Beachgoers in Portugal mistook a large wall of clouds for a tsunami, expressing their fears on social media. This phenomenon was confirmed to be roll clouds, a type of low-level cloud associated with powerful thunderstorms. Roll clouds, or arcus clouds, differ from shelf clouds as they are not attached to the storm cloud. They form from cold downdrafts of air interacting with warm moist air, often bringing strong winds, heavy rain, and thunderstorms along with them.
Images shared across social media showed a huge horizontal wall stretching from the horizon towards the shore, accompanied by a violent gust of wind when it reached land.
Roll clouds are low-level, wide-ranging clouds that are typically associated with powerful storm clouds and thunderstorms.
When a cold downdraft from a cumulonimbus cloud reaches the ground, the cold air may spread rapidly along the ground, pushing existing warm moist air upwards.
Arcus clouds usually form with cumulonimbus clouds and downdrafts, meaning they tend to be associated with strong, gusty winds, heavy rain or hail showers, as well as thunder.
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