Unusual weather conditions made for a spectacular natural phenomenon in the sky over Berlin this week. 

Mammatus clouds cause a storm over Berlin

“Like hundreds of glowing jute bags hanging from the sky”: a special weather phenomenon over Germany’s capital city caused a stir on the internet this week. Following a brief thunderstorm on Thursday, which brought heavy rain and strong gusts of wind, the sky was suddenly lit up with a glorious swathe of unusual and distinctive cloud formations. 

Residents of the city were quick to take to Twitter and Instagram to share their snaps of the clouds, which shimmered in the evening light, hanging densely in the sky and making the cityscape look like a curious mix of cartoon and hyper-realism. 

Users described them as “candy floss clouds”, “Super Mario clouds”, “crazy” and “surreal”. And everybody had the same question: what was the natural phenomenon, and what causes it? 

According to ntv weather expert Björn Alexander, the unusual cloud formations were what are known as mammatus clouds. The word is derived from the Latin term “mamma”, which means “breast” or “udder” - a perfect description for the cloud’s unusual “lumpy” or “pouchy” undersides. 

“These clouds usually arise in the area of thunderstorms, showers or cold fronts,” said the weather expert. This means that they are observed more frequently in the summer than in winter, but they also occur in the colder seasons. 

What are mammatus clouds and how are they formed?

Exactly how they are formed is not yet fully understood. It is thought that they are the result of convective processes in connection with rain clouds or larger thunderstorm complexes, as cooled air from higher layers tries to descend through the warmer air beneath it. This results in turbulence within the cloud and therefore causes mammatus to form on the underside, thus reversing the usual cloud-forming process of upward growth and making the cloud base appear uneven. 

According to the German Weather Service (DWD), mammatus clouds are a relatively frequent phenomenon - but they are generally short lived, lasting only around 10 minutes. They also normally cover areas no larger than a kilometre in diameter, which explains why the phenomenon attracted so much attention this week over the densely-populated Berlin, where an unusually large number of people were able to see the clouds at the same time.