Earth from space: Mysterious, slow-spinning cloud 'cyclone' hugs the Iberian coast

This striking photo shows an unusual cloud spiral nestled perfectly along the west coast of the Iberian Peninsula in 2017. 

The spiral consists of moist, cloudy air from the sea that's been swirled together with clear, dry air from the land by a phenomenon known as cyclonic rotation — the same mechanism that is responsible for generating tropical storms such as hurricanes, typhoons and cyclones, according to NASA's Earth Observatory. 

However, in this case, the rotation was much slower and weaker than the rotation during tropical storms, so the cloudy and dry air did not fully mix together, which prevented a proper vortex from forming. This also stopped any clouds from passing over the land. (The image has been edited slightly to include more infrared light, in order to accentuate the difference between the land and cloud.)

"Precisely what caused this [unusual] mid-latitude rotation remains somewhat of a mystery," Stephen Jospeh Munchak, a meteorologist at NASA's Mesoscale Atmospheric Processes Laboratory told NASA's Earth Observatory. However, it could have been caused by an eddy — a temporary, swirling current of water that extends deep below the ocean's surface, he added. 

An extreme heat wave that swept across Southern Europe in July 2017 also created an usually large temperature difference between the cloudy, sea air and the dry, land air. This may have further prevented the two fronts from mixing, helping to create the stunning spiral shape. When this photo was taken, temperatures in Spain and Portugal were above 104 degrees Fahrenheit (40 degrees Celsius), according to NASA's Earth Observatory.

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The clouds in the spiral are marine stratocumulus clouds — common low-lying clouds that usually stay below 6,000 feet (1,830 meters) and can cover up to 6.5% of Earth's surface at any one time, according to the National Oceanic and Atmospheric Administration (NOAA).