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 That Thing That Hangs Off a Turkey’s Face?

NCHANT/iStock via Getty Images
NCHANT/iStock via Getty Images

That thing is called a snood. And it's there to let the other turkeys know that its owner is kind of a big deal.

When a male turkey—known as a tom—wants to mate, he faces two hurdles. One is his potential mates, the female turkeys (a.k.a. hens). In the realm of turkey mating, the hens wield the power of choice and the toms have to get a hen's attention and win the opportunity to reproduce. Come mating season, a tom will strut around, gobble, puff out his chest, fan his tail, and drag his wings to attract the hens, who then pick which of the toms they’ll mate with.

The second problem for a tom looking for love is the other toms in the area. They’re all competing for the same limited number of hens. Sometimes a good mating display isn’t enough to win a mate, and toms will attack and fight each other to secure a hen.

This is where the snood comes in. That goofy-looking piece of dangling flesh helps a tom both with choosy hens and with competition from rival males. Having a long snood almost always means that a hen will want to mate with him and that another tom will back down from a fight.

Dudes and their snoods

When two toms are trying to establish dominance, they’ll size each other up. Then they'll either fight, or one will flee.

In the late 1990s, Dr. Richard Buchholz, an animal behaviorist who focuses on turkeys, wanted to figure out which, if any, characteristics of a tom turkey could predict how they fare in dominance fights. That is, did bigger turkeys tend to win more scuffles? Did older ones? He also wanted to see if the turkeys used any of these predictive cues when sizing each other up. He looked at various characteristics of dominant toms that fight and win, and compared them to those of subordinate toms that lose fights or run from them. Of all the characteristics he looked at, only “relaxed snood length” seemed to be a reliable predictor of how a tom would do in bird-vs-bird combat. The dominant males, the ones who won fights and got a choice mate, had longer snoods.

With that in mind, Buchholz looked at how toms reacted to other toms with snoods of varying sizes. The birds tended to avoid confrontation with other males with longer snoods, and wouldn’t even feed near them. A big snood, this suggests, says to the other turkeys that this is a tom you don’t want to tangle with. Buchholz noted that snood length correlates with age, body mass, and testosterone, so, to competitors, the snood could be a good indicator of a tom’s aggressiveness, age/experience, size, and overall condition and fighting ability.

In the snood for love

Once the males have established who’s going to have a chance to mate, the final choice goes to the hen. While the mating display is the main draw for getting a hen to check him out, a tom’s snood helps him out again here.

Like it did for the other males, a tom’s snood signals a lot of information to a female assessing potential mates—it indicates how old and how big he is, and even says something about his health. In another study, Buchholz found that longer-snooded toms carried fewer parasites. If a hen wanted to choose a mate with good genes that might help her offspring grow large, live long, and avoid parasites, a tom’s snood is a good advertisement for his genes. In that study, hens showed a clear preference for toms with longer snoods. In another experiment years later, Buchholz found that healthy hens again showed a strong preference for long snoods and that hens with their own parasite problems were less picky about snood length and checked out more potential mates—perhaps, Buchholz thinks, because the hens recognized their own susceptibility to infection and were willing to invest more time searching for a tom with genes for parasite resistance that would complement their own—but still showed some preference for longer ones.

While a snood might look goofy to us, for a turkey, it’s integral to the mating game, signaling to other toms that they should get out of his way and letting hens know that he’s got what they’re looking for.

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What's the Difference Between Yams and Sweet Potatoes?

Julia_Sudnitskaya/iStock via Getty Images
Julia_Sudnitskaya/iStock via Getty Images

This Thanksgiving, families across the country will enjoy a traditional meal of turkey, stuffing, and sweet potatoes ... or are they yams? Discussions on the proper name for the orange starchy stuff on your table can get more heated than arguments about topping them with marshmallows. But there's an easy way to tell the difference between sweet potatoes and yams: If you picked up the tuber from a typical American grocery store, it's probably a sweet potato.

So what's a sweet potato?

Sweet potato and yam aren't just different names for the same thing: The two produce items belong to their own separate botanical categories. Sweet potatoes are members of the morning glory family. Regular potatoes like russets, meanwhile, are considered part of the nightshade family, which means that sweet potatoes aren't actually potatoes at all.

Almost all of the foods most Americans think of as yams are really sweet potatoes. The root vegetable typically has brown or reddish skin with a starchy inside that's orange (though it can also be white or purple). It's sold in most supermarkets in the country and used to make sweet potato fries, sweet potato pie, and the sweet potato casserole you have at Thanksgiving.

Then what's a yam?

Yams.
Yams.
bonchan/iStock via Getty Images

Yams are a different beast altogether. They are more closely related to lilies and grasses and mostly grow in tropical environments. The skin is more rough and bark-like than what you'd see on a sweet potato, and the inside is usually white or yellowish—not orange.

They're a common ingredient in parts of Asia, Africa, and the Caribbean. Because the inside of a yam is less moist than the inside of a sweet potato, they require more fat to make them soft and creamy. They're also less sweet than their orange-hued counterparts. In many regions in the U.S., yams aren't sold outside of international grocery stores.

Where did the mix-up come from?

Also sweet potatoes.
Also sweet potatoes.
Kateryna Bibro/iStock via Getty Images

So if yams and sweet potatoes are two totally different vegetables that don't look or taste that similar, why are their names used interchangeably in the U.S.? You can blame the food industry. For years, "firm" sweet potatoes, which have brown skin and whitish flesh, were the only sweet potatoes grown in the U.S. In the early 20th century, "soft" sweet potatoes, which have reddish skin and deep-orange flesh, entered the scene. Farmers needed a way to distinguish the two varieties, so soft sweet potatoes became yams.

Nearly a century later, the misnomer shows no signs of disappearing. Many American supermarkets still call their orange-fleshed sweet potatoes yams and their white-fleshed ones sweet potatoes, even though both items are sweet potatoes. But this isn't a strict rule, and stores often swap the names and make things even more confusing for shoppers. So the next time you're shopping for a recipe that calls for sweet potatoes, learn to identify them by sight rather than the name on the label.

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

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