Horseshoe cloud spotted above Navada, USA on Thursday.
Horseshoe cloud spotted above Nevada, USA, on Thursday.Chrisy Grimes.

In a rare occurrence, a rather unusual looking cloud appeared on the Nevada sky in the United States on Thursday. In an onlooker's photo, the cloud is shaped like a horseshoe magnet.

The photo went viral online after the National Weather Service in Elko, Nevada tweeted it and thanked Chrisy Grimes for taking the photo. As the name suggests, the cloud looked like a horse's shoe set upon a background of other clouds.

People all over Twitter were left shocked with the bizarre occurrence and discussed how it could be caused by some alien involvement. However, the buzz didn't last long as the NWS Elko explained the phenomenon via tweets.

They explained the cloud's formation and said it goes through a process involving the movement of the wind. Essentially, the cloud's formation happens in either a vortex or an area of spinning air – something that when happening in a vertical vortex leads to the formation of a cyclone, or a tornado.

When the said vortex is horizontal, it leads to the formation of a sort of crescent cloud, according to the Cloud Appreciation Society.

Many urban legends about the cloud claim to bring luck to people that see it. Some people consider the cloud's downward facing position – the one that was seen in the Nevada sky – will bring luck to people down below, as per The Society.

Some Twitter users even begged to differ from the majority that claimed the cloud's shape was that of a horseshoe. These people believed it looked more like a mustache floating in the middle of the sky.

Another felt it looked more like a staple for papers than a horseshoe or a mustache.

User Mika McKinnon even contributed to the on-going debate with her own two cents, using a little drawing to explain the cloud as a "weak & sideways cousin of a waterspout or tornado".

Be it a stapler, or a mustache, or even a horseshoe - the cloud has definitely sparked the curiosity of many – just like the 'rainbow cloud' over Rio not so long ago.