Hurricane season: What is the Saffir-Simpson scale; how does it work; is there a Category 6?

One was a structural engineer who thought in the ways engineers are trained to — logically and result-oriented.

The other, was a meteorologist who, at age 6, had survived one of the deadliest hurricanes to ever hit the United States, and was eager to warn others of the destructive potential of a tropical weather system.

Together, engineer Herbert Saffir and meteorologist Robert Simpson developed a system that offered people who live in storm-prone areas a clear early warning of trouble to come.

The Saffir-Simpson Hurricane Wind Scale was developed in 1971 and unveiled to the public in 1973.

Here’s a look at the system that ranks tropical cyclones by their potential destructive power, how it works and the men who invented it.

What is the Saffir-Simpson Hurricane Wind Scale?

The scale rates the potential for damage from hurricanes based on the storm’s sustained wind speed.

Who were Saffir and Simpson?

Herbert Saffir was a structural engineer who moved to Florida to become a county engineer after graduating from Georgia Tech and serving in World War II.

After living in South Florida for a while, Saffir became interested in the effects of hurricane-force winds on coastal structures, and in 1959 opened a structural engineering firm in Coral Gables, Florida.

He quickly became an expert on the forces that damage buildings during a storm and was asked to help develop building codes for the region.

His expertise led to an appointment to head a United Nations project looking for a way to reduce damage to low-cost buildings in hurricane-prone areas. The work he did on that project became the basis for Saffir's scale of wind damage.

Saffir continued to work in structural engineering until four weeks before his death at age 90 in 2007.

Robert Simpson had first-hand knowledge of hurricanes from an early age. In 1919, when he was 6, he and his family survived a massive hurricane that made landfall near Corpus Christi, Texas. The family had to swim down the streets of the town to safety as the waters rose to 8 feet above street level.

“The family had to swim — with me on my father’s back — three blocks in near hurricane force winds to safe shelter in the courthouse,” Simpson said. “A lot of what I saw frightened me, but also supplied a fascination that left me with a lifelong interest in hurricanes.”

After graduating from Southwestern University in Georgetown, Texas, and then earning a master's degree at Emory University in Atlanta, he worked as a music teacher in Texas high schools because he could not find work as a physicist. Finally, in 1940, he was hired by the U.S. Weather Bureau as a meteorologist. Simpson worked all over the world for the Weather Bureau, with stints in New Orleans, Panama, Miami, Hawaii and Washington D.C.

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In the 1950s, he lobbied officials at the Weather Bureau (the forerunner of the National Weather Service) to do more research into tropical systems and the effects they have on coastal areas. His arguments worked, and in 1955, he was appointed to lead the National Hurricane Research Project.

He headed up the project for four years then left to get a doctorate at the University of Chicago. In the 1960s, he was in charge of Project STORMFURY, an experiment in which clouds were seeded with silver iodide in the hopes of diminishing hurricane intensity.

In 1967, Simpson became the deputy director of the National Hurricane Center. In 1968, he was named the center's director. He stayed at NHC until 1973.

He retired to Washington to start a weather consulting firm with his wife, Joanne.

How was the Saffir-Simpson Hurricane Wind scale developed?

The system of categories that became the National Hurricane Center's way of conveying the strength and destructive potential of a storm did not start out as an NHC project.

Saffir’s United Nations project work led him to create a rating system for hurricanes that the U.N. could use to try to match buildings with their potential risks for damage. At the time, hurricanes were classified as either “minor” or “major” storms. In 1969, Saffir came up with a rating system that included five categories using wind speed, barometric pressure, likely flooding and storm surge as determining factors.

Saffir took his work to Simpson, who was the head of the NHC at the time. Simpson wanted to have a system that gave people common sense information about storms to help them make a decision about staying put or evacuating a coastal area.

Neil Frank, who succeeded Simpson as NHC director, told The Washington Post that Simpson was “very sensitive to being able to communicate to the public in meaningful terminology.”

Simpson and Saffir worked together. Simpson assigned a range of wind speeds and storm surges for each category, and the Saffir-Simpson Hurricane Wind Scale was born.

The NHC released the scale to the public in 1973 and began classifying storms the following season.

The system remained as it was developed until 2009, when the NHC eliminated storm surge, pressure and potential flooding from the factors that make up the categories. Those factors, the NHC explained, did not always match up with the damage that storms can inflict.

Another change was made in 2012, when the wind speed for a Category 4 storm was changed by 1 mph at both ends of the category. That was done because winds speeds are measured in 5-knot increments by the NHC, and the conversion to a miles-per-hour-measurement was incorrectly classifying storms as either Category 3 or Category 5.

How does the Saffir-Simpson scale work?

The scale has five categories ranging from Category 1 — with winds from 74 mph to 95 mph to a Category 5 — with sustained winds in excess of 155 mph. The National Hurricane Center uses a one-minute averaging time to establish a measure of sustained winds. In other words, the highest winds speed maintained for a full minute would be the highest sustained wind speed for a storm.

Here, from the National Hurricane Center, are the categories for the scale:

Category 1: Maximum sustained winds are at 74-95 mph. 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: Maximum sustained winds are at 96-110 mph. 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: Maximum sustained winds are at 111-129 mph. 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: Maximum sustained winds are at 130-156 mph. 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: Maximum sustained winds are at 157 or higher. Catastrophic damage will occur: 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.

Can there be a Category 6 hurricane?

With stronger storms of the past decade, some have questioned whether there should be another category for hurricanes, a Category 6 that would comprise of storms with sustained winds of 158 mph-180 mph.

Before his death in 2014, Simpson argued that there was no need for another category since what is measured is the potential damage a hurricane’s winds can inflict on human-made structures. Simpson once told The Washington Post that “when you get up into winds in excess of 155 mph (249 km/h) you have enough damage if that extreme wind sustains itself for as much as six seconds on a building, it’s going to cause rupturing damages that are serious no matter how well it’s engineered.”

In other words, winds from a Category 5 storm will be sufficient to severely damage or destroy most man-made structures.

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