Thursday, May 5, 2016

What a Load of Hot Air

Kansas is smack dab in the middle of one tornado alley in
the US. Even if The Wizard of Oz was an allegory for the
gold/silver standard debate, the tornado is frightening
enough just because it has a witch in it. The cyclone was
supposed to represent the populist storm that was free
silver, while the yellow brick road represented… well,
you can guess that one.
It’s that time of year again – tornado season! If you think baseball is America’s favorite spring pastime, just ask Oklahomans about dodging cows and other items thrown into the air by Mother Nature. And tornadoes are as purely American as baseball as well – no place on Earth gets as many tornadoes as the eastern part of the USA. In order to be the best-informed tornado victim you can be, let’s look at the science behind the twister – you know it’s no coincidence that Dorothy lived in Kansas, home of the 300 mph fastball that took her to Oz.   

What Makes a Tornado?
Tornadoes are born from thunderstorm clouds. The Sun heats air over water, causing some of the water to evaporate and rise. Warm is lighter than colder air, and will rise over cold air if it holds much more water, but it only rises so far. The cooling of the upper atmosphere causes the water vapor to coalesce into droplets – a cumulonimbus cloud that can reach as high as 4000 meters (10,000 ft.) in the air. The air continues to gain more water vapor, to the point where the cloud collapses on itself and rain falls. The more energy in the atmosphere, the stronger the storm.

This is how a thunderstorm forms, but not every thunderstorm produces a tornado. If the winds above the warm, moist, rising air are strong enough and tend to change directions, this can cause a wind shear in the rising moist air, starting a horizontal vortex, usually in the counterclockwise direction for the Northern Hemisphere. (called cyclonic flow). If the shear is strong enough and the two temperatures are different enough, the shear can turn vertical and start to rotate faster and more broadly. Rotation of the air in the atmosphere is called a mesocyclone – which is not the same thing as a tornado. However, mesocyclones do make up the cores of supercells, and these are the storms that can spawn tornadoes. Occasionally (about 1 in 100), rotation of some air is clockwise (anticyclonic), but these are usually little sisters that form around a larger mesocyclones, and if they produce tornadoes, they are usually smaller, shorter in duration, and weaker.

This graphic shows how a tornado is born. The green line
is the warm moist air rising, with the red counterclockwise
circles showing the rotation due to wind shear. The blue
arrow is the downdraft of the cooler air helping to create
the rotation and the forward gust. Notice that the rain
front is ahead of the tornado, this goes along with the hook
echo we talk about below.
The rising of the warm, moist air creates a low-pressure area below it, and some of the colder, drier, heavier air will rush in to take its place. In truth, this is all that wind really is, cooler air rushing in to an area vacated by rising warmer air. The low pressure can also pull down some of the cumulonimbus cloud several hundred meters (maybe a thousand or more feet) into a formation called a wall cloud. Seeing a wall cloud doesn’t guarantee that there will be a tornado, but the chances are going up. A bigger pressure difference will pull the mesocyclone down into a funnel cloud. A yet bigger pressure difference, in a small confined area, will pull it all the way to the ground – a tornado.

There are actually three different types of supercells (classic, low precipitation, and high precipitation) with the classic form producing the most tornadoes. Classic cells often have specific features on radar that predict tornadic activity. Doppler radar is different from regular radar because it doesn’t just capture reflected radiowave (microwave) signals that bounce off the water in the clouds. Doppler radar can detect this reflectivity, but it also shows velocity of the wind. Remember that velocity is a vector quantity so it includes the direction of the wind as well. This allows Doppler radar to determine which masses of air are moving away from source and detector because their wavelengths get lengthened, and differentiates them from masses of air that are moving toward the radar, returning concomitantly shortened wavelengths. It is just like the Doppler Effect that makes the pitch of a train engine lower a few notes as it passes you and makes light from galaxies moving away from Earth become red shifted (red light has a longer wavelength).

By looking at the Doppler radar, one sees not only the shape of the storm, but where the wind shears and rotating masses of air might be located. On the back end of a supercell, especially a classic supercell, there may be an echo notch or hook where the pressure difference is the greatest and the rotation and wind speed the greatest. This is where funnel clouds are most likely to form and where they most often reach the ground to become tornadoes. The tight area of the notch or hook shows the high energy movement of the air being confined to a small volume, and this promotes tornado formation. Beware the hook.

Tornado Alley
Now that we know how tornadoes are formed from thunderstorms, the question becomes, can we use that to tell where tornadoes are most likely to form? The answer is yes, and if you live in the USA, then the answer is just about anywhere you find yourself.

Each blue dot represents one tornado occurring between
1950 and 2013. Florida has many, but they are usually
weak. The Rockies have many fewer than the Smokies, but
that has more to do with the fact that the Smokies are east
of the Gulf while the Rockies are to the west. In general, if
you live east of the Continental Divide you have a great
chance of being in a tornado.
The warm air coming up from the Gulf of Mexico, and the cooler, drier air coming down from Canada in the form of the jet stream, can often meet up to give just the right conditions for the formation of supercells, especially in the Lower Plains and Midwest. Just about anywhere in the USA can be considered tornado alley on a global scale; the United States gets about ten times as many tornadoes as any place else on Earth.

Within the country, Oklahoma, Kansas, Texas, and Mississippi are right there where the warm air comes off the Gulf and meets the jet stream if it dips to the south. Iowa, South Dakota, Missouri, Illinois, Indiana are the states further away that see the most impact of the jet stream moving back north and sucking the arm gulf air with it. In Europe, some areas get some warm air from the Mediterranean and some cool air from up north, but the temperature differences and wind directions don't reach big enough differences to produce many strong tornadoes or very strong ones. Europe hasn't had an EF-3 or higher tornado in the last 20 years. No place on Earth get as many or as strong of tornadoes as the American corn belt.

That being said, the image above and right shows that just about anywhere east of the Continental Divide is tornado alley. Mountainous areas are less likely to produce conditions for supercells, and therefore are less likely to have tornadoes, but it does happen, even in places with such varied terrain as California. In 1987 there was a tornado called the Teton-Yellowstone in Wyoming was a high altitude tornado that rated an F4 (out of 5 on the Fujita scale) that did a significant amount of damage. Because it is harder to get big pressure differences at higher altitudes and to get large amounts of warm moist air higher in the mountains, tornadoes in these areas are much more rare.

Therefore, if you want to avoid tornadoes, live just about anywhere other than the USA, but if your heart is set on being an American, then stick to the west coast – although they have natural disaster troubles of their own (forest fires, earthquakes, egomaniacal entertainers and baristas). What else can you do to reduce your chances of being hit by a strong tornado – Florida might be a decent place. They have lots of warm moist air, but little cold dry air on top of it. They do have a fairly high number of tornadoes, but they are usually smaller and weaker; many are waterspouts that come ashore for short times.

In 2007, the Fujita scale was revised to the Enhanced Fujita
Scale. This was meant to align winds speeds determination
more directly with damage estimates. An EF-0 is the most
common tornado seen in California, with the highest
recorded tornado wind speed being 318 mph in a 1999
tornado in Oklahoma City. Now, anything above 200 mph
is considered an EF-5.
Do tornadoes target mobile home parks? It may seem so, but the reason you hear about them so often is that less wind speed is required to do damage there. Small tornadoes that would otherwise not tear things up and wouldn’t be reported will overturn trailers, so this makes it seem like the tornadoes are aiming for them – the tornadoes occur other places, but on the power scale, don’t do any damage and therefore get ignored. How about cities, do tornadoes occur less often in cities? Well, yes, but that is only because cities cover a relatively small area compared to farmland, forests and plains. There is nothing inherently different about the geography of cities that makes tornadoes less likely to form or travel there. In 1999, an F5 tornado plowed through Oklahoma City, and more recently, an F4 tornado traveled from Tuscaloosa to Birmingham, AL, striking both downtowns without ever leaving the ground.

How Much Energy is There in A Tornado?
Different tornadoes contain different amounts of kinetic energy. A study from 2015 showed that most tornadoes contain between 108 joules (100 MJ) and 1014 joules (100 TJ) of total kinetic energy (TKE). There are many more small tornadoes than bigger ones, so the average TKE is skewed to the low end, so that the mean TKE for a tornado from 2007 – 2013 was 61.2 GJ. The tornado over that period with greatest TKE occurred in Yazoo City, MS in 2010 – 516.7 TJ! That is 100x more energy than the average tornado. So how much energy is a GJ or a TJ? A tornado of mean energy (61 GJ) releases energy equivalent to fifteen tons of TNT. On the other hand, the Yazoo City Tornado let loose with about the same energy as nine Hiroshima atomic bombs.

An EF-5 tornado can strip the ground right off the earth.
These images represent something called “ground scour,”
where the dirt, grass, trees, and even roads are pulled up by
the power of the tornado. 
Like noted above, not all tornadoes carry the same amount of energy, and this is affected by time of year. Early season tornadoes tend to have more energy than late summer storms. April and May have both the highest number of storms and the highest total kinetic energy, with April displaying more energy than May despite having fewer tornadoes. At the low end, September has the lowest TKE for any month, despite having twice as many tornadoes as November or December. The bottom line – you have the most chance of being in a tornado in the spring, and those tornadoes are likely to do the most damage, while September seems to be the celebration month for people terrified of being sucked up into the clouds and thrown into the next county. Over the period of time from 2007 to 2013, the state with the highest total kinetic energy dissipation from tornadoes is Alabama, while the lowest tornado energy states are Nevada and Utah. Plan your vacations accordingly – apparently what happens in Las Vegas stays in Las Vegas because it doesn’t get blown anywhere else.

Contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules

Fricker, T., & Elsner, J. (2015). Kinetic Energy of Tornadoes in the United States PLOS ONE, 10 (7) DOI: 10.1371/journal.pone.0131090

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