Claire Stewart, from Kincorth, Aberdeen, saw the "flying saucers" from her back garden at 9.30am.
She told news.stv.tv: "It looked like a UFO and I had to double take before I realised it was actually a cloud. I've never seen anything like it before ... It's very weird."
Other residents took to Twitter about their sightings, with one writing: "Beam me up, Scotty."
STV weather watcher Sean Batty said the cloud formations are actually quite common. He explained: "In any country that has decent sized mountains there will also be occasions when these lovely clouds are seen. Sometimes mistaken to be UFOs, these clouds are called altocumulus lenticularis, and are formed by air flowing over the mountains."
He added: "As the moist flow of air pushed in from the Atlantic this would have given a little rain on the western slopes of the Highlands before the air dried out as it moved east.
"During this drying process, the cloud would have broken up and the air would have started to become turbulent over the mountains, causing a bouncing effect, and in turn was able to create these cloud over eastern areas in the drier air."
These lenticular clouds form when stable moist air flows over a range of mountains, and on Monday, the air was flowing from the Atlantic into the North Sea, over the Cairngorms.
A Met Office spokeswoman confirmed to the Daily Mail that these spaceship clouds were likely to have been formed over the Cairngorms.
She told the paper: "These photos show an excellent example of a cloud called Altocumuls Lenticularis.
"Lenticular clouds can form at heights between 6,500 and 16,500 ft, and are called lenticular because of their lens shape."
She added that these clouds are the most common explanation for UFO sightings across the world.
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