Ocean Sciences News

Does U.S. Hurricane Rating Scale Get the Danger Right?

Some scientists think it's time to retire the Saffir-Simpson scale and start fresh.

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When Hurricane Katrina made landfall 10 years ago tomorrow, it claimed the lives of more than 1800 people and caused $108 billion in damages. Decades prior, in 1969, Hurricane Camille devastated the same region, although it resulted in only 262 deaths and about a billion dollars in damage ($6.2 billion in 2015 dollars).

So why was Katrina categorized as a 3 and Camille a 5?

The answer has to do with the physical differences between the storms, which is part of the reason why some scientists think that the current way that the National Oceanic and Atmospheric Administration (NOAA) categorizes hurricanes—using the Saffir-Simpson hurricane scale (SSHS)—is inadequate.

“There’s a strong feeling that the current scale doesn’t accurately communicate hazard,” said Chris Davis, a hurricane specialist at the National Center for Atmospheric Research in Boulder, Colo.

Size and Speed Matter

The SSHS was developed in the 1970s by Herb Saffir, a structural engineer, and Bob Simpson, a meteorologist, who attempted to quantify the physical damage incurred by hurricane-force winds and calibrated the numbers to a simple-to-read 1–5 scale, in which each category represents a range of wind speeds.

But winds alone don’t make a hurricane dangerous. Hazards include the potential for flooding, from both rainfall and seawater inundation. A hurricane’s powerful winds push waves toward the shore at speeds that are sometimes faster than the storm itself, piling up water near the shore before the storm itself even arrives.

Size was a major factor that differentiated relatively compact Camille from sprawling Katrina, said Mark Powell, a scientist formerly with NOAA’s Hurricane Research Division who now runs Hwind Scientific, a private company based in Tallahassee, Fla., that provides real-time hurricane analysis.

At its largest, Camille’s hurricane-force winds only stretched 60 miles (97 kilometers) from the storm’s center, whereas Katrina extended twice as far. Although Camille’s maximum wind speed (more than 200 miles (322 kilometers) per hour) far exceeded Katrina’s (130 miles (209 kilometers) per hour), Katrina’s size allowed the storm to push 4 times as much water toward the shore.

When comparing storms like Camille and Katrina, “it’s not important how strong the peak wind is, what’s important is how big the storm is,” according to Powell.

Davis noted that the speed at which a storm moves across the ocean can also affect its hazard potential. “It turns out that slower-moving storms can create more damage because the adverse conditions last longer,” he said.

The SSHS scale takes into account neither a hurricane’s size nor the speed it travels, Davis said.

Side-by-side diagrams of Hurricanes Camille (1969, category 5) and Katrina (2005, category 3) show the vast difference in their sizes. Credit: Mark Powell, NOAA Hurricane Research Center
Side-by-side diagrams of Hurricanes Camille (1969, category 5) and Katrina (2005, category 3) show the vast difference in their sizes. Credit: Mark Powell, NOAA Hurricane Research Division

Other Shortcomings

The SSHS also falls short, critics say, because any hurricane with wind speeds exceeding 156 miles per hour (251 kilometers per hour) remains a category 5, no matter how much faster the wind blows. Given that scientists expect global warming to make hurricanes more intense, a new rating scale should not lump all the highest–wind speed storms into a single, open-ended category, said Lakshmi Kantha, a scientist at the University of Colorado in Boulder.

Another one of Kantha’s main criticisms of the scale is that because each of the SSHS categories encompasses a wide range of wind speeds, a small difference in wind speed at the edge of a storm’s range can dramatically alter a hurricane’s rating.

For instance, a category 3 hurricane sustains wind speeds between 111 and 130 miles per hour (179 and 209 kilometers per hour), whereas a category 2 storm’s winds blow at 96 to 110 miles per hour (154 to 177 kilometers per hour). So a wind speed a few miles per hour lower could bump a hurricane down a whole category but not necessarily affect its strength or potential to cause serious damage, Kantha said. That’s why he calls for a hurricane scale with more precise numbers, like 2.4 or 3.8, for communicating hurricane strength.

THE SAFFIR-SIMPSON SCALE

  • Developed by Herb Saffir and Bob Simpson
  • Quantifies wind damage on structures
  • Range 1–5
  • Katrina was a category 3

ALTERNATIVE HURRICANE SCALES

Integrated Kinetic Energy

  • Developed by Mark Powell and colleagues
  • Represents wind force on the ocean; accounts for storm size and speed
  • Range 0.00–5.99
  • Katrina would have been a 4.9

Hurricane Intensity Index (HII) + Hurricane Hazard Index (HHI)

  • Developed by Lakshmi Kantha
  • Uses fluid dynamics to rate hurricane strength; for use with additional scales
  • Range 0.3 to above 5.0 for HII, 0.3 to about 15.00 for HHI
  • Katrina would have been a 3.0 on HII and a 14.5 on HHI

Cyclone Damage Potential

  • Developed by George Holland, James Done, and colleagues
  • Uses coupled atmosphere-ocean dynamics for interactions of wind, waves, currents
  • Range 1–10
  • Katrina would have been a 6.6

Alternate Scales

Kantha urges use of multiple scales—an intensity scale like SSHS with others that rate the potential damage from, say, winds or storm surge. Because such factors are hard to combine into one simple-to-use scale, Kantha suggests providing them separately but simultaneously. Kantha’s proposed intensity scale, which he calls a “hurricane intensity index,” uses laws of fluid dynamics, which he says more accurately portray a hurricane’s intensity.

Powell, meanwhile, analyzes hurricanes using a concept called “integrated kinetic energy”—a measurement of the hurricane’s total energy based on wind speed as well as size. Applied to a numerical rating system akin to the SSHS, these measurements might be able to better reflect how much damage a hurricane could cause, Powell said.

Even if hurricane experts all agreed that an alternate scale is needed, the challenge lies in devising one as easy to understand as the SSHS, Davis noted. Even though agencies like NOAA make supplemental information about the size and the speed of storms available, he said, the data are useless to most people “unless you know where to look for it, and how to interpret it.”

Going Beyond the SSHS

Scientists at NOAA recognize that the SSHS alone inadequately communicates hurricane hazard, said Jamie Rhome, a scientist in the National Hurricane Center’s Storm Surge Unit. Instead of starting from scratch and creating a new scale, the center has begun to develop other measures and warnings of storm surge and flooding to provide what SSHS leaves out.

“We’re about 7 years into a 10-year journey to bring storm surge into the forefront because that’s what’s killing the largest number of people in hurricanes,” he said.

The team is “effectively de-emphasizing the Saffir-Simpson scale to communicate the hazards of a hurricane,” Rhome said. “We’re breaking out the hazards individually and then dealing with them with their own separate communication mechanism.”

The research center has started testing the effectiveness of flood risk maps that communicate how high water could get during a storm and how far inland it could go. Additionally, to supplement the SSHS, center scientists rolled out prototype alerts to be issued if a life-threatening storm surge is imminent. This warning system should go into effect in 2017.

—JoAnna Wendel, Staff Writer

Citation: Wendel, J. (2015), Does U.S. hurricane rating scale get the danger right?, Eos, 96, doi:10.1029/2015EO034841. Published on 28 August 2015.

© 2015. The authors. CC BY-NC 3.0
  • Thomas H Pritchett

    In essence, if the Saffir-Simpson scale did not have the arbitrary cut-off at level 5, we would have already had at least on Category 6 storm – Tropical Storm Haiyan

    • Alexandre Araújo Costa

      Yes. Also Patricia.