NWS meteorologist Paul Frisbie told BuzzFeed News the horseshoe cloud forms when a cumulus cloud dissipates in a warm, moist atmosphere with changing wind speed and direction.
"The cloud is in the process of dying and this is like the last stage before it disappears," Frisbie said.
This type of cloud is so rare because they're created under conditions that typically don't occur at the same time. They also don't last very long, Frisbie said.
Christy Grimes, the woman who took the photos, told BuzzFeed News the cloud vanished after about a minute. She spotted it outside her Battle Mountain, Nevada, home while walking to her car on Thursday.
"I just decided to take some pictures to show everyone," she said. "I didn't think they would believe me."
Her sister sent the photo to the NWS because they wanted to know what kind of cloud it was.
"This was the first one I’ve seen like that," Grimes said.