Have you ever seen those circular, hovering clouds in the sky? They resemble UFOs, but they're actually Lenticular clouds.
"Lenticular clouds are hovering, lens-shaped clouds that are formed in the troposphere, normally perpendicular to the direction of the wind," according to AccuWeather. "When fast-moving air is forced up and over a topographic barrier, waves in the atmosphere develop and that's when the cloud takes an odd shape."
Man-made objects, like buildings and bridges or natural features like valleys and mountains, "can interrupt the flow of the air," AccuWeather says, "crafting the swirls that make them this mysterious formation in the sky."