These are Mammatus clouds hovering over the Promenade des Anglais, drooping like mysterious celestial bubbles ready to burst.

I don’t recall ever seeing them before, and hoped that my Sunday evening spotting turned out to be a rare sighting of a rare formation. Mammatus clouds are sometimes described as rare, but that’s a perception based on their relatively short life span, just some 10 to 15 minutes, so when they do form, they’re easy to miss. Thus, not rare. But still quite amazing.

EarthSky.org describes them like so:

Mammatus clouds are pouch-like protrusions hanging from the undersides of clouds. You’ll usually find them under thunderstorm anvil clouds, but you might see them under other clouds as well. They’re composed primarily of ice. And groups of them can extend hundreds of miles in any direction. But they’re fleeting, remaining visible in your local sky for perhaps 10 or 15 minutes at a time.

Most clouds are formed by rising air. But mammatus clouds are formed by sinking air. They appear ominous.

They do. And then they’re gone.

Another shot below cropped a little bit closer, snapped, like the main image, from near the west end of the Promenade des Anglais. That spread wasn’t hundreds of miles, but as you can see, it was wide.

Photos from 8:53pm, 5 July 2026.