We feel prepared to go out on a limb and say none of these images are of flying saucers.But since mankind first gazed up and saw one of these clouds float past, their uniform and tight circular shapes have caused wonder, curiosity - and even fear - in the eyes of their beholder.
The clouds look so artificial that, over the course of history, they may well have prompted early thoughts about visitors from space.
Instead, these are lenticular clouds - smooth, tightly-bordered circular formations that form high in the atmosphere over the tops of mountains, before drifting off to prompt curiosity in those that behold them.
The clouds generally form over mountains - although in rare instances they can be caused by shear winds, and have been spotted over the UK on occasion. The clouds generally form when stable air flows over the top of a mountain. The moisture droplets are pushed up a steep slope, condensing into cloud on their way, and forming in a spiral formation over the top.
For people living near mountains, these sightings are a common occurrence. But when visitors see one - or the cloud forms away from mountains - they are a startling shape. Straight lines rarely form in nature, so to see such a tightly packed shape is a curious event.
Other than UFO-watchers, there are other people who seek these out - glider pilots. Lenticular clouds imply rising air around the bow of the cloud, and gliders can seek these spots and get a free lift to some very high altitudes.This type of gliding, known as 'wave gliding', can lead to some impressive results. Gliders Steve Fossett and Einar Enevoldson set a world record by using a lenticular cloud to get enough lift to get up 15,453 metres over Argentina.
Another glider, Klaus Ohlmann, travelled 3,008 kilometres (1,869 mi) in 2003 thanks to the help of a lenticular cloud in South America.
source: dailymail
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