Sunday, February 12, 2006

Column 4: Tornado or straight-line wind? (originally posted September 26, 2005)

I got asked a question by Rain, one of the moderators at North American Met, which I thought deserved a good, long answer. An answer which I could easily make into my next column! (And if you couldn't tell by now, I **love** long answers!) :)

She asked me about storm damage surveys. How it's determined whether damage is from a tornado or straight-line winds.

Now, I've only done a couple of damage surveys, but it only took the first time to recognize how to tell the difference.


First off, surveys are done after the fact, so it's a good idea to look at the archived RADAR information from the region and time in question. It's not always feasible, as the RADAR network across the country is such that many tornadic storms are only "seen" above about 20 000 feet. But we do what we can. We use this, as well as all other available information, to determine the range of possibilities of storm type.

Then we get to the site and map out the damage, taking pictures and copious notes.

Rule number one is that if there are pictures and video of a tornado, that's pretty strong evidence that a tornado was to blame.

But on the ground, there are a number of damage signatures which are almost exclusively attributable to tornadoes, and there are some which are erroneously attributed to tornadoes. Knowing these patterns is half the battle. Great. Now I'm quoting G.I. Joe.

I was asked specifically about circular patterns as opposed to straight-line patterns. Well, this one is a bit of a toughie, with a lot of "if"s in there. For example, I was witness to a tornado near Pilot Mound, MB, on July 2, 2005. (I was chasing.) Two days later I was one of the damage surveyors. Now our mandate wasn't to determine whether or not there was a tornado--my personal observation of it and the mounds of pictures and video from the storm were conclusive proof. Our aim was to determine what Fujita scale the storm was.

Circular patterns of damage are very rare and almost impossible to see from the ground. This idea is around because of a couple of photos taken from the air by the legendary man, T. Theodore Fujita. He took pictures of a damage path with circles embedded in forward motion (think a pattern drawn by a spirograph) to further his assertion that some tornadoes have multiple vortices. As it turns out, if you're lucky (!!!!) enough to see this pattern, odds are the damage was caused by a tornado. And a multiple-vortex one, at that.

The other version of "circular damage pattern" much talked about is twisted trees. Twisted trees are **not** a definite sign of a tornado. All it shows is that when the twisted branch or trunk was formed, it wasn't as strong on one side as it was on the other. The weak area broke first, using the strong area as a pivot point, and the branch or trunk got twisted around that point. Straight-line winds, as well as tornadic winds, can do this kind of damage. As well, the radius of your average tornado is such that you wouldn't see circular motion on that scale anyhow.

This was very evident on the Pilot Mound damage survey. The tornado happened to blast through a forested area. For about a half kilometre, all the trees were bent or snapped off, and they were all laying down toward the south. As we drove farther along, seemingly at the blink of an eye, all the trees switched to leaning toward the north. This is an example of a pattern that **is** consistent mainly with tornado damage: convergence. It's one of the things we look for.

Next, we'll look for a long, (relatively) thin path, pretty much in one direction. This is easy to find especially if you're using maps of the area and plotting the damage on it. On the prairies, this is even easier to see due to the relative dearth of trees. There are trees usually alongside roads, but nowhere else. So if a tornado has come by you'll see a row of trees that looks like it had a bite taken out of it. And that bite is the width of the tornado.

Other, less-publicized debris patterns tend to lead us to believe that a tornado was the culprit. For example, insulation. You know, the pink stuff. Tornadoes tend to rip this stuff out of houses and hang it in trees. Straight-line winds don't do that as much.

And mud. After a tornado you can often see many sides of a house splattered with mud. This, again, doesn't happen often with straight-line winds.

Straight-line winds can do considerable damage. They can rip roofs off houses, snap trees, and take down power lines. Too often it seems such damage is immediately assumed to have been from a tornado. "It **must** have been a tornado," people say.

But it takes more than substantial damage to discern that. It takes a careful eye, a skeptical mind, and attention to detail, to the little clues.


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