After the Hurricane

Perhaps your attention has drifted over to football, but the aftermath of hurricane Isaac has given way to another national sport. Isaac was the category-1 storm that just pummeled New Orleans and neighboring areas. Isaac landed close to the spot where Category-5 hurricane Katrina landed, back in 2005. We now know that Isaac’s destruction and havoc – and especially the rainfall – was higher than what was expected. Many were apparently unprepared for the storm’s intensity– as was evident after the fact, even though they had taken the seeming necessary recommended precautions. Put simply, it seems many people were unpleasantly surprised by Isaac –despite having received and acted on the advance warnings provided by the authorities.

Predictably, the chorus of complaints impugning the hurricane scale rankings has surfaced. And with the outcry one hears calls for revising the hurricane warning scales – presumably, in a direction that will convey better information. Even the New York Times joined the fray – hosting a “debate” on the matter titled “How Could the Storm Ratings be Improved?” I argue that to replace the current system with a more complex one is inappropriate. And I argue in fact, that to advocate replacing the scale is to call attention to the wrong issue.

The by now familiar hurricane scale is the Saffir-Simpson scale – a 5-point category ordering originally created by Howard Safir. The scale was later enhanced and popularized by Robert H. Simpson of the National Hurricane Center. It was an effort at prediction in a manner intended to inform decisions both by individuals and by cities, states and regions.

Mr. Saffir’s ordering was a storm-destruction ranking system keyed primarily on wind speed. To be considered a Category 1 hurricane, the system’s wind speed must clock within the 74-95 mph range. At the other extreme are Category 5 monster cyclones where wind speed exceeds 155 miles per hour. Three other thresholds between 74 and 155 mph winds separate Category 2-4 hurricanes.

Although it might seem somewhat obvious, the beauty of Mr. Saffir’s innovation was not only in establishing the threshold (thereby creating the categories) but also in associating each category with the damage the winds may cause. To formally do this he examined historical data. He documented the positive relationship between wind-speed and property and other physical damage. Thus, trees and unanchored mobile homes receive the primary damage in a Category 1 storm whereas – at the other end of the scale – a Category 5, involves the complete failure of roofs and some structures. The other three descriptions of destruction were then matched with the sustained wind speeds that would produce the corresponding damage. What follows then is to recognize that past is prologue.

What Mr. Safir and Mr. Simpson created was a model, a simplification of a real event intended to convey decision-making information (material destruction) based on a particular, distinctive attribute – like wind-velocity. By design, because it is an abstraction of reality, it will (practically) always be mistaken. To expect a model to perfectly convey as much information as the actual event it aims to describe is a mistake. Jorge Luis Borges most elegantly captured this fallacy in his Exactitude in Science. As an aside, it’s worth noting that this beautiful story stands to be one of the most influential (in my opinion), if not the most influential, in the history of the written word given the number of words expended (it has less than 150 words). Borges tells of a map-making competition where different generations of cartographers aimed to surpass the previous generation by building more and more accurate maps in the pursuit of the perfect map. They ended the map competition by building a map that replicated the world. A map that was useless.

In threshold setting – which is at the core of the Safir-Simpson method and other ordered scale warning systems – there can be two types of errors. Suppose that the authorities announce a Category 1 storm (and to keep it simple, assume there are only two levels). When the cyclone finally arrives we find the property damage ends up being what would normally be associated with a Category 2 hurricane (for sake of argument) – an unpleasant mistake. On the other hand, suppose the authorities announce a Category 2 hurricane – and when the winds are quieted we find material damage of Category 1 magnitude. The error, this time, is a more welcome one. In principle, we could move the thresholds to try to maximize the “welcome” error. Lower the wind-speed threshold to 80 miles per hour. Thus, any storm with winds between 75 and 79 miles per hour will be a Category 1 and now anything above 80 and 110 is a Category 2. But moving the thresholds we encounter several unintended consequences. We would reduce the chances of incurring the unpleasant mistake and enhance the chances of the welcome mistake. But over time since everything is going to be a Category 2 hurricane – any information contained in the warning is lost – it becomes meaningless – like Borges’ map. Think about it – why not avoid unpleasant mistakes at all by eliminating the various categories and only have 1 category. In this case you can only be pleasantly surprised.

So what is being left out in the map-building – the Saffir-Simpson scale – and thereby a potential source of error? There is more to a hurricane’s damage potential that high winds. The storm surge is wall of water at the leading edge of a storm – is especially destructive in low lying areas. The rainfall associated with storms presents flooding threats – especially in regions with saturated soils or already swollen rivers. Two other things (at least) surge and rainfall in addition to a cyclone’s wind speed are associated with the ultimate concern material damage. The association between these three measurable features of a cyclone and material damage – aside from being positive – is different and distinct. Thus, relying on one aspect of a hurricane wind-speed to categorize them in a manner that tells us how much damage to expect will imperfectly capture the association between the other aspects as well – practically guaranteeing a source of error.

Will a more complex model – perhaps one that combines wind-speed, surge and rainfall – do better? Not likely – error and therefore judgment cannot be avoided.

Thus, a model builder has to tradeoff error and accuracy. How much error? Alan Alda’s character in Nothing but the Truth arguing a point of law set forth the parameters: “is this mistake like wearing white after labor day or is it like invading Russia in winter?” Scale constructors often opt to minimize errors – and associated costs of errors. And to educate the population – so that we understand the nature of the underlying decision-making model and thereby take its efficacy into account into our own decision-making and importantly, ex post performance appraisal.

– Arod



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