There was great excitement this week when the Ministry of Defence released files on UFO sightings from 1986 to 1992 (The Times, October 20). Among the many reports were stories of phantom objects seen on radar that suddenly disappeared, or strange missile-shaped objects. But there could be explanations for both of these spooky happenings.

Weather radar sometimes picks up echoes from what seem to be small clouds that explode into a doughnut-shaped ring and then vanish. But there is a clue to how these phantom rings appear because they are often seen around sunrise. This is when huge flocks of birds all leave their roost in one big surge, the flock expands as the birds disperse to begin feeding, and so the radar image fades away.

The atmosphere can also do some strange things. Usually the higher the atmosphere, the colder it is. But if layers of warm air sit over cold air a sharp difference in temperature can bend light like a glass prism and create some fantastic mirages. One amazing incident was on March 4, 1937, in the North Pacific when the crew of a ship saw another steamer going the same way upside down. Ten minutes later they saw another horizon, which had appeared above the real one, with a weirdly distorted image of the steamer upside down and now joined to the real ship funnel-to-funnel. A short while later there appeared to be three ships sailing along on top of each other with the middle one inverted. Small wonder that mirages in the sky can create unbelievable sights easily mistaken for UFOs.