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What You Need to Know About Tornadoes

It’s tornado season in the United States once more. Since tornadoes are largely unpredictable and can be incredibly dangerous, it’s important to know as much as possible about them so that you can be prepared when bad weather strikes. It’s important to remember that, while tornadoes are most common in the center of the U.S. and throughout the South, they have been reported in every state (and on every continent except Antarctica) and that most states have had deadly tornadoes. Slate has a cool but somewhat sobering interactive map of every deadly tornado in the U.S. from 1950 through the Moore, Oklahoma tornado on May 20, 2013. However, even if you live right in the heart of Tornado Alley, the odds are overwhelmingly in your favor that you’ll never actually be hit by one.

Tornadoes frequently develop from supercell thunderstorms. These storm clouds contain an area of rotation called a mesocyclone that forms several miles up in the atmosphere. With heavy rainfall, a downdraft can pull the mesocyclone toward the ground, where the cool air within the storm mixes with warm air coming up from below. This intensifies the rotation of the storm, which can cause a wall cloud, and when an area of low pressure forms in the updraft area below the cloud, a funnel cloud can form. If it reaches the ground, we call it a tornado. The funnel may not always be visible, however. What we usually see is a condensation funnel where water vapor in the air condenses in the area of rotation, but if the air is dry this can’t happen. We may also see debris in the air after the tornado has been on the ground.

There are three different alerts that are given out for tornadic activity.

  • Tornado watch: Conditions are right for possible tornado formation. Watch boxes generally cover large areas and last for several hours. If you’re in a watch area you should periodically check for updates, but there’s usually no cause for real alarm as there’s no guarantee that tornadoes will form at all, much less in any particular spot within the area.
  • Tornado warning: A tornado has either been detected on radar or a funnel cloud has been spotted. Warnings cover a much smaller area; meteorologists can usually predict the path of the storm fairly accurately to tell you what areas need to take cover and the approximate arrival time of the tornado based on the speed at which the storm line is moving. Since radar can detect rotation in the clouds that never reaches the ground, there’s still a chance that the storm won’t cause any damage, but you should take cover immediately if you’re in the predicted path of the storm.
  • Tornado emergency: Rarely issued, a declaration of a tornado emergency means that it has been confirmed that a violent tornado is on the ground and that there is a great risk of destruction. If you are in an emergency zone, prepare for the worst.

When discussing tornado watches and warnings, there are two common radar patterns that are frequently mentioned. A bow echo occurs when strong straight-line winds push part of a storm system faster than the surrounding storms, producing a radar echo that resembles an archer’s bow as the front curves forward. Tornadoes are rare within bow echoes (though there may be straight-line winds capable of producing severe damage), but they can sometimes form in the storm cells on either end of the bow. Since a bow echo encompasses multiple storm cells, they can cover a fairly large area.

Radar image of a bow echo
Bow echo near Kansas City on May 2, 2008
Image via Wikipedia

Hook echoes, however, may indicate that a tornado is forming or has already formed, so meteorologists will generally declare a tornado warning as soon as they detect one on radar. A hook echo can usually be found on the southern edge of a single supercell in the northern hemisphere (on the northern edge below the equator). It looks like a fishhook because rotation within the clouds can start to wrap rain around the center of rotation but it won’t be raining within the funnel itself. A hook echo doesn’t necessarily mean that a funnel is on the ground yet or even that it will reach the ground, but if you’re in the path of one, take all necessary precautions. If I find out I’m in a warning area, the first thing I do is check the radar for hooks and then see where the storm is heading.

Radar image of a hook echo, with relevant parts labeled.
Hook echo near Oklahoma City on May 3, 1999
Image via Wikipedia

Even if you don’t have immediate access to weather alerts, there are still warning signs you can look for. A visible funnel is the most obvious sign, though they may sometimes be obscured by heavy rains in front of the tornado or not even form a visible funnel. The most severe storms can have a base that’s half a mile to two miles wide and may look more like a low-hanging cloud. You may also see flying debris or bright flashes of light at the base of the storm as it pops power lines. Tornadoes also usually produce a loud, persistent roaring sound, commonly compared to a freight train.

If you’re in the path of a tornado, you need to take immediate steps to protect yourself. A comprehensive list can be found on the NOAA’s Storm Prediction Center website, but the most important things to remember are:

  • If you’re in a building, think low and central. If you can get to a designated storm cellar or basement, great! If not, go to the lowest floor you can or hide under a staircase. Try to find a room that has four sturdy walls, no windows, and that isn’t below something heavy that could fall on you if the building collapses. A bathtub will work nicely if you’re at home.
  • Cover yourself the best you can. Get under a sturdy table, cover yourself with a mattress, or wrap up in a thick blanket or sleeping bag. The most important thing to protect is your head; most fatalities are caused by head trauma from flying or falling debris. If you have a bike helmet or something similar and have sufficient warning that you can grab it, put it on. At the very least, duck and cover: kneel down and wrap your arms over your head.
  • Supplies: If you have time, grab a weather radio (highly recommended if you live in an area prone to severe weather), your cell phone (so you can call for help if you’re trapped), and some food and water. Put on your shoes in case you need to climb out through debris. If you have a storm cellar that you can keep fully stocked, add copies of important papers, a battery-operated phone charger, spare medication, a first-aid kit, pet food if applicable, and spare car keys.
  • If you’re in a mobile home, leave if at all possible. Try to get to an underground shelter or a more stable building.
  • If you’re driving, try to drive away from the storm if it’s far enough away that you can outrun it (drive at a 90° angle to the storm to get out of its path instead of trying to race ahead of it). If you can park and run to a building, do so. If neither of those is an option, pull off the road, get out of the car, lie down in the lowest point you can find, preferably away from any cars or trees so they don’t get tossed on top of you, and cover your head as best you can. You do not want to be in a car if a tornado strikes, it’s incredibly dangerous since the windows will probably blow out and the car may get lifted off the ground and/or rolled. However, do not try to hide under a highway underpass; the wind increases in speed as it passes through a tight space and you’re highly likely to get sucked out.

Unlike a tropical storm or hurricane, which last long enough that meteorologists can fairly accurately measure their strength as they’re happening and use computer models to guess how much they can be expected to weaken or strengthen over time, there’s no way to know ahead of time how strong a tornado will be. After all, you can fly special planes through the eye of a hurricane to measure the windspeed surrounding it, but you can’t do that with a tornado. Instead, we have to look at the damage caused by the storm and try to estimate the tornado’s windspeed. Tornadoes are now rated on the Enhanced Fujita scale, which is a revised version of the Fujita scale that was used from 1971-2007 in the U.S.

  • EF0: Winds approximately 65-85 mph, minimal damage
  • EF1: Winds approximately 86-110 mph
  • EF2: Winds approximately 111-135 mph
  • EF3: Winds approximately 136-165 mph
  • EF4: Winds approximately 166-200 mph
  • EF5: Winds over 200 mph, total destruction

Stronger tornadoes are seemingly becoming increasingly common, but this is partly due to the new scale. If a tornado’s path didn’t bring it in contact with any buildings, it was hard to determine the windspeed under the old scale, but the new one also provides rubrics to assess damage to trees and power lines. It also provides more accurate descriptions of how buildings made from different materials are affected by the same strength storm. In addition, under the original Fujita scale an F5 tornado had winds between 261-318 mph, while one with 201 mph winds (currently an EF5) would still fall within the F3 range. By redefining the range, the same tornado looks much worse on paper. Tornadoes are also more likely to cause damage to homes these days simply because some areas that were farmland 50 years ago are densely populated suburbs now (this was the case in Moore, Oklahoma).

One thing we cannot say with any certainty is whether global warming has played a role in causing stronger, more frequent tornadoes. There just isn’t enough data yet, and there are so many different factors that go into tornado formation that it’s currently impossible to say what effect climate change will have on tornadic activity (they may even become less common). We definitely can’t pin any particular storm on global warming.

By [E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

13 replies on “What You Need to Know About Tornadoes”

I grew up in a place with mountains, so we never had tornadoes. I did experience one when I was in college in Ohio, though (and I had no idea what to do!) I was studying in the library and they announced that we all had to go to the basement. At my college, we had to do a senior thesis so everyone was super panicked about their papers (we were NERDS!) and we grabbed our work and went down to the basement.

The tornado was considered minor, but it uprooted several trees. One girl got caught in a building with revolving doors, and the doors revolved, creating a wind tunnel and she had lots of cuts and bruises from leaves and branches making it into the building she was in.

Suffice to say, if that was a minor tornado that only resulted in some bruises, I have major respect for it.

Wow. I never knew that there were tornadoes so close to me. I know I’m in tornado alley, but for some reason I thought that Lake Michigan protected me, since I’m only a few blocks from it. I don’t know why I thought that – maybe just because the lake tends to create more temperate weather? I don’t know. Thanks for informing us.

Yikes. Never been so glad to have lived (mostly) in mild temperate climates… The worst weather-related things here are floods (rare) and everyone losing the head when it snows 2 inches.
Still, if I ever find myself in Tornado Alley during the season, I know what to do…!

Thanks for this, Hillary. I had no idea about the new rating system; however, I live in Florida, so I know the hurricane/tornado drill very well. (They start doing tornado safety in kindergarten around here, if not earlier.) I also was around for this:
http://en.wikipedia.org/wiki/1998_Kissimmee_tornado_outbreak

I was in fifth grade, and I remember waking up one day to the sun shining, which was unusual because it was a school day. My mother called to me from the living room of the mobile home we were living in at the time: “Amanda, you can go back to sleep. There’s no school. There was a tornado last night.”

I didn’t know anyone who got hit in the most devastating of those on this list, although the place was only four or five miles from where we lived. I look at it now and based on the winds, it would have been an EF4 of not an EF5 on the new Enhanced Fujita. My dad was working at a moving company at the time, and he had to help move some of the survivors to new places, if anything could be salvaged at all. It was heartbreaking. To this day, my dad still worries about any sort of weather alert because of what happened here.

We had to hide out in my mom’s closet or do duck and covers at school a few times when I was a kid in Texas, but none ever hit my town in the 22 years I lived there. There was a big one in ’53 that killed over 100 people downtown and then not another one until a few years ago (and it was much smaller and I think managed to hit open areas). Living in NY, though, I’ve been under warnings several times! Because I am a huge dork I kept sneaking out on my balcony to try to watch the ones that hit the city in 2010. The tornadoes missed us, but my neighborhood got slammed by a downburst that knocked over a ton of trees. It was fucking scary when that burst of wind hit my building! The tree right below my window fell into the street. And my husband’s commute was long enough that he was on the subway before shit started happening and I couldn’t get ahold of him to tell him to stay underground. It was over before he got home, though, and he managed not to notice any of the wreckage on his walk back to the building. I mean, it was just around the corner but there was a damn tree in the street just down the hill from our front door!

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