Are violent tornadoes becoming more common?

After the rapid succession of devastating tornado outbreaks in 2011 that ultimately produced 6 EF-5 tornadoes and killed over 500 people – the most tornado-related deaths the country has seen since the advent of modern Doppler radar – a national conversation started on whether or not this dramatic increase in highly damaging tornadoes was “the new norm.”

The short answer is no, not really. The long answer is that while the average number of reported tornadoes has increased since accurate records started in 1950, the average number of highly destructive (EF-4 and EF-5) tornadoes has actually gone down in the last 62 years.

Number of tornadoes

The number of tornadoes officially recorded during the 1950s (4,908) nearly tripled by the 2000s, with a whopping 12,972 tornadoes reported during the ten year period between 2000 and 2009.

 

(NWS SPC)

(NWS SPC)

Examining these numbers alone, it stands to reason that the number of tornadoes in the United States is increasing dramatically, but there’s always a catch. While the number of tornadoes may be increasing somewhat, the Storm Prediction Center only records tornadoes confirmed by National Weather Service officials who go to the scene to survey the damage.

Back in the 1950s, before Doppler weather radar (which didn’t come into operational use by the NWS until 1992), the only way forecasters would have known a tornado happened was to receive a report that one was spotted or that one caused damage. Weather radar wasn’t used by the weather service until 1957, and even then they could only see possible tornadoes by identifying clear hook echoes in precipitation, so the vast majority of tornadoes in the United States went unreported.

As the country’s population exploded (152 mil. in 1950, 226 mil. in 1980, almost 314 mil. today, according to Census data) and spread out into the countryside, it provided more opportunities for people to see and report tornadoes to the NWS.

The widespread use of the internet in the 1990s and social media in the late 2000s helped increase awareness and reports, so much so that many major tornadoes nowadays are instantly known and documented as a result of sites like Facebook and Twitter.

At the very least, thanks in large part to advanced technology, we now know about almost every tornado that occurs in the United States. But are they getting stronger?

Tornado strength

Meteorologists rate tornadoes on the Enhanced Fujita (EF) scale based on the damage they cause. A result of extensive scientific testing, the EF scale allows meteorologists to use damage done to structures, vehicles, and vegetation to determine how strong the winds were at that particular location. Some tree limbs torn down and shingle damage to roofs could be considered EF-0 damage, whereas heavy damage to a well-built, modern school building with rebar-infused concrete walls could be considered EF-5 damage.

The scale was originally called the Fujita Scale, named after its designer, legendary meteorologist Theodore Fujita, but was “enhanced” in 2007 to correct estimated wind speeds based on factors like the quality of building construction.

It is extremely hard to get an actual measurement of a tornado’s winds, though it has been done on a couple of occasions. Thanks to mobile Doppler radar technology (like the widely-known Doppler on Wheels), meteorologists have been able to use these mobile radars to get a pretty close estimate of how strong the winds are within a tornado.

Just recently, a tornado in El Reno, Oklahoma was originally rated an EF-3 based on structural damage, but later upgraded to an EF-5 after a mobile Doppler radar revealed that its winds reached 295 m.p.h. at one point (tornadic winds over 200 m.p.h. are considered to be EF-5).

According to data collected from the Storm Prediction Center, the average number of EF-4 and EF-5 tornadoes has actually gone down in the time between 1950 and 2012.

(NWS SPC)

(NWS SPC)

Even after the “fluke” years of 1965, 1974, and 2011 are removed – all years that lay claim to historic tornado outbreaks – the results are the same; the average number of EF-4 and EF-5 tornadoes is going down.

(NWS SPC)

(NWS SPC)

Every time tragic pictures emerge from a major tornado anymore, it looks like a nuclear bomb went off. News cameras pan over street after street of utter devastation. Trees stripped barren, tattered pieces of dozens (and sometimes hundreds) of homes scattered about.

So why is there so much destruction now, while the number of highly rated tornadoes is actually going down?

It’s likely several factors. As noted previously, the population of the United States has more than doubled in the time between 1950 and today. Those extra 150 million or so people have to live somewhere, so they spread out.

People who have lived in the D.C. area for a few decades can remember when places like Prince William County had a fraction of the development it has now.  This rapid expansion into previously unpopulated areas is occurring almost everywhere in the United States. When tornadoes form, they’re simply hitting  and destroying more things. Neighborhoods that get wiped out today wouldn’t have existed 40 or 50 years ago.

(NWS SPC)

(NWS SPC)

The population explosion may account for why there’s so much destruction when a major tornado occurs, but why isn’t there as much EF-4 or EF-5 damage as there was back when this type of recordkeeping began?  The answer likely lies partially in luck, and partially in the quality of building construction. It’s rare for the strongest part of a tornado to hit any building, let alone a well-built one.

The amount of damage it takes for a tornado to be classified as an EF-5 is immense – the destruction of a public school building or a hospital, for one, or an anchored bank vault being tossed – so it’s a good thing that they aren’t too common.

More science is needed to determine exactly what kind of long-term trends are developing in tornado activity, but until better technology is developed to measure the exact wind speed in every tornado, the only thing meteorologists have to go on is the damage they produce.

* Guest contributor Dennis Mersereau, who grew up in Woodbridge, Va., writes a weather column for Daily Kos.

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