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On the 4th of July, Americans like to celebrate the things that make the United States unique, and a lot of those things have to do with our geography. That remarkable geography is also responsible for some pretty unique weather, and unfortunately for the millions of people living in the Midwest, that weather includes tornadoes. In this episode of SciShow, Hank explains why scientists think the U.S. is prone to so many tornadoes.

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Hank Green: I grew up thinking that tornadoes were just a thing that happened all over the world -- just a fact of life, like earthquakes and hurricanes, but it turns out tornadoes are a fairly American problem. Now, of course they're not exclusive to America; tornadoes have been recorded on every continent except Antarctica, and even there scientists say tornadoes are possible. Tornadoes regularly strike in Bangladesh, Australia, and China; the United Kingdom actually has more tornadoes relative to its size than any country in the world. However, of the roughly 1500 tornadoes that occur globally each year, between 65 and 80% of them occur in the US, and almost all of the big, dangerous, powerful ones happen here. So, why? Why do tornadoes hate us? [intro music] Well, like many things that we really should know, we just don't. Scientists do recognize a few key conditions for tornado genesis, and the plains of the United States between the Rocky and Appalachian Mountains, an area dubbed "Tornado Alley", have proven to be the perfect environment for them, especially in late spring and early summer. So, what are those conditions? First, there has to be a lot of low elevation moisture. Here in the US, the moist air is supplied by the Gulf of Mexico. When it comes over land, it stays in the lower part of the atmosphere and becomes heated by the Sun, but it can't rise to form clouds because it's trapped by a layer of hot, dry air that blows in from the southwestern deserts. This layer of hot dry air is called the convection cap, and it acts like the lid on a pot of boiling water, keeping the warm, humid air from rising. Above the convection cap, cold air comes in from the Rocky Mountains. That hot air wants rise up and the cold air wants to drop. This potential energy builds up as the convection cap starts to weaken. Then there needs to be some kind of trigger to quickly lift that moist, high-pressure air. In some cases, the trigger is a cold front; in other cases, it's a low-level area of converging winds. Whatever the case, when the cap breaks, the moist air bursts through fast. We're talking speeds of up to 160 km per hour. So, now you've got the air moving fast up, how do you get it to start spinning? Most tornadoes are the product of supercell thunderstorms, large convective storms with a persistent rotating updraft known as the "mesocyclone". When the warm air rushes upwards, it creates a powerful and unstable updraft. As it rises, it can produce a storm as much as 15,000 meters tall within a few minutes. We're still trying to figure out the complex interactions that take place between the updraft and the surrounding winds within the mesocyclone, which is around two to six miles in diameter. What we know is that, in order to spawn a tornado, the updraft has to successfully raise, sustain, and tighten the central vortex of the mesocyclone. If these winds achieve balance between the inward and outward flow of air, there is the potential for a funnel cloud. Add it all up, and the central US is an all too perfect place for tornadoes for at least two reasons. First, it's between 30 and 50 degrees north of the equator. At these mid latitudes, air tends to flow at different speeds and different directions throughout the troposphere, which is the lowest level of the Earth's atmosphere. This aids in creating the wind rotation of a supercell. And second, Tornado Alley is geographically unique in that no other place on Earth has moist air coming from the equatorial region and tall, wide, north-south running mountain ranges to provide triggering cold fronts. So, for all you out there in Oklahoma, Texas, Kansas, Missouri, or any other state in Tornado Alley, you live in the part of the world that is perhaps best suited to this amazing and terrifying act of nature. Sorry. Thank you for watching this episode of SciShow. If you have any questions or comments or ideas, we're on Facebook and Twitter and down in the comments below, and if you want to keep getting smarter with us here at SciShow you can go to youtube.com/scishow and subscribe. [outro music]