As Hurricane Isaac lashes the Gulf coast, I’ve been taking in a lot of coverage from both national and local outlets as well as on Twitter. The size, strength, and potential for Isaac to be a bad storm have been part and parcel of conversations between meteorologists and during the broadcasts leading up to the storm for more than week. Yet, even before the storm has made its final landfall, I’m hearing and seeing comments to the effect of, “I had no idea it would be this bad. It was only a category 1 storm!”
Not ‘Justa’ Category 1
At one of his last official functions as director of the National Hurricane Center, Bill Read spoke to the North Carolina Hurricane Workshop at East Carolina University. He told a room of more than 100 meteorologists, emergency managers, researchers, and other officials that there’s “no such thing as ‘justa’ category 1 storm.” Just ask anyone in the path of Irene, to take just the most recent example.
Yet, the fact that a storm is “‘justa’ category 1″ continues to be a reason for people not to evacuate, to under-prepare, and generally to ignore the risks that even ‘justa’ tropical storm would bring, much less a stronger system. Then-Director Read even suggested the categories we apply to storms of this ilk may be doing us more harm than good.
I’m starting to think he’s got a point.
Being able to summarize a storm into one of five (or seven) categories can be a useful thing. For example, assigning historical storms to these categories can help us sort through a large dataset and make quick, usually high-level judgements about which storms to study further. Even in a forward-looking sense, it may be helpful to get us in the ballpark of what kind of damage we might expect.
But the ballpark is all we can expect such a categorization to give us. We must never forget that when we reduce storms to a category, this process is indeed a reduction – we’re removing data in exchange for the simplicity of a single, easily-comparable number. When we’re dealing with a single storm that’s threatening a populated coastline, that simple number belies the multitude of details that absolutely matter – details we don’t see when looking at a single category number. It also masks the truth that not all category 1 hurricanes are created equal.
It doesn’t help this at all that for most of a hurricane’s life cycle, wind, pressure, and physical size are all we really can talk about with any confidence. Recon flights and satellite analysis methods such as the Dvorak technique can help us determine or at least estimate the minimum pressure, maximum winds, and overall size of a storm. Until that storm begins bearing down on a particular area, however, we can’t begin to address the details of how those winds are distributed within the storm, or more importantly, who is likely to see how much storm surge, where the heaviest rains will fall, and where the biggest tornado threat is likely to be.
In other words, if a hurricane lasts for 10 days before making a single landfall and dissipating a day or two later, we will likely spend 80% or more of its life cycle talking about the winds (and whether it’s a category 1 or a category 4). Only in the last 20% or so can we talk about the aspects of the storm that kill the most people and do the most damage, and if those latter aspects are out of sync with the category assignment (as seems to be the case with Isaac and was definitely the case with other storms like Irene and Ike), it will be difficult to convince people who may have previously written off the risk to acknowledge the threat and reconsider their plans.
I think scales such as the Saffir-Simpson Hurricane Wind Scale, as it known today, have their place. The clearest case for them is in a historical sense, where examining the fine details of all storms (and including storms in a dataset where those details may not be known) is not possible. However, when we are trying to motivate people to take protective action ahead of a specific storm threatening a specific area, the picture gets quite muddy. If your storm is at the high end of the scale, it can certainly help motivate people to act! However, as category 1 storm after category 1 storm reminds us, it doesn’t take a category 5 hurricane to kill dozens or hundreds, do billions of dollars in damage, and disrupt the lives of thousands for weeks. Assigning a low number to a storm only serves to hide (or at least make it easy for people to ignore) the dangers of these complex weather phenomena.
Should we stop assigning categories to hurricanes? How can we better communicate the threat that even “low-end” storms bring?