WEB EXTRA: The Formation of a Tornado

Severe Weather Handbook

 (WBRE/WYOU-TV)  We have learned all too quickly this year that tornadoes can and will happen in northeastern and central Pennsylvania.  So what we’re going to do is take everyone through the formation of a tornado, and how we try to detect them using our radars during severe weather events.

Let’s talk about that tornado formation. What we start off within severe weather a lot of time what we see is a change in the wind direction based off the height.  The wind at the surface may be blowing in one direction but then once we go higher into the atmosphere that wind is shifting and blowing in the opposite direction. The word we use for that is sheer, you’ll hear us talk about that sometimes in severe weather.  All it is are two different kinds of air blowing in two different directions.

As that happens, the air starts to rotate. So that change in the wind direction with height causes horizontal rotation in the atmosphere. This is actually the start of a tornado, it’s the inner workings of it. That spinning, that rotating, actually starts horizontally. 

And then think of thunderstorms as a big vacuum, they suck in a lot of air and have a lot of upward forcing with them. So as that happens let us watch what happens to the rotation. As that thunderstorm starts to bring in the air a lift it, it starts to lift that rotation vertically. So now we’ve gone from that horizontal spinning to vertical and now all of a sudden that’s how we get that tornado to form.

 

So how do we determine this during a tornado warning?  There’s a couple of different things that we look for, the first of which is that shear, that’s that change in that wind direction with height. 

On this map here it’s that red little area here that you’re looking at right near Gibson and Jackson, this is from a tornado that touched down in April is Susquehanna County.  So there’s that shear, that’s the first sign that there’s some rotation in the atmosphere.

Then we look at our rotation detector.  What we’re looking at here is the way that the wind blowing both toward and away from the radar. 

So these green returns everything is blowing toward the radar, that red return everything is blowing away from it.  

Right in the middle, that’s where we would see that rotation. That’s another very good sign that there’s at least rotation in the cloud cover and a possible tornado. 


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