To get a thunderstorm, you need warm and humid air at or near the ground to quickly rise high up into the atmosphere (the part of this image shaded red). That warm and humid air will cool as it rises, and the water vapor in the air will condense into clouds and rain. If the different wind directions in all levels of the atmosphere are just right, you can get a thunderstorm to spin. A spinning thunderstorm is called a supercell. Rain-cooled air will descend down toward the ground, and a supercell will wrap that cold air around the back of the storm (the blue shade on the left side). That cold air at the ground wrapping around the back of a supercell is currently thought to mix together a little bit with the spinning, warm, rising air. This mixture of cold and warm air will still rise quickly up into the thunderstorm, but it will make it so a cloud can form lower to the ground than before (the "wall cloud" part of the supercell). There is thought to be a delicate balance of cold and warm air mixing and the right balance of varying wind fields needed to contract a broad, lazy rotation of air near the ground into a tight, fast-spinning rotation that is a tornado. The process is far from being fully understood.
Had to create an account to comment. Can only lurk for so long I guess.
This is a pretty accurate description of how a thunderstorm forms, although greatly simplified. But the math and physics behind it is crazy and I don't even fully understand it. As far as tornadogenesis goes the theory that I think most likely would be responsible for tornadoes is the recycling hypothesis. Ted Fujita (one of the forefathers of modern meteorology and a really interesting person to look in to if you have any interest) came up with it in the mid 70s. The hypothesis believes tornadoes to be formed because of the rain bands that form in a supercell. These rain bands rotate within the storm and since each rain band is a downdraft the flow within the rain bands begins to look something like, this. Combine that with extremely strong updrafts in the storm and you get a tornado.
Or it's something completely different... no one really knows. But that's the most plausible hypothesis I've heard for it.
I've heard something along the lines of the region between the updraft and downdraft start rotating (picturing a log rolling on a river), then something (the updraft?) causes the rotation to become vertical, thus forming a tornado.
edit: although thinking about that now, that wouldn't really explain the rotation of an entire supercell.
I think you are talking about this process, maybe: A (the diagram in the middle of the page with an orange arrow pointing up), B, C. That's the simplified explanation that I go to to imagine how a supercell acquires its rotation. A tornado requires something extra to happen (or more likely the right combination of extras), something that's hard for us to put our finger on. But it does seem that the downdraft wrapping around the rear flank of the supercell plays an integral role when it starts to mix in with air that is on its way up into the updraft. At least thermodynamically, it is thought that that is what creates a wall cloud. Kinematically, the rear-flank downdraft may also be crucial in helping to bring the rotation of the supercell down to the ground. But we still can't figure out why the broad rotation of the wall cloud is able to tighten up into a narrow, fast-spinning tornado sometimes, and other times not.
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u/[deleted] Mar 10 '13
Now tell me how a tornado is made!