The dramatic mass moving ashore one afternoon in Portugal this week looked like something out of a movie: dozens of people who had gathered at a beach to escape an oppressive heat wave stared up at the sky as it inched toward them.
Was an alien ship about to emerge? Should someone call Jeff Goldblum?
But it was a cloud that stopped people in their track — a “roll cloud,” to be specific, which more typically form from thunderstorms.
In this case, the cloud formed at the intersection of the ocean and the land as a cool, moist air mass moving over the Atlantic Ocean collided with dry, hot air over southwestern Portugal, which, like much of Europe, has been sweltering in a weeklong heat wave.
The roll cloud is created by a wave in the atmosphere that causes a rising and sinking motion over adjacent areas, allowing the clouds to form and appear to roll forward. You can see that below in an aerial image, where the crest and trough of the cloud are visible and resemble an ocean wave.
Sean Waugh, a severe storms expert with the National Severe Storms Laboratory who typically encounters such clouds as thunderstorms move over the Plains in the United States, compared the clouds to what happens when a rock is dropped in a pond. The cloud is just the leading edge of a ripple in the atmosphere.
While the cloud in Portugal occurred in the afternoon, it was most likely a “morning glory,” a specific type of cloud often associated with roll clouds that more commonly occurs after sunrise in the Gulf of Carpentaria in Australia.
This type of roll cloud can stretch many hundreds of miles long, which is why it was seen up and down much of the central coast of Portugal.
The heat wave that has swept Europe this week has led to multiple deaths and helped fuel wildfires in Greece, Turkey and Portugal. High temperatures are expected to spread east in the coming days.
Judson Jones is a meteorologist and reporter for The Times who forecasts and covers extreme weather.
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