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.
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.
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.
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.
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.
Contributed by Mark E. Lasbury, MS, MSEd, PhDFricker, T., & Elsner, J. (2015). Kinetic Energy of Tornadoes in the United States PLOS ONE, 10 (7) DOI: 10.1371/journal.pone.0131090
As Many Exceptions As Rules
As Many Exceptions As Rules