Ask A Met: How Do Tornadoes Work?

This week's question comes from Morning Brief reader Robert, who asks, "Do tornadoes rotate clockwise or counterclockwise? And why?"

Meteorologist Jonathan Belles: The answer is both.

Low pressure systems in the northern hemisphere typically will rotate counterclockwise. That's because of the Coriolis effect, which is created by the spinning of the earth.

If the Earth decided to spin the other way all of a sudden, then we'd have low pressure systems rotating clockwise.

The reason that tornadoes are able to rotate both ways is that they're so small that friction and very small scale factors can play into which way they rotate.

Thunderstorms often split. They separate right down the middle. In meteorology, they're called the left split and the right split.

In the right split, with the counterclockwise spin of the low pressure system, tornadoes are favored because of the rotation of the earth. It's a little bit harder for tornadoes to spin the opposite direction, which is why clockwise tornadoes are much less common.

On occasion, you get tornadoes on both sides and because of the way the air flows through those splits, you'll often get one that spins clockwise, and one that spins counterclockwise.

Typically tornadoes will spin counterclockwise about 90% of the time.

The Coriolis effect is one of the fundamental systems that drives winds. All low pressure systems, all high pressure systems are spun by that Coriolis effect.

That's also the reason that in the southern hemisphere, low pressure systems spin the opposite direction. And, in fact, tornadoes also spin clockwise in the southern hemisphere for all of the same reasons that they spin counterclockwise in the northern hemisphere.

The vast majority of tornadoes in the world, probably around 75%, occur in North America, though. That’s a function of our geography.

There are heat and moisture sources in the Plains and the Gulf that fuel the rising air for tornadoes, but elevation also plays a big role.

When storm systems come directly off of the Rockies, the elevation drops rapidly. Let’s say by 1 mile for this example. That means that storm systems have to stretch to reach down towards the ground again and, by stretching, they end up spinning faster and intensifying.

Imagine an ice skater:

When we see a figure skater in the Olympics pull their arms in and stretch upwards, they spin faster. They’re bringing all of their weight to the center, which accelerates rotation.

The exact same thing can happen with low pressure systems.

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