Hurricane Maria (left) and Hurricane Lee (right). Maria’s winds were much weaker than Lee, but it’s a huge storm. (NOAA/NASA RAMMB/CIRA)

Samantha Durbin was born and raised in Silver Spring, Md. She graduated from the University of Maryland Baltimore County with a degree in Environmental Science in May 2017. Currently, she is a graduate student researching severe storms and societal impacts at the University of Maryland. 

The 2017 hurricane season was exhausting. In its peak, 10 hurricanes formed in 10 weeks. Three of those made devastating landfalls in the Caribbean and United States.

As we recover, it’s a good time to talk about how we rank these storms, the Saffir-Simpson scale, and whether there are better alternatives that would more clearly convey the threat. Each major hurricane this season — Harvey, Irma, Maria — had characteristics that were markedly different from the previous.

  • Harvey: Long-duration heavy rain, deadly flooding
  • Irma: Large wind field and power outages
  • Maria: Torrential rain, surge and wind in mountainous island terrain

Between urban developments and population growth, our society has changed dramatically since 1971 when Saffir-Simpson was created.

There are obvious benefits to leaving things the way they are. The Saffir-Simpson hurricane scale is well known. It’s simple and easy to understand — a five-point scale that denotes the increasing threat of catastrophe from Category 1 to Category 5. People understand that a Category 5 hurricane will be devastating, and a Category 1 hurricane is still a risk, albeit considerably weaker.

But the current scale is also flawed.

A Category 2 can do more damage than a Category 4

The Saffir-Simpson scale assumes every storm is similar. It ignores where the storm is making landfall and disregards the actual size of the storm.

Compare, for example, Hurricane Charley and Hurricane Ike.

Charley was a Category 4 when it made landfall in Southwest Florida in 2004. The storm killed 15 people and caused about $16.3 billion in damages.

On the other hand, Hurricane Ike was a Category 2 when it made landfall near Galveston, Tex., in 2008. Hurricane Ike killed 195 people and caused $35.5 billion in damages.

Ike took far more lives and did far more damage than Charley, a Category 4. The difference was Hurricane Ike’s giant size and storm surge.

Storm surge is an aspect of a tropical storm that is better predicted by combination of size and wind speed rather than wind speed alone. Larger storms tend to produce larger surges. They also affect a greater portion of the coast.

In general, storms weaken as they expand. Although this means weaker maximum winds, there is now possibility for wind damage over a larger area. Bigger storms are also over any given location for more time, which leads to a longer duration of impacts.

Superstorm Sandy was a monster when it blew into the Northeast in 2012. The storm only had Category 1 strength winds at landfall, but because of its large size, it devastated several Mid-Atlantic and Northeast states.

In the case of Sandy, location was perhaps its main destructive factor. It hit the most densely populated corridor of the United States.

Room for change

The challenge is to find a way to categorize a storm that takes into account all of the factors that influence its potential for damage. Otherwise, how do you explain to people that a Category 1 hurricane could be more destructive than a Category 4?

Some meteorologists have proposed alternatives.

StormGeo is a consulting company based out of Norway. In 2006, Chris Hebert and Bob Weinzapfel proposed using what they coined the Hurricane Severity Index (HSI).

This scale takes into account not only the wind speed but the size of the storm and the area the winds cover. It uses a rating scale from 1 to 50, with 50 being the most destructive. The “points” are assigned by size and wind speed, with 25 being the highest in each category.

HSI scale chart. (StormGeo)

Points are assigned quantitatively. More points are assigned to storms that have regions with hurricane-force winds, since they are more damaging.

A tropical storm with 35 mph winds can receive one intensity point, and a hurricane with winds above 175 mph would receive the maximum 25 total intensity points.

Hurricane Dolly (2008) was a small category 2 storm and Hurricane Ike (2008) was a large category 2 storm. Dolly’s storm surge was small and damages cost around $1.35 billion. Ike produced a surge of 22 feet and, as stated before, caused $35.5 billion dollars in damage.

The image below shows the track, intensity, and size of Ike and Dolly. This illustrates how size and intensity significantly change over the course of a storm’s lifetime. The size difference when they both made landfall is clear.

Using the Saffir-Simpson scale, the storms appear equal. However, using HSI, Dolly is a 12 near landfall and Ike is a 28. Indeed, Hurricane Ike caused more damage than Dolly.

A comparison of Hurricane Ike and Hurricane Dolly using the HSI scale. (Samantha Durbin)

Greg Holland and James Done, researchers at UCAR in Boulder, Colo., also proposed a new hurricane scale in 2016. The Cyclone Damage Potential Index (CDP) gives a better picture of the destructive power of a storm based on a rating from 1-10.

CDP uses an equation to relate the maximum surface wind speed, the radius of hurricane force wind and how fast the storm is moving. This provides more insight into the duration of the storm.

Hurricane Andrew (1992) was a Category 5 and Hurricane Irma (2017) was a Category 4 at landfall in Florida, according to the Saffir Simpson scale. You would expect them to cause similar devastation however, Andrew affected the southwestern corner of Florida, while Irma wreaked havoc on the entire peninsula. With CDP, Irma is ranked a 5.9 at landfall and Andrew a 2.9.

You can use the CDP of previous storms to assess what kind of damage to expect from future storms. This index provides valuable information to insurance companies, weather reporters and the public.

The equation is not simple, but the resulting 10-point scale is clear and easy to understand.

Better knowledge of a coming storm means more lives saved, more money saved and more information to help FEMA and insurance agencies prepare. It’s time for an upgrade.