Wx Savvy | Tornadoes

What Is A Tornado?

"A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. " -American Meteorological Society glossary of terms.

Things to remember about this definition:

1) A tornado is air. What hurts or kills people is the debris swirling around a tornado.

2) A tornado is by definition not necessarily visible. Rotating air in contact with the ground usually becomes visible due to the dirt it picks up and/or the condensation of water in the air forming a funnel like cloud giving the tornado it's visible appearance.

3) A tornado is by definition in contact with the ground.

In fact, if it is not touching the ground, it is often referred to as a funnel cloud. However, you'll learn why that may be a misleading term if you read further.

Tornadoes can last for seconds or close to an hour, though most last less than 10 minutes (1). Most tornadoes in the U.S. form between March and August, when atmospheric conditions are most favorable for the severe thunderstorms that produce tornadoes. However, the tornado season never truly ends. In fact, winter tornadoes are not all that uncommon, but they usually occur in the Southern US where it is much warmer than say...Minnesota. In Iowa however, there has been a tornado recorded in the state in every month of the year. So although January tornadoes are rare in the Midwest, they have occurred.

Here's a great web site to answer many of your tornado questions: http://www.spc.noaa.gov/faq/tornado/

Above I have discussed the simple and widely accepted definition of a tornado. However, atmospheric science is very complex and sometimes you need to dig deeper to truly understand what's happening in the atmosphere. What am I getting at? I will attempt to intrigue you with my understanding of the concept and then leave you with a web link for further exploration.

Above I said a tornado is "in contact with the ground" and not necessarily visible. So, if a tornado by definition does not have to be seen, how do you know it's in contact with the ground?

For starters, to better understand the definition of a tornado let's think about what is physically happening in the atmosphere. A rotating thunderstorm is causing a large area of wind to rotate around the surface and up to the middle levels of the atmosphere. This rotation is over a large area, not the small area and intense rotation of a tornado. However, this is where it all starts. The rotation, or vortex, is not yet intense enough to pick up dirt, cars or cows as a tornado would, but it likely would be visible as you looked up at the parent thunderstorm. What you would see is a slow swirling in the large thunderstorm clouds. Although you can't see it in the clear air surrounding you, the slow swirling of the air is happening all around you too!

What happens to form a tornado is pretty cool. Just like a figure skater who wants to spin faster and faster would pull their arms inward toward their body, the spinning vortex of the thunderstorm starts to come closer together and intensify. This follows the law of conservation of angular momentum. If the speed increases enough, a small but violently rotating column of air will develop, and this would be a tornado.

It often appears as if a tornado comes down from the thunderstorm, first as a funnel cloud and then making contact with the ground. But this is a deceiving observation. As I just mentioned the rotation is already in contact with the ground and not coming down from the thunderstorm. What is happening is that the vortex around you is not intense enough to pick up dirt or form that classic tornado condensation cloud at that moment, but nevertheless it is in contact with the ground. But as the vortex tightens (like the figure skater pulling their arms inward and spinning faster) it's just easier for water to condense near the cloud base and that's why the classic looking funnel cloud forms out of the thunderstorm first.

As the rotation intensifies even more, the water in the air near the surface also condenses around the twister and forms the classic looking tornado. So a tornado doesn't necessarily stretch down from the clouds as much as it intensifies around a vertical axis (a vertical axis is kind of like an imaginary pole that stretches from the ground up). In the picture above the condensation funnel is not in contact with the ground, but a debris cloud near the ground shows the rotation is intense near the surface. (The debris cloud is the dirt swirling just left of the water pump or whatever that thing is in the middle of the picture.) So this is a tornado!

A tornado's definition is somewhat vague because the rotation is not always visible and yet in order to be a tornado the rotation must be in contact with the ground. Typically if the winds are indeed violently rotating you will see at the very least a debris cloud as in the picture above. But digging deeper into what's actually happening in the atmosphere is truly the best way to understand the definition of a tornado.

The reason I'm explaining this is because I read an interesting article by Chuck Doswell on this very issue (See his web article here). Although the article represents Mr. Doswell's opinion, he's a very well respected meteorologist and his ideas help to further our understanding of how we conceptualize and define atmospheric phenomena.

(1) http://www.spc.noaa.gov/FAQ/tornado/

Pictures are from the National Severe Storms Lab (NSSL) Photo album