In Cape Town, South Africa this weekend, clouds that looked like UFOs hung low over the city and surrounding areas, appearing as if aliens might land and start abducting residents.
Of course, these clouds weren't real spaceships - they were simply the product of changes in air temperature.
As NASA explains, "lenticular clouds" form when warm air rises while passing over a mountain or other rough topographic feature. This warm rising air quickly cools, chilling the water in the newly formed cloud so it condenses into droplets that we can see. Then, if a strong wind blows in from the side it can give the cloud a layered, stack-of-pancakes-looking shape, creating a lenticular cloud formation.
These clouds look stationary, as seen in this time-lapse GIF below, but that's only because they are continuously forming and disappearing. The air continues on it's upward path, but is no longer white because denser clouds appear gray (usually because of ice crystals). The side wind keeps blowing and forming the disk shapes as more air cools and floats upward.
Here's some more shots of the lenticular clouds above Cape Town:
