This extremely rare horseshoe vortex cloud was photographed in the sky over Endicott, New York on February 3, 2016.
This cloud forms in a region of rotating air, or vortex. Such vortices usually form vertically, sometimes leading to waterspouts or even tornados. But, occasionally, they can develop with a horizontal axis and give rise to a gently rotating crescent of cloud.
The Cloud Appreciation Society ranks the horseshoe vortex cloud as one of the rarest of all.
Some say that a horseshoe brings luck only when it is pointing upwards and holding the luck in. Others claim that, when pointing down, it allows luck to fall onto those below.
Here a 2012 timelapse video of such a cloud forming before fading away over Patagonia:
As horseshoe vortex clouds tend to point downwards and so any cloudspotters already fortunate enough to see one will want to stand below it to top up on their luck.