Mysterious clouds that 'shine' so brightly they can be seen at night have finally been explained by scientists.
The blue-white clouds, known as noctilucent clouds, glow even after sunset because they form at such unusual heights - around 50 miles above sea level.
At that height the clouds can reflect the sun's light even after sunset.
But the rare sight has baffled people since they were first observed by astronomers more than a century ago, because according to simple models clouds should not be able to form that high in the atmosphere.
The first description of the phenomena was made after the massive eruption of the Krakatoa volcano in 1883, leading some to think there was a link between eruptions and the clouds.
But now scientists think they have another explanation: meteors.
According to a team at the University of Hampton, in Virginia USA, smoke from meteors burning up in the atmosphere has been detected in the clouds.
"This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form," said James Russell, a scientist at Hampton University.
The dust from burnt-up meteors attach themselves to water molecules in the region that the clouds form.
By merging with the dust, the crystal are able to form at the edge of space where air pressure is close to a vacuum.
The particles are also smaller than those lower in the atmosphere, and scatter more short wavelengths of light - which is why the light they reflect is blue.
About 3% of the ice crystals in the clouds originated from a meteor, the team said.