WHEN Hurricane Harvey churned into Texas last month, it made landfall as a Category 4 storm, meaning its sustained wind speed was between 209kph and 251kph. The categorisation is determined via the well-known Saffir-Simpson Hurricane Wind Scale, developed in 1971. Category 4 implies “catastrophic damage will occur” as winds severely damage houses, snap power poles and uproot trees.
The storm did cause catastrophic damage, but it wasn’t due to its winds. Harvey rapidly weakened on the coast and was quickly downgraded to a tropical storm, booting it down and off the hurricane scale. Its malice came from spinning in place while it dumped several feet of rain. It devastated Houston by water instead of speed. Damages are estimated to be up to US$180 billion (RM756 billion), rivalling 2005’s Hurricane Katrina for the costliest natural disaster in America’s history.
In this instance, the Saffir-Simpson category ranking did not accurately capture the storm’s potential for destruction. Neither did it work for Hurricane Sandy in 2012, which only ever reached a Category 2 rating, and in fact, was unranked when it made landfall in the northeastern United States. One of the reason’s Harvey was so destructive was because it lingered for so long — but the potential durability of a storm doesn’t factor into the Saffir-Simpson metric. Yet, duration is one of the main cost drivers of hurricane devastation — especially for lower category storms like 1 or 2, or, in Harvey’s case, storms not on the scale at all. Which is why we need a better way of measuring hurricanes.
One storm metric meteorologists have used for decades is
the “Accumulated Cyclone Energy”, or ACE. It is proportional to the square of the wind speed, measured and summed every six hours. The square is used, because, like the kinetic energy of a car or cat, energy of motion increases as the second power of velocity. It is tabulated religiously every day in hurricane season, for each storm, then totalled for the year. But, one immediate problem with ACE is that, despite its name, it can’t convey the energy of a storm, because it doesn’t take note of a storm’s size in part because Hurricane Katrina made landfall in Mississippi as only a Category 3, with 201kph sustained winds.
Many in Katrina’s way expected it to be nothing like the monstrous Hurricane Camille of 1969, still the second most intense hurricane to ever strike the US, which came ashore as a Category 5 with breathtaking winds of 305kph.
There are metrics that include storm size to better estimate a storm’s total energy. But, Katrina’s maximum winds went out to a radius of 48km, double that of Camille’s, so the volume of water it put into motion was four times larger. As a result, Katrina’s storm surge was higher than Camille’s all across the coast, and economic losses 14 times greater.
Sandy, busting Asbury Park’s Madame Marie and a great deal else, was the physically largest storm to ever hit the US, with a wind field 1,770km across. It caused about US$79 billion in damages.
There are metrics that include storm size to better estimate a storm’s total energy, like the IKE (integrated kinetic energy), developed in 2007. Another is the Hurricane Severity Index, developed by the private company ImpactWeather in 2006. But, these still miss important factors — in particular, how long a storm might spend over your house.
But, the best metric to replace the current category system would be one that can measure what we most need to know about an impending hurricane — how damaging it will be. And that’s what the Cyclone Damage Potential (CDP) measures — the capacity to cause damage. It includes wind speeds and storm size, but also storm duration and how the storm’s wind field will move. James Done of the National Center for Atmospheric Research and colleagues at NCAR and in the reinsurance industry developed it. CDP predicted that Harvey would be bad, much worse than did Saffir-Simpson. It ranks storms on a scale from 0 to 10 — Harvey was a 5.2 at landfall, Irma a 5.9 when it made landfall in the Florida Keys, and Maria a 4.2 upon landfall in Puerto Rico. Katrina was a 4.9, Sandy a 3.0, and Hurricane Andrew of 1992 came in at 2.9.
One of the more pressing reasons behind our need for a better metric is the worry that climate change will increase the frequency and intensity of hurricanes in the future. Done’s CDP work is funded by a global reinsurance broker, Willis Towers Watson — a company he says is specifically interested in climate change because “they understand it will impact their bottom line”. This is a reassuring converse to Pulitzer Prize author Upton Sinclair’s pithy line that “It’s hard to get a man to believe something when his salary depends on his not believing it”.
It’s hard to know how much a more accurate prediction could have helped Houston prepare for the damage heading its way last month. Transitioning from the simple and widely known 1 to 5 scale to a subtler index built on more information might be tricky for the people who have lived in hurricane zones for years and have their own internal barometers of how to respond. But, as we brace for an uncertain future, better metrics can only help.-- NYT