Most of the pictures are from Indonesia but there have been similar sighting in Moscow, Romania, Florida and elsewhere in 2009.
Possible theories circulating include aliens, secret military experiments or natural consequences of climate change.
In the film ‘Independence Day’, extraterrestrial spaceships hid behind odd round clouds. UFO expert and author Hartwig Hausdorf said: “This film idea is based on reports that we have from an uncountable number of eyewitnesses.”
Weather forecaster Karsten Brandt looked at the photos. Her opinion is that they are “not fakes but explainable natural phenomenon”.
In Indonesia a very high thundercloud could have been filmed in the sunlight, almost at the edge of the atmosphere.The Moscow photos looked to him like two layers of clouds lying on top of one another, where the upper cloud was illuminated.
And in Romania he said that the air could have fallen into a circular shape around a middle layer of cloud, like a slow twirl.
And experts warn that competing radio transmissions could be confusing to aliens and are little more than space spam, AFP reported.
The messages – which include images of genitals and appeals for money – may even be deemed as a threat by aliens.
European Space Agency (ESA) astrophysicist Malcolm Fridlund advised caution about drawing attention to ourselves.
“I’m not lying awake at night worrying about the overlords of the galaxy or anything like that,” he told AFP, “but when you don’t know of anything that’s out there, you should maybe be a little careful, you should know something about the (star) system first.”
His comments echo those made at a conference on alien life at the Royal Society in London this week.
“A lot of the stuff is very responsible, but I do wonder about some of the other stuff that’s being transmitted,” Albert Harrison, a professor of social psychology at the University of California at Davis, said at the meeting.
“There’s pictures of celebrities, of two political candidates – one identified as good, the other identified as evil – snack-food commercials, love letters to rock stars and so on.”
He added: “When you start broadcasting and drawing attention to yourself, you have to be very cautious about the image you give. We might appear as a threat to them.
The thirst for contact with alien civilizations has a long history.
The US probes Pioneer 10 and 11, launched in 1972 and 1973, bear plaques of a naked man and woman and symbols seeking to convey the positions of the Earth and the Sun.
Voyager 1 and 2, launched in 1977 and now on the outer fringes of the Solar System, each carry a gold-plated copper phonogram disk with recordings of sounds and images on Earth.
But it will take around 40,000 years for Voyager 1, the most distant man-made object in space, to get anywhere close to a star.
Space enthusiasts have since discovered active SETI, which uses active powerful radio astronomy transmitters to beam out messages to interesting stars or extrasolar planets. The transmitters are operated by space agencies or institutions, which in some cases are paid for the service.
Messages range from the earnest and the philosophical to the cerebral, such as an “Interstellar Rosetta Stone” of symbols that give information about Earth and Homo sapiens.
The missives include the jokey: “Please send money. Any kind of money. Universal money is OK. Alien currency OK. Meteorites are good. Gold, Moon rocks, space junk also good. Send to: Maura, Planet Earth.”
And there is the political: an image of George W. Bush as the personification of evil, juxtaposed against Barack Obama as the embodiment of good, sent out by X-Files actress Gillian Anderson.
Any life forms at Epsilon Eridani and Tau Ceti, meanwhile, will receive recordings of the vaginal contractions of ballerinas with the Boston Ballet, a renegade 1980s art project aimed at giving the galaxy an idea of human conception.
Fridlund said that in the absence of any evidence so far that extraterrestrial life exists of any kind, active SETI may well be a waste of time – but still recommends a cautious approach to dealing with the unknown.
Those who share his concern include the British cosmologist Stephen Hawking, who suggests “we should keep our heads low,” given any possibility of encountering a hostile, technologically superior civilization.”
Here is some video’s that show’s ”UFO clouds”.