Saturday, May 28, 2011

Tornado Season: A Little Perspective Please

There are certain people who always like to trumpet something as being worse than it is. And we are getting that from some environmentalists who are using the tornado season to once again hype their claims. This year's tornado season has been the "deadliest" on record but, and this is a big one, NOT REALLY. It all depends on how you want to look at the numbers.

The sad death toll this year is 520, the highest confirmed total previously was in 1953 ,when it was 519. Notice the use of the word "confirmed." Records indicate previous years with higher death totals, but in the past numbers were always estimates. For instance, in 1917 they estimated 551 deaths, 540 in 1927, and 552 in 1936. In fact, if those estimates are close, then 2011 was not the deadliest year, but the fourth deadliest year. And even that is not quite accurate.

What people are forgetting is that the population of areas hit by tornadoes is much larger than it was at any time in history. Consider an imaginary town called Tornadoville. In 1953 the town is wiped out and every single resident, within a one square mile area is killed. In 1953 the population might be 100 people. By 2011 that same square mile might have 300 people living there. If another tornado hits the town and kills 150 people it is the "deadliest" year but the death rate went down from 100% to 50%.

The more people there are in area, the greater the likelihood of some deaths, not because the tornadoes are necessarily worse, but because the population density is higher, making it more likely that some people will be in the wrong place at the wrong time.

By comparison let us take 1953 and compare it to 2011. The US population in 1953 was 160,184,192. Today, it is estimated to be 307,006,550. That is almost double what it was in 1953. The number of deaths per million in 1953 was almost twice as high as it is today. In reality the number of people killed per year by tornadoes has been declining if you take into account the greater number of people who live in the path of tornadoes. The National Oceanic and Atmospheric Administration has put together this chart showing the death rates from tornadoes, normalized to take population factors into account.

You can click on it to enlarge it if you wish. Here is how they explain it:

The purple points are the annual death rates, the red line is a simple smoother, the solid black line is a long-term trend in two sections (1875-1925, 1925-2000) and the cyan lines are estimates of the 10th percentile and 90th percentile from 1925-2000. Brooks and Doswell (2002) have an extensive discussion of the record and its possible implications.
But life is always more complicated that it appears on the surface. The death rate can decline simply because we have better houses to protect people, more advanced warning systems, better ability to evacuate an area, faster communication, etc. What about the number of tornadoes? Are they increasing due to evil climate change? Some would want you to think so, and openly claim that is happening. And, once again, on first glance it could look convincing. Consider the chart below, also from NOAA, about the number of tornadoes per year since 1950. It shows a clear increase.

Yet again, life is not so simple. NOAA says: "The number of tornadoes increased dramatically in the 1990s...." Now, some want you to think it was global warming. NOAA, however, finishes the sentence thusly: " the modernized National Weather Service installed the Doppler Radar Network." They also "began the Warning Coordination Meteorologist program increasing partnerships with media and Emergency Management across the United States." This included "the training of storm spotters" across the country. NOAA writes, "With more people trained to relay information on storm activity to the Weather Forecast Office and improved communication and digital technology, more tornadoes could be reported."

Consider a tornado in 1900. It might be a very lonely thing, spinning around in the middle of some rural county. Local folks would see it. It might even hit a farmhouse or two. They'd talk about it and the cow that went missing as a result. If the area had a local newspaper it might get mentioned there as well. No one would phone it in and to a central agency. They didn't have phones. Aunt Edith might write her sister about it. But certainly the weather forecaster on the local TV station didn't catch it, there were none. Neither did the radio, it wasn't really around either. Methods for reporting such things were scarce, and that reduced reporting. In some cases a tornado could briefly touch down and no one even realized it. Today, the radar systems pick them up, even if no one actually sees the damn thing.

Joshua Wurman, of the Center for Severe Weather Research admits the frequency of reported tornadoes has increased but says "there's pretty good evidence that it's due to improved reporting efficiency." In the end, since we are using better technology, and have better reporting, we report more tornadoes than our parents, or grandparents did. We don't actually have any evidence that tornadoes are more frequent, only that we are more likely to catch them when they do occur.

But, there is one more fallback for the alarmist: the level of intensity for the tornado. We have a system called the Enhanced Fujita Scale (EFS), which is used to determine storm intensity. And while it tries to include factors like wind speed it also relies heavily on "damage indicators." That means "the type of structure which has been damaged." Since the number and type of buildings damaged, along with the degree of damage, are used to determine the EFS rating, there are some drawbacks. As Wurman noted: "I've seen strong tornadoes going through open fields, and they don't really do a whole lot of damage."

Along with population growth, which increases the raw number of deaths, you get an increase in the number of buildings. More buildings mean more structural damage, which would tend to push up EFS scales. Wurman's tornado in an empty field couldn't easily be rated high on the EFS scale. Two years later a similar tornado, hitting a newly built shopping mall, would register on the scale in ways the previous tornado did not.

Life can be a bitch, a very complicated bitch. It is surely much easier to claim that tornadoes are more frequent, we are all at risk, and global warming is to blame. That at least, will help sell papers and improve broadcast ratings.

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