A stunning and rare weather event known as a “tsunami roll” cloud left beachgoers in Portugal awestruck as it swept over the coastline on Sunday, June 29, according to the New York Post.
The massive, wave-like cloud formation appeared to roll across the skies above the ocean, creating the illusion of an approaching tsunami. Stretching an estimated 150 kilometers (93 miles), the dramatic scene was captured on video and quickly went viral on social media.
One eyewitness shared footage on X (formerly Twitter), comparing the surreal sight to a “tsunami out of a movie,” noting that the cloud spanned from Figueira da Foz up to Vila do Conde. In one clip, the cloud appeared like vast snow drifts; in another, it began to resemble a towering wave as it neared the shore.
Meteorologists identified the phenomenon as an arcus cloud, more specifically a roll cloud—a rare type of low, horizontal formation often seen ahead of thunderstorms. According to the National Weather Service, these clouds are “relatively rare” and typically form independently from the main thunderstorm base or other cloud systems.
Though visually intimidating, arcus clouds are harmless and not associated with actual tsunami activity.
