[♪ INTRO].

Nearly half the people in the world live within 100 kilometers of the coast, and for many of those 3 billion people, tsunamis pose a serious threat. Fortunately, scientists these days have developed pretty reliable early warning systems that can help evacuate people before a giant wave reaches shore.

The problem is, these warning systems specifically detect tsunamis triggered by earthquakes, and these aren’t the only kinds of tsunamis out there. In some cases, there can also be tsunamis triggered by… the air. These are much harder to predict with today’s technology.

But the better we understand how they happen, the better we can protect coastal populations from these devastating waves. Now, generally speaking, a tsunami is any wave caused by some sort of physical disturbance that displaces a large body of water. For example, seismic tsunamis happen when an earthquake on the seafloor gives part of Earth’s crust a jolt and displaces a lot of seawater.

It’s sort of like if you had a bucket of water sitting on a brick patio. If a brick underneath it suddenly pushed upward, the bucket would tip, and the water would rush out in one big wave. Except, when it happens in the ocean, this wave can be several stories tall instead of a few centimeters.

But there’s also another type of tsunami called a meteotsunami. These waves are caused by a change in air pressure instead of shifting land. So, if we’re sticking with the bucket analogy, it’s kind of like you took a leaf blower to a bucket and blasted the water with air so it spills over the edge.

In both cases, the disturbance triggers a wave of water, but with meteotsunamis, that disturbance comes from the sky instead of the seafloor. It tends to happen when there’s fast-moving weather over the ocean, like thunderstorms, squalls, or storm fronts. Now, if you’re wondering why you’ve never heard of a meteotsunami before, it’s because they’re not usually that big of a deal.

A wave doesn’t have to be especially big to be a tsunami. It just has to be made of water being displaced by some outside force. And most meteotsunamis aren’t that big.

That’s because the storms that cause them move a lot less water than earthquakes do. So while seismic tsunamis can spread across the entire ocean, meteotsunamis just tend to affect the area right around a storm. They also usually top out at 2 meters high.

In fact, most meteotsunamis are so small that they’re harmless… but under certain conditions, they can become dangerous as well. For example, in 1954, a wave around 3 meters tall swept over Chicago’s waterfront, killing eight people. In 1979, a 5-meter-high wave killed several people in Japan’s Nagasaki Bay.

And in 2013, a meteotsunami injured two New Jersey swimmers after sweeping them away from the shore. Fortunately, large meteotsunamis are pretty rare, and they’re usually less destructive than the seismic kind. But they do happen.

So scientists are working on understanding how some of them get so dangerously large and how to predict them. One important piece of the picture seems to be the geography of the place a wave strikes. Areas with features like harbors or long, narrow bays are typically more prone to meteotsunamis than wide open coasts.

And scientists think that has to do with the way the land interacts with the waves passing by it. As ocean waves bounce off the land, they interfere with incoming waves. And in special cases, that interference can create unusually strong waves.

In particular, if the peak of a reflected wave lines up with the peak of an incoming wave, the two waves combine into one bigger wave. This is called constructive interference, and each time one wave lines up like this with another wave, it gets bigger. A lot of factors have to be just right for constructive interference to happen, like the speed and angle of the waves and the shape of the coastline.

And in certain places, it’s more common for all those factors to align than in others. For instance, meteotsunamis are more likely to develop in narrow waterways or bays with narrow entry points. That’s because in these areas, waves tend to bounce off the shoreline and crash into each other, creating more opportunities for constructive interference.

So as a result, certain areas are more prone to meteotsunamis. Places like Japan’s Nagasaki Bay and the eastern Adriatic Sea, as well as the western Mediterranean, which are all pretty narrow. But even in an ideal environment, not every meteotsunami will turn into a giant wave.

The angle of that wave also matters, and that depends on where the storm is. So it takes many different elements coming together to make a meteotsunami, which makes them hard to predict. But researchers are making some headway.

In a paper published in 2020, scientists observing a meteotsunami in the Great Lakes figured out that the atmospheric conditions that caused that wave could have been predicted by weather forecast models in advance. It suggests that, basically, the computer models that tell us to expect sunshine or a chance of thunderstorms could be repurposed to warn of a chance of tsunamis. But the problem isn’t completely solved, because these models work better for certain weather patterns than for others.

So scientists aren’t predicting meteotsunamis yet. But they’re moving in the right direction, and hopefully one day, weather forecasts will actually be able to protect people from tsunamis from the sky. Thank you for watching this episode of SciShow, and thanks as always to our patrons for helping to make it possible.

If you’d like to join a great community, and get to see some cool stuff like our newsletter and monthly bloopers, you can get started at Patreon.com/SciShow. [♪ OUTRO].