© Popperfoto
This picture, taken by a coastguard through a window in Salem Massachusetts, US, in the summer of 1952,shows four unidentified flying objects as bright lights in the sky.
Sixty years ago, America was shaken by a wave of UFO sightings in Washington, DC. Some appeared over the White House itself: were these extra-terrestrials preparing to land, or did the lights in the sky have something to do with the weather?

The sightings lasted from 12 July to 25 July 1952. Objects were sighted by many observers, including airline pilots, and picked up on military and air traffic control radar. On several occasions F-94 Starfire jets were sent to intercept the intruders, but UFOs vanished when an aircraft came near. The objects were variously described as orange balls of fire, clusters of red or orange lights, or white lights. President Truman took a personal interest and demanded an explanation from the air force.

On 28 July two USAF major generals held a press conference at the Pentagon. They explained that the UFO sightings had been caused by a temperature inversion, where a layer of warm air is trapped under cold air. This can give rise to mirages, both visual and radar. A distant light on the ground can be reflected in the sky, giving the impression of a flying, shimmering object.

This did not convince some people, who scented a cover-up and argued that pilots and radar operators were familiar with temperature inversions. However, when flying saucers subsequently failed to land on the White House lawn, the sightings dropped out of the public awareness and into UFO history.