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.

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Apple
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Why Do Supreme Court Justices Serve for Life?

Chip Somodevilla/Getty Images
Chip Somodevilla/Getty Images

There are few political appointments quite as important as a nomination to the U.S. Supreme Court. Unlike a cabinet secretary or an ambassador, justices serve for life. In the modern era, that often means more than three decades on the court. Thanks to increased lifespans, justices appointed in the next century are expected to sit on the Supreme Court for an average of 35 years, compared to the average of around 16 years that judges served in the past. Because of this shift, some scholars have begun to question whether lifetime appointments are still appropriate, as the definition of “for life” has changed so much since the Constitution was written. But why do justices serve for life, anyway?

Well, for one thing, the U.S. Constitution doesn’t exactly specify that justices and the court are in a “’til death do us part” relationship. Article III says that judges (of both the Supreme Court and lower federal courts) “shall hold their offices during good behavior.” So technically, a judge could be removed if they no longer meet the “good behavior” part of the clause, but there are otherwise no limits on their term. In practice, this means they have their seat for life, unless they are impeached and removed by Congress. Only 15 federal judges in U.S. history have ever been impeached by Congress—all lower court judges—and only eight have been removed from office, though some have resigned before their inevitable removal.

The only Supreme Court justice Congress has tried to impeach was Samuel Chase, who was appointed by George Washington in 1796. Chase was an openly partisan Federalist vehemently opposed to Thomas Jefferson’s Democratic-Republican policies, and he wasn’t afraid to say so—either in his role as a lower court judge or once he was appointed to the Supreme Court. In 1804, the House of Representatives, at then-president Jefferson’s urging, voted to impeach Chase, accusing him, among other things, of promoting his political views from the bench instead of ruling as a non-partisan judge. However, he was acquitted of all counts in the Senate, and went on to serve as a Supreme Court justice until his death in 1811.

The point of giving justices a seat on the bench for the rest of their lives (or, more commonly nowadays, until they decide to retire) is to shield the nation’s highest court from the kind of partisan fighting the Chase impeachment exemplified. The Supreme Court acts as a check against the power of Congress and the president. The lifetime appointment is designed to ensure that the justices are insulated from political pressure and that the court can serve as a truly independent branch of government.

Justices can’t be fired if they make unpopular decisions, in theory allowing them to focus on the law rather than politics. Justices might be nominated because a president sees them as a political or ideological ally, but once they’re on the bench, they can’t be recalled, even if their ideology shifts. Some data, for instance, suggests that many justices actually drift leftward as they age.

The lack of term limits “is the best expedient which can be devised in any government, to secure a steady, upright, and impartial administration of the laws,” Alexander Hamilton wrote in the Federalist No. 78. The judiciary, he believed, “is in continual jeopardy of being overpowered, awed, or influenced by its coordinate branches,” and “nothing can contribute so much to its firmness and independence, as permanency in office.” Without lifetime job security, he argued, judges might feel obligated to bow to the wishes of the president, Congress, or the public, rather than confining their work strictly to questions of the Constitution.

While lifetime appointments may be a longstanding tradition in the U.S., this approach isn’t the norm in other countries. Most other democracies in the world have mandatory retirement ages if not hard-and-fast term limits for high court judges. UK Supreme Court justices face mandatory retirement at age 70 (or 75 if they were appointed before 1995), as do judges on Australia’s High Court. Canadian Supreme Court justices have a mandatory retirement age of 75, while the 31 justices of India’s Supreme Court must retire by the age of 65. Until her passing at the age of 87 on September 18, 2020, the oldest justice on the current U.S. Supreme Court was Ruth Bader Ginsburg. Oliver Wendell Holmes Jr., the oldest justice in U.S. history, retired in 1932 at age 90.

Though the U.S. Supreme Court has never had term limits before, there have recently been serious proposals to implement them. Term limits, advocates argue, could combat partisan imbalances on the court. Presidents wouldn’t get to appoint justices purely based on whether someone died while they were in office, and the stakes for political parties nominating a justice would be slightly lower, possibly leading presidents and Congress to compromise more on appointments. One popular suggestion among political analysts and scholars is to impose an 18-year term limit, though critics note that that particular plan does bring up the potential that at some point, a single president could end up appointing the majority of the justices on the court.

In any case, considering such a change would likely require a constitutional amendment, which means it’s probably not going to happen anytime soon. For the foreseeable future, being on the Supreme Court will continue to be a lifetime commitment.

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