If you are enjoying a nice day at the beach, the last thing you want to see is a massive tsunami heading your way. Tsunamis can be extremely deadly, and if you don’t flee to higher ground quickly enough, they can be almost impossible to escape.

With that in mind, it is completely understandable that when beachgoers in Portugal recently saw what appeared to be an impossibly big wave heading their way, many of them packed up and left in a panic. Fortunately, however, what appeared to be a tsunami was actually what is known as a ‘roll cloud’. While rare, roll clouds are entirely harmless.

This type of cloud can form when cold air comes into contact with high-temperature air, but there isn’t enough moisture in the air to cause rain. When they occur, they often stretch for hundreds of miles across the ocean (they can’t last once they hit land).

Since they form out in the ocean and can move in toward land, they can have a similar appearance to a giant wave coming toward the shore. This is because the sky is often clear and blue ahead of the roll cloud, and under the cloud, it is in deep shadow. The cloud seems to blend with the ocean, giving it an ominous look that many people who aren’t cloud or tsunami experts will mistake for a wave.

You can see a video of the roll cloud that moved in toward Portugal on video here:

It is easy to see why one might be frightened and want to get out of the area. And just to be clear, if a tsunami is even suspected to be coming toward a beach, the best course of action is indeed to run to higher ground.

For cloud experts, however, seeing a roll cloud move in is something not to be missed. They are very rare and impressive events that most people will never get to witness. So, while scary, the beachgoers that day really did get to experience something amazing. Fortunately, it was also completely safe.

Also, some climate scientists believe that as temperatures rise due to climate change, this type of cloud formation may become more common.

If you thought that was interesting, you might like to read about a quantum computer simulation that has “reversed time” and physics may never be the same.