FACTBOX: Explanations for UFO sightings

LONDON (Reuters) - Here are 10 possible explanations for UFO sightings.

1) Weather balloons - Gas-filled balloons that measure conditions in the upper atmosphere to help forecasters.

2) Clouds - Saucer-shaped or "lenticular" clouds that form at high altitudes have been confused with UFOs.

3) Meteors - Space debris can create a spectacular lightshow when it burns through the Earth's atmosphere.

4) Civilian or military aircraft - Planes can look mysterious at night or in certain light conditions.

5) Ball or bead lightning - Strange forms of lightning that are rarely seen and are the subject of scientific debate.

6) Stars or planets - Heavenly bodies can be confused with UFOs.

7) Sundogs - Also called "mock suns", these are bright spots on the solar halo that appear as balls of light in the sky.

8) Airships or advertising balloons - These can look like flying saucers from some angles, especially at night.

9) Mirages - A natural phenomenon where cold or hot air bends light rays to produce an image of a distant object in the sky.

10) Magnetic fields - A Ministry of Defence report said people exposed to certain electrical or magnetic fields can suffer from "vivid, but mainly incorrect" recurring memories.

The full MoD report of possible scientific explanations for UFO is online at www.mod.uk/DefenceInternet/FreedomOfInformation/PublicationScheme/SearchPublicationScheme/UapInTheUkAirDefenceRegionExecutiveSummary.htm

(Source: Ministry of Defence; Reuters archives)