.

And we have it.

Why Irma got so big, intense, and scary

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The strength of a hurricane is determined by three main factors: water temperature, wind shear, and moisture in the atmosphere. Warmer water and atmospheric moisture give the system energy. A low wind shear — i.e., sharp changes in wind directions as you go higher and higher in the atmosphere — keeps a hurricane from dissipating.

All these conditions have been ideal for Irma. As for the wind shear, Klotzbach says there’s “almost zero.” That’s just bad luck. “If you plot shear averages over the Atlantic over the past 30 years, there’s really no trend,” he says.

The Atlantic, meanwhile, reaches its hottest temperatures for the year in September (that’s why the peak of hurricane season is September 10). Klotzbach says the Atlantic is a degree or two Fahrenheit hotter than it usually is this time of year, which is providing Irma with some extra fuel. (The surface area of water in the Atlantic topping 82.4 degrees in September has been growing slightly since the 1970s, which also could contribute to Irma’s strength.)

Irma reached a top speed of 185 mph Tuesday, and maintained those winds for 37 hours. That’s “close to the maximum intensity you’d ever get out of an Atlantic hurricane for that long,” Klotzbach says.

What’s more, Irma only encountered more hot water — approaching 90 degrees Fahrenheit — on its path to Florida. (The Gulf of Mexico, on the other hand, has been about 4 degrees above normal, which helped Harvey intensify so rapidly.) And, according to Klotzbach, “the atmosphere is more saturated than normal too.” So there’s plenty of energy and moisture to keep this system spinning quickly.

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Also fortuitous for Irma: It hasn’t made much direct contact with land. Yes, it devastated several islands in the Eastern Caribbean. But small islands “don’t create dry air to entrain into the system to slow it down,” Weber said. Irma’s eye has also traveled just North of islands like Puerto Rico and Hispaniola, which has allowed it to keep up its momentum. “It’s a perfect path [meteorologically speaking]” he adds. Irma made brief landfall on the North shore of Cuba, which sapped some of its strength.

But since, Irma has intensified. And has been following a runway of hot water all the way to Florida.

There are currently three hurricanes in the Atlantic and Gulf. Is that normal?

Within the past two week, two other storms — Katia and Jose — have achieved hurricane status in the Atlantic and Gulf. And what are we to make of the fact that three hurricanes are currently active at the same time?

This also isn’t unprecedented. It happens on average once every 10 years, Klotzbach explains. “In the satellite era, since 1966, this has happened in: 1967, 1980, 1995, 1998, 2010 and now in 2017.” So it’s fairly common, especially near the peak of the season, like we are in now.

Is climate change to blame for Irma’s strength?

This is always a tricky question when it comes to hurricanes. These storms are powerful, but they’re not necessarily much more powerful than storms we’ve seen in the past. Also, meteorologists have only been tracking hurricanes by satellite since the 1970s. Hurricanes have ravaged the Atlantic coast for hundreds of thousands of years. Scientists are still figuring out what, exactly, is “normal.”

But climate change does likely play a role, albeit a subtle one.

Vox’s Dave Roberts explained it best in a piece about Hurricane Harvey:

Say I turned up Earth’s gravity by 1 percent. More people around the world would trip and fall. Does it make sense to say, of a particular person tripping and falling, that the increase in gravity (“gravity change”) caused it to happen? No. Does it make sense to say that gravity cause it to happen? No. …

Increased gravity is a causal condition in every fall, but it is not the primary causal agent in any one fall. Similarly, increased heat energy is a causal condition in every storm (not just the bad ones) — every storm forms and travels in the same global climate — but it is not the primary causal agent in any storm.

Florida has been hit by intense hurricanes before. But the damage today can be greater because there are more people and fewer natural protections.

Climate change science predicts that in the future, hurricanes will intensify. There will be simply more energy available for them to form, and more moisture in the air to sustain them. Both Klotzbach and Weber agree the current consensus on whether hurricanes today are more intense due to climate change is messy.

But we don’t need the threat of climate change to know that hurricanes are a dangerous threat to Florida and the surrounding area. We know this because hurricanes have ravaged the state before. (If anything, it’s weird that the United States hasn’t more hurricanes in the past 10 years, as the Washington Post explains.)

In the 1940s and into the ’50s, “South Florida was hit by five Category 4 or 5 hurricanes in six years,” Klotzbach says. “Just what’s happened in the past should be scary enough to realize this stuff can be bad.”

But yet the South Florida coastline has only grown denser, reaching a population of 6 million for the first time in 2016. And Tampa, on the Gulf, is also one of the fastest growing cities in the country.

In 1926, Miami was hit with a storm that was likely a Category 4, and it devastated the young city, killing 375. If that same storm struck today, Brad Plumer explains at the New York Times, it “would inflict more than $200 billion in damage.” That’s an amount greater than Hurricane Katrina. Hurricane Andrew, the worst storm in the state’s history, caused $26.5 billion in damage. Today, it “would be far more catastrophic,” Plumer writes.

What makes hurricanes like Irma more dangerous than ever before is not climate change. It’s hubris.

“I would maintain that the actions humans have done to the land, such as paving roads and blocking natural estuaries, taking away sand barriers, and things of that nature, is a far greater impact than anything we’ve done in the warming world,” Klotzbach says. “If Miami was not completely paved and not much forethought was put into drainage when they built the city 100 years ago, this would not be as bad of a situation.”

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