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.