My six-year-old nephew spent the weekend at my house, which delighted my son to no end. The end result was the sort of excited chaos you might expect – lots of six-year-old bickering, and toys strewn everywhere, and strange non sequitur laced conversations. At one point, tornadoes came up. I don’t know why, because I wasn’t paying attention to the start of that conversation. But my nephew declared that he’d seen a tornado, and that his mom had made them get in the closet. And my son responded that you didn’t get in the closet, you got in the tub, because we’d had a tornado warning once and our front bathroom is the safest place in our condo for that sort of thing. So they argue the merits of bathroom versus closet for a few minutes, and then my son looks at me. I smile, preparing my “it depends on the house” explanation for the question I’m sure is coming.
“Dad?” my son asks. “Why do tornadoes suck things up?”
All right. So that isn’t the question I expected.
What is a tornado?
The National Oceanic and Atmospheric Administration has a page titled The Online Tornado FAQ, and I’ll be referencing it a lot over the course of this question. To begin with, I’ll just quote their answer to the question:
According to the Glossary of Meteorology (AMS 2000), a tornado is “a violently rotating column of air, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud.” Literally, in order for a vortex to be classified as a tornado, it must be in contact with the ground and the cloud base. Weather scientists haven’t found it so simple in practice, however, to classify and define tornadoes (per this essay by Doswell). For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also disagreement as to whether separate ground contacts of the same funnel constitute separate tornadoes. Meteorologists also can disagree on precisely defining large, intense, messy multivortex circulations, such as the El Reno tornado of 2013, compared to the parent mesocyclone and surrounding winds of damaging intensity. It is well-known that a tornado may not have a visible funnel. Mobile radars also have showed that tornadoes often extend outside an existing, visible funnel. At what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.
In other words, a tornado is a vortex, very much like an atmospheric whirlpool. In an oversimplified fashion they form under basically the same conditions – our atmosphere is described by the same fluid dynamics that describes the behavior of water, after all. Moving air hits a barrier – in this case, denser colder air – and twists back on itself. The air is still moving into the barrier, however, so the air that is deflected back picks up speed thanks to the conservation of angular momentum, creating a vortex.
But, like I said, that’s the oversimplified explanation. Here’s what NOAA says on the subject:
The truth is that we don’t fully understand. The most destructive and deadly tornadoes occur from supercells–which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX programs suggest that once a mesocyclone is underway, tornado development is related to temperature changes across the edge of downdraft air wrapping around the mesocyclone (the occlusion downdraft). Mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind these theories are given in several of the Scientific References accompanying this FAQ
What this means is that they’re vortices, and they form just like any other vortex. But, like most things in nature, they’re super complicated and we don’t really quite understand what makes them start.
So how do these tornados suck things up?
The famous “sucking tornados” are “multiple vortex tornados“, and they create what is called a “suction vortex“. Interestingly, the suction vortex has little to do with air pressure, and everything to do with wind speed. That is, the lower air pressure within the vortex isn’t low enough to “suck” things up. Tornadoes aren’t straws. Instead, the speed of the winds traveling up the vortex funnel create the “suction” effect.
Here’s what happens. The secondary vortices of a multiple-vortex tornado orbit the axis of the primary vortex, increasing the wind speed around the primary vortex. When the wind from and around the secondary vortices “turns the corner” – that is, enters the primary vortex and suddenly changes from horizontal to vertical flow – angular conservation causes the wind to pick up an enormous amount of speed. It is this wind speed that lifts objects – cows, trucks, people, roofs, whatever – and hurls them into the air. Note that all tornadoes have this “turn the corner” effect, but it takes the secondary vortices to really get the wind moving fast enough to lift really heavy objects.
Waterspouts and fire tornadoes
My son had heard of waterspouts, and guessed that they were water tornadoes. He was right. A waterspout is literally just a tornado that forms over water and that sucks up water.
Fire tornadoes are a little different. They are similar to actual tornadoes in appearance – except, you know, for the fire – but are formed by rising surface winds (usually generated by the heat from the fire) that meet turbulent winds to form a spiral of rising flame. They aren’t formed by supercell thunderstorms and aren’t tornadoes. Doesn’t make them safe, mind.