To mark the end of the hurricane season means to not only take stock of the lessons learned from this year’s storms, but to explore the progress made from lessons learned and debated in past years following major storms. One issue that has gained urgency in the scientific community over the past few hurricane seasons has been the continued use of the Saffir-Simpson Hurricane Scale.
The Saffir-Simpson Hurricane Scale was developed by civil engineer Herbert Saffir and meteorologist Bob Simpson. Saffir was commissioned to study low-cost housing in regions of the world that were prone to tropical cyclones and hurricanes by the United Nations in the late 1960s. Realizing that no system existed to describe the effects of a hurricane, he developed a 1–5 scale based on wind speed that showed expected damage to structures. Later, Robert Simpson, then director of the National Hurricane Center, modified Saffir’s work, adding measurements for flooding and storm surge. The scale was introduced to the general public in 1973. Saffir, who died in November 2007, defended his system in recent years by stating that adding too many variables into a rating system would make it too complex, and that the system’s longevity was due to it being easy for the public to understand.
But critics argue that the current scale doesn’t take a hurricane’s size, rainfall or location into account, and is too simplistic. For example, Hurricane Ivan (2004) and Hurricane Dennis (2005) were both categorized as Category 3 storms when they made landfall in the United States, 30 miles apart from each other. However, the differences in each hurricane’s size, speed and fluctuations in strength resulted in extreme differences in the total destruction left in the wake of the hurricanes’ paths. Dennis caused US$4 billion in total damages in all affected countries, while Ivan resulted in US$3 billion in damages in the Carribean alone, and an additional US$13 billion in the United States.
Also, as with Ivan in the inland United States, the rain associated with a hurricane often leads to flooding that causes just as much or more death and damage than the wind. In coastal areas, the storm surge is often the most devastating component of a hurricane, but this is determined more by the size of a hurricane rather than its maximum wind speeds. Hurricane Katrina (2005) hit wind speeds that categorized it as a Category 5 storm for part of its lifespan, as did Hurricane Camille in 1969. However, Katrina’s hurricane-force winds extended 105 miles from its center while Camille’s only extended 60 miles out, resulting in much greater damage from Katrina.
Fast forward to this year: Hurricane Ike made landfall in the United States as a Category 2 storm, but became the most destructive storm of the 2008 season and for the United States, ranks as the third-costliest tropical storm behind Hurricanes Katrina and Andrew. Again, officials are questioning the value of the Saffir-Simpson hurricane scale and discussing whether and how the system should be revised. Local officials and citizens often make decisions based on the ranking of a storm on the scale, and as a Category 2, Hurricane Ike was not considered a major hurricane. Revisions to the current scale system could result in more accurately predicting a storm’s potential destructiveness by taking all data into account that modern meteorological equipment can provide.
One alternative system is the Hurricane Severity Index, employed by the private forecasting service ImpactWeather. The HSI rates storms on a scale from 1 (weakest) to 50 (strongest), accounting for both the intensity of a storm as well as the size of its wind fields. Another alternative is Integrated Kinetic Energy, a concept developed by Mark Powell, an atmospheric scientist for NOAA’s Hurricane Research division, and Timothy Reinhold, a former Deputy Director of FEMA. A storm’s IKE (not to be confused with Hurricane Ike) calculates its total wind energy to determine its destructive potential. These systems would not be as easy for the public to understand, but supporters argue that it is more important that emergency personnel understand the numbers and react accordingly.
In the face of a storm, though, the trust and cooperation the public is the one of the most important determinants in the outcome of a storm’s destructiveness in loss of life and property. If new systems are used, the scientific community will have to work to establish the public’s trust in their effectiveness.