Why Should I Care?

By Angela Fritz

When you want to determine how intense or how strong a hurricane season was, there are many ways you can calculate it.  An obvious place to start would be summing the intensity of the individual tropical cyclones in that season.  When a cyclone is defined as a tropical storm or a hurricane by the National Hurricane Center (NHC), it is given a ranking on the Saffir Simpson Hurricane Scale (SSHS).  It can be a TS (tropical storm) or a category 1, 2, 3, 4, or 5, with 5 having the strongest winds .  Category 5’s that we can remember well would be Hurricane Andrew of 1992, or Katrina of 2005 (although I should note, Katrina was a Category 3 when it made landfall).  When we take a step back and look at the number of strong hurricanes in a given season, we begin to get an idea of how bad that particular season was.  That is, until we dig in a little deeper.

Naturally, one asks at this point “why does that matter?”  It’s a really good question, and it’s one that I’m trying to investigate.

Hurricane Andrew (top) and Hurricane Ike (bottom) before their respective landfalls. Andrew was 300 miles wide and a Cat 5, and it did 41.1 billion (2010) USD in damage when it made landfall in Miami, FL. Hurricane Ike got as large as 480 miles wide and was a Cat 2, and damages totaled 38 billion (2010) USD. Of all U.S. landfalling hurricanes that we have data for, Andrew was the second costliest and Ike was the third, even though Andrew’s landfall wind speeds were 165mph versus Ike's 110mph.

Let’s use an example.  Hurricane Ike of 2008 was a moderately intense hurricane.  When it made landfall on the Texas coastline, it was a Category 2 on the SSHS.  This means it had a maximum wind speed somewhere in the hurricane of at least 96 mph (154 kph)¹.  That’s nothing to sneeze at, especially when you’re living on the coastline that’s in the way.  However, it’s definitely not the strongest storm that coastline has ever seen.  So why did Ike do $38 billion in damage, and why does it rank 3rd in costliest U.S. landfalling hurricanes?¹ There are many variables that go into what makes a hurricane a “very costly” hurricane, but I’m going to argue that one in particular had a pretty large influence: size.

When Hurricane Ike approached the Caribbean, it was a Category 4 hurricane.  It slammed into Puerto Rico, Hispaniola, and then washed over Cuba.  The affect of making multiple landfalls on these islands was that the hurricane decreased in intensity.  But it did something else – it increased its size.  Meaning, when you looked at the storm for instance in  a satellite image, it got bigger; it took up more space.  So when the storm began to strengthen again into a Category 2 hurricane over the Gulf of Mexico, it was not only stronger, but those strong winds covered much more area.

So when we look at how strong a hurricane season was, it is my suggestion that we not only look at how strong the winds were, or how long it lasted, but we take into account how big it was.  There are currently 2 ways of measuring the intensity of the seasons over time, the Accumulated Cyclone Energy index (ACE)² and the Power Dissipation Index (PDI)³.  Both of these calculations take into account maximum wind speed and the duration of the storm, however, neither include size.  There’s a third way of doing it, called Integrated Kinetic Energy, or IKE^4,5.  The IKE of a storm uses three things: maximum wind speed, duration, and size.  Using this index rather than the conventional ACE or PDI gives us better insight into

So if you ever hear on the news that one hurricane season was worse than another, or that hurricanes are getting stronger or weaker over time, I encourage you to ask the question “how did they measure that?”  Chances are, since IKE is a relatively new method of making this determination, they didn’t include size.  Which, I would argue, matters.