Angelika Pickardt

July 2, 2025, 11:29 am | Read time: 2 minutes

Videos are currently circulating online that were taken on the north and central coast of Portugal and are causing a stir: They show a kilometer-long, cylindrical cloud forming over the sea, moving ominously toward the mainland. “This cloud rolled in like a real tsunami,” writes a user on the social media platform X about his video. It shows the full extent of the natural phenomenon, known as a roll cloud. How do roll clouds form?

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How Do Roll Clouds Form?

“Roll clouds form when cold air masses—such as after a thunderstorm or along a cold front—slide under warmer, more humid air and push it upward. This can create a horizontal cloud cylinder over several kilometers,” explains Dominik Jung from the weather service wetter.net in response to a TRAVELBOOK inquiry. “The key: Different wind directions in the air layers above and below set the cloud into rotation. This creates the impression that it is rolling across the sky—calmly, evenly, and often completely detached from the other clouds in the sky.”

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Are Roll Clouds Dangerous?

“Even though they can appear threatening, a roll cloud is harmless on its own,” explains weather expert Jung. However, it often indicates that strong gusts or a weather change are imminent nearby. “The phenomenon is particularly impressive when it moves like a white wall over a city—such scenes almost resemble a science fiction film.”

Can Roll Clouds Also Form in Germany?

According to the meteorologist, roll clouds are rare in Germany but do occur. “Especially along the coast, such as near Rostock or on the North Sea islands, they can appear under certain weather conditions. In July 2015, a spectacular roll cloud over Mecklenburg-Vorpommern made headlines and produced fascinating images on social networks.”