We’ve had a relatively calm hurricane season this year. According to the National Hurricane Center, we’ve had eight storms: two hurricanes, five tropical storms, and one tropical depression. That’s a far cry from the past eleven years in which the number of storms has been in the teens. Except for 2005, when we had a whopping 28 storms, including 15 hurricanes with four of them reaching Category 5. A Category 5 storm is rare indeed: there have been 32 of them recorded since 1851.
The National Weather Service or its forebears has been keeping information on hurricanes and tropical storms since 1851. Perhaps before, but the records available from the NHC only go back that far. You can download the raw data from 1851 through 2008 from the NHC’s Hurricane Research Division. The data isn’t in the most convenient format—a text file of fixed fields—but parsing it with a C# program proved to be easy enough, and within an hour or so I was examining the data.
The last ten years of hurricane activity has raised many concerns that climate change is increasing the number of storms and the intensity of the storms. Looking at the historical record, it’s impossible not to draw the conclusion that 11 consecutive years with 10 or more storms is anomalous. It’s way out there. No other 10-year period has had even close to that number of storms.
I created a dump of the data to a comma-separated format so that I could massage it with Microsoft Excel. I’ll be the first to admit that I’m not an Excel expert, but I can do the basics and create usable if not especially nice looking charts. In Excel, I created two charts: one examining the number of hurricanes each year, and another showing average storm intensities. If you like, you can download my Excel file with the raw data and the charts.
Let’s talk a bit about those charts. This first chart has two sections: storms per year, and storms per year that hit the U.S. There are also lines for the 5- and 20-year rolling averages.
(Sadly, the charts are gone.)
The top part of the chart is the storms per year, and the bottom part is storms that hit the U.S. Looking at the chart, there’s no doubt that recent hurricane activity is far out of the ordinary. Even the 20-year rolling average shows a sharp uptick starting around 1995. And in general we seem to be having quite a few more storms over the past 60 years than previously. But there are a few things about the data that make me wonder how much I can trust it.
It’s interesting that, although the average number of storms increased sharply from 1935 to 1953 or so and then plateaued, the average number of storms hitting the U.S. doesn’t show a similar increase. This makes me wonder if global weather patterns tend to cycle, pushing hurricanes towards the U.S. in some years and pushing them away in other years. The other possible explanation is detection bias. That is, it’s quite likely that prior to 1935 we just didn’t have the technology to detect hurricanes that didn’t hit any major population centers. That wouldn’t explain the increase since 1995, but it certainly could explain the increase from 1935 to 1953.
The second chart shows average storm intensities, along with 5- and 20-year rolling averages. Excel did a poor job of converting this graph to a .GIF. Download the Excel file if you want more readable text.
I computed the average storm intensity by multiplying the number of tropical storms by 1, category 1 hurricanes by 2, etc., adding it all up and dividing by the total number of storms. So if you had two tropical storms and three Category 1 storms in a year, the average intensity would be ((2 * 1) + (3 * 2))/5, or 1.6. The 5- and 20-year averages summed the number of storms in each category, did the multiplication, and then divided by the total number of storms. I didn’t fall for the “average of averages” trick.
The interesting thing about this chart is that recent average storm intensity is at or below the historical norm. The high point in the 20-year rolling average is in the late 1960s.
I’m not sure what conclusions, if any, to draw here. Well, I can draw one for sure: the recent hurricane activity is way above normal. Why we’re getting that activity is something I’ll leave to climate scientists. Other than that, I just have questions. Two in particular:
- I am curious about the sharp increase from 1935 to the plateau starting in 1953. The 20-year rolling average is almost completely flat from 1953 until 1995. That’s pretty strange.
- I’d sure like to know why the average number of storms hitting the U.S. doesn’t show an increase similar to the total number of storms.
I was a little surprised to find just how far current activity is from “normal.” I expected it to be perhaps a little unusual. I’m not yet ready to call it a trend, although if the next ten years are as active, I’ll start wondering what the heck is going on.
I’d be interested in any comments you have about the raw data, my methods, or my interpretations of the data presented. I am not prepared, though, to have a discussion about what’s causing the recent spike in hurricane activity, because I haven’t yet seen a credible hypothesis. The honest climate scientists have said that they don’t know yet. Perhaps more on that another time.