How Does a Tornado Form?

We don’t know exactly. How's that for an answer. That’s the simple answer and the truth.

A lot of time, effort, and money is and has been spent trying to decipher the "tornado code". The trouble is that unlike in the movie twister, we can't all drive our dodge pickup right up to the tornado and take measurements.

However, not to worry, we do know a lot about what type of conditions make tornado development favorable.

The figures I made below (I majored in weather not art, so I'm sorry the figures are crude) shows you roughly where I believe tornado alley could be defined. Because the US happens to be a nice large flat piece of land that tends to be the battle ground for Polar airmasses and a Maritime airmasses, the topography is one reason why we have so many thunderstorms and tornadoes in the Midwest.

Caveat: This is a rough estimation of tornado alley. I don't believe anyone has precisely defined tornado alley.

But warm & moist air colliding with cool & dry air is just the primary setup for strong thunderstorms. So why don't we call it "strong thunderstorm alley"? Too many syllables and it's not as flashy as tornado alley.

So moisture is a key component and typically we need cool & dry air colliding with the warm & moist air to get strong thunderstorms. But there are more ingredients needed to make tornado development favorable. Most tornadoes develop from a storm that is rotating. In fact, the most destructive tornadoes are spawned from a rotating thunderstorm called a supercell. Well, how does a storm get it's rotation?

The rotation develops due to something called wind shear. This means that the wind at the surface is a different direction and speed then the wind aloft. This wind shear aids in developing the rotation. To understand this further, imagine a pencil in between your hands. If you pull your bottom hand toward your body and push your top hand away from your body, the pencil rotates, right? This is a grossly simple demonstration that doesn't necessarily accurately reflect what's happening in the atmosphere. However, it may help you understand that when winds in the atmosphere that are moving at different speeds and different directions it is a more favorable atmosphere for rotation and thus tornado development.

When this rotation first develops it may actually be horizontal (meaning parallel to the ground, not vertical like a tornado.) It's the strong updrafts and downdrafts in a storm that likely turn this horizontal rotation vertical. This doesn't automatically create a tornado, but it gets us to the point where we have a rotating storm, which as mentioned above is one of the ingredients needed to create a tornado. (I have provided some links which graphically explain this at the end of this article)

So, a thunderstorm that is rotating (like the supercell in the picture above) and develops in or near a boundary where cool & dry air is colliding with warm & moist air are all the basic ingredients for our tornado. But just like baking a cake, you can't just put flour, eggs, sugar and water together and get a cake. You need to follow the instructions on how to put those ingredients together in the right order. Well, we don't have the instructions on how our atmospheric ingredients go together to produce a tornado. But we can forecast a potential for tornadoes when we see the ingredients are all present.

According to the Storm Prediction Center (SPC), the VORTEX program (Verifying the Origins of Rotation in Tornadoes Experiment) suggests that tornadic development is associated with the temperature difference along the edge of the downdraft air wrapping around the supercell. So, scientists are getting closer to finding out what variables in a storm trigger a tornado, but so far it can't be narrowed down to one trigger. Likely it isn't one trigger, but several all acting together.

If you'd like to learn more in depth about what we do know or theories about tornado development, here are a few links for your edification.

This link shows you in general and simple terms how the rotation develops in the first place. http://www.crh.noaa.gov/lmk/tornado1/fslide9.htm

This link has a nice animation of how the rotation starts horizontal and then turns vertical. http://www.weather.com/newscenter/specialreports/tornado/inside/about.html

Here's some info on Project VORTEX: http://www.nssl.noaa.gov/noaastory/book.html