How Are Hurricane Categories Determined?

Satellite photo of Hurricane Dorian, a Category 5 storm, tracks toward the Florida coast on September 1, 2019.
Satellite photo of Hurricane Dorian, a Category 5 storm, tracks toward the Florida coast on September 1, 2019.
NOAA via Getty Images

Residents of the Bahamas are in the midst of Hurricane Dorian, a Category 5 storm that prime minister Hubert Minnis said has already caused "unprecedented" devastation to the area. Meanwhile, residents of Florida, Georgia, and the Carolinas are being evacuated ahead of the storm's potential arrival in the U.S. At this point, we've become accustomed to hearing about hurricanes, and to predicting what sort of damage they might cause based on their category number. But how do meteorologists categorize these often-deadly storms, and how does that scale work?

First, a quick primer: Hurricanes are tropical cyclones that occur in the Atlantic Ocean and have winds with a sustained speed of at least 74 mph. A tropical cyclone, in turn, is a storm system that develops in the tropics and is characterized by a low pressure center and thunderstorms that produce strong winds, rain, and storm surges. Tropical cyclone is a generic name that refers to the storms' geographic origin and cyclonic rotation around a central eye. Depending on their location and strength, the storms are called different things. What gets dubbed a hurricane in the Atlantic, for example, would be called a typhoon if it happened in the northwestern Pacific.

What's the difference between a hurricane and a tropical storm?

Simply put: Wind speed. When tropical cyclones are just starting out as general areas of low pressure with the potential to strengthen, they’re called tropical depressions. They’re given sequential numbers as they form during a storm season so the National Hurricane Center (NHC) can keep tabs on them.

Once a cyclone’s winds kick up to 39 miles per hour and sustain that speed for 10 minutes, it becomes a tropical storm and the NHC gives it a name. If the cyclone keeps growing and sustains 74 mph winds, it graduates to hurricane.

Once we call it a hurricane, how do we categorize it?

In order to assign a numeric category value to a hurricane, meteorologists look to the Saffir-Simpson Hurricane Wind Scale, which was developed as a classification system for Western Hemisphere tropical cyclones in the late 1960s and early '70s by structural engineer Herbert Saffir and his friend, meteorologist Robert Simpson, who was the director of the NHC at the time.

When Saffir was working on a United Nations project to study low-cost housing in hurricane-prone areas, it struck him that there was no simple, standardized way of describing hurricanes and their damaging effects, like the way the Richter scale is used to describe earthquakes. He created a five-level scale based on wind speed and sent it off to Simpson, who expanded on it to include the effects on storm surge and flooding. Simpson began using it internally at the NHC, and then in reports shared with emergency agencies. It proved useful, so others began adopting it and it quickly spread.

How does the scale work?

According to the NHC, the scale breaks down like this:

Category 1 storms have sustained winds of 74 to 95 mph. These “very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding, and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days."

Category 2 storms have sustained winds of 96 to 110 mph. These “extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks."

Category 3 storms have sustained winds of 111 to 129 mph. This is the first category that qualifies as a “major storm” and “devastating damage will occur: Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes."

Category 4 storms have sustained winds of 130 to 156 mph. These storms are “catastrophicand damage includes: “Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

Category 5 storms have sustained winds of 157 mph or higher. The catastrophic damage entailed here includes: “A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

While the Saffir-Simpson scale is useful, it isn’t the be-all and end-all for measuring storms, as the National Oceanic and Atmospheric Administration (NOAA) pointed out on Twitter in 2013:

Is there anything worse than a category 5?

Not on paper, but there have been hurricanes that have gone beyond the upper bounds of the scale. Hypothetically, hurricanes could up the ante beyond Category 5 more regularly. The storms use warm water to fuel themselves and as ocean temperatures rise, climatologists predict that potential hurricane intensity will increase.

Both Saffir and Simpson have said that there’s no need to add more categories because once things go beyond 157 mph, the damage all looks the same: really, really bad. Still, that hasn't stopped several scientists from suggesting that maybe the time has come to consider a Category 6 addition.

Timothy Hall, a senior scientist at NASA's Goddard Institute for Space Studies, recently told the Los Angeles Times that if the current global warming trends continue, he can foresee a time—likely by the end of the century—where wind speeds could blow past 230 mph, which could create conditions similar to a F-4 tornado (which has the power to lift cars off the ground and send them hurtling through the air with relative ease).

“If we had twice as many Category 5s—at some point, several decades down the line—if that seems to be the new norm, then yes, we’d want to have more partitioning at the upper part of the scale,” Hall said. “At that point, a Category 6 would be a reasonable thing to do."

An earlier version of this article appeared in 2013.

What's the Difference Between Stuffing and Dressing?

iStock
iStock

For carbohydrate lovers, nothing completes a Thanksgiving meal quite like stuffing—shovelfuls of bread, celery, mushrooms, and other ingredients that complement all of that turkey protein.

Some people don’t say stuffing, though. They say dressing. In these calamitous times, knowing how to properly refer to the giant glob of insulin-spiking bread seems necessary. So what's the difference?

Let’s dismiss one theory off the bat: Dressing and stuffing do not correlate with how the side dish is prepared. A turkey can be stuffed with dressing, and stuffing can be served in a casserole dish. Whether it’s ever seen the inside of a bird is irrelevant, and anyone who tells you otherwise is wrong and should be met with suspicion, if not outright derision.

The terms are actually separated due to regional dialects. Dressing seems to be the favored descriptor for southern states like Mississippi, Tennessee, South Carolina, and Georgia, while stuffing is preferred by Maine, New York, and other northern areas. (Some parts of Pennsylvania call it filling, which is a bit too on the nose, but to each their own.)

If stuffing stemmed from the common practice of filling a turkey with carbs, why the division? According to HuffPost, it may have been because Southerners considered the word stuffing impolite, and therefore never embraced it.

While you should experience no material difference in asking for stuffing or dressing, when visiting relatives it might be helpful to keep to their regionally-preferred word to avoid confusion. Enjoy stuffing yourselves.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

Why Do Tires Have to Be Filled With Air?

BookyBuggy/iStock via Getty Images
BookyBuggy/iStock via Getty Images

Paul Misencik:

This is an issue that has perplexed me for most of my life, because pneumatic tires filled with air seem like the last anachronistic, 19th-century component of a modern automobile, and an idea which should have disappeared many decades ago. In an era where even the internal combustion engine itself is giving way to electric motors, and where a new economy hatchback has exponentially more computing power than the Space Shuttle, pneumatic tires don’t seem to make sense any longer.

(And before I get flamed, I know modern tires are vastly more advanced and reliable and capable than their 1930s counterparts. Blowouts, which were a common occurrence when I was a kid, are pretty much unheard of today. Modern tires are great, but they are still vulnerable and maintenance-intensive in a way that doesn’t make any sense to me.)

Companies have experimented with non-pneumatic passenger vehicle tires in the modern age—one of the primary drivers was Michelin. But the tires weren’t filled with solid rubber. In fact, they didn’t even have sidewalls. They were open on the sides, and they had a support lattice of structural polyester ribs, with a ton of air space between the contact patch and the (now deformable) wheel.

One of the big problems with switching from pneumatic tires to non-pneumatic tires is the fact that the current air-filled tire is an important component of the suspension of a vehicle. The flex in the sidewall is a critical part of the compliance of the suspension and substantially affects a vehicle's ride and handling. (Which is why race car drivers sweat tire pressures at each corner of the vehicle so much, as even a small change in tire pressure can have a big effect on the handling and grip of a vehicle.)

If a company like Michelin wants to make a non-pneumatic tire, they'll improve their chances of finding success with it if the new design mimics the compliance and flex characteristics of the outgoing, air-filled models as closely as possible. That way, Michelin would be able to sell the new, non-pneumatic design as a retrofit to older vehicles whose suspensions were originally designed with pneumatic tires in mind. And that is hugely important because if they can’t, it becomes much more difficult to convince manufacturers to change over to the new design—particularly after the mild debacle of Michelin’s failed “TRX” metric tire idea of the 1980s, which required the use of a special wheel and which, despite being by most accounts a superior design in almost every way, never really took off. (Owners of 1980s Ferrari 512 Berlinetta Boxers and some Saab 900 turbos will know what I’m talking about here.)

Non-pneumatic Michelin tires are also rather weird looking, and it’s not clear which manufacturers, if any, would take the risk of being the first to offer them on a new car.

So that is the real issue: Any non-pneumatic tire design must be not only clearly superior to the pneumatic designs of the past, but it must be functionally identical to the outgoing models they would replace, and they must be visually acceptable to consumers.

I hope it happens, though. I hope someone cracks the nut. Pneumatic tires are a 19th-century application still being used on 21st-century vehicles, and at some point that needs to change.

This post originally appeared on Quora. Click here to view.

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