What is the rare ‘tsunami’ roll cloud phenomenon?

On June 29, 2025, Portugal witnessed a rare "tsunami" roll cloud. This strange and scary weather event caused a massive roll cloud to stretch over 93 miles (150 kilometers) of the coastline.

Resembling a giant tsunami wave, beachgoers were seen fleeing in panic, and as pictures began to circulate, curiosity spread as to what could cause such a meteorological spectacle and how dangerous it could be if it happened again.

So, click on to find out about this rare phenomenon.

The rare roll cloud

The rare roll cloud, known as a Volutus, looks something like a tsunami and can be terrifying to experience. The bizarre weather event causes a cloud to form into the shape of a long, low tube.

Cloud species

In fact, the weather event is so unique that in 2017, it was officially added to the International Cloud Atlas and became its own cloud species. The Atlas is maintained by the World Meteorological Organization (WMO) and recognizes over 100 distinct cloud types. 

A solitary cloud

The roll cloud typically appears as a single, isolated band that is unconnected to any other clouds. The tube of cloud then stretches across the horizon in a long, cylinder-shaped cut through an otherwise clear or stormy sky.

Rough and rolling

At other times, however, it takes on a much scarier appearance. When the wind beneath the cloud is turbulent, it can make the cloud appear rough or wrinkled. The front edge then appears to climb upward while the tail dips down—like a long, rolling barrel.

Weather change

Whilst the cloud is hanging over the area you are in, it is unlikely to bring rain. However, as it passes over you, you will probably feel a large gust of wind or notice a sudden change in air pressure. 

A cloud blanket

Most often, however, they form in the stratocumulus genus—a category of low-level clouds that usually form below 2,000 meters (6,600 feet). They appear like layers and can create the illusion of a blanket of clouds being left hanging in the path of the rolling cloud. 

Thunderstorm downdraft

Sharp shifts in wind speed and temperature occur when there is a downward rush of cool air from a thunderstorm. This is called a downdraft, and it pushes cooler air outwards, like cold air racing out from the storm. 

What makes it happen?

This terrifying and unexpected event can occur for a number of reasons. However, two main scenarios are likely to make it happen, and you should be aware of them to make sure that you stay out of their path.

Coastal breeze

The second is a coastal breeze, and it occurs, as the name implies, near the coast. Typically, it happens at nighttime when the late land breeze pushes cool air towards the warmer sea air. This then creates the roll cloud, which rides the “bore wave” of rising air. 

Is it dangerous?

There is no denying that this apocalyptic-looking cloud is terrifying, but the question is—should you be worried if you see one? It might be hard to believe, as they appear so much like a tsunami, but Volutus is not actually dangerous. 

Global warming

As with all weather events, global warming has an impact on roll clouds as it can influence specific atmospheric setups that lead to roll clouds forming. However, the connection is indirect and still being studied.

The "Morning Glory"

The exception to this, however, is in Australia. One of the few places where roll clouds consistently show up is Australia’s Gulf of Carpentaria, which hosts the now-famous "Morning Glory" clouds every year. 

Around the world

However, it can also occur outside of Australia, on coastlines, and storm fronts in North America, Europe, South America, and parts of Africa. However, it’s so rare that aside from the Morning Glory cloud, there have only been five major events in the past decade.