The shimmering clouds form at an altitude of around 55 miles above sea level and are made up of tiny ice droplets. Because they are so high up in the atmosphere the sun is able to illuminate the clouds from below the horizon.Called 'noctilucent' clouds, which literally means 'night-shining' in Latin, they are normally spotted in polar regions during the summer months. But stunned residents spotted a rare glimpse of the clouds lighting up Leicester's skyline shortly after midnight on Thursday morning
Noctilucent cloud formations are the highest on Earth where temperatures can plunge below -130C (-200F) and winds peak at 300mph. They appear in the mesosphere, which is between 30miles and 50miles above the Earth's surface.
Clouds are made up of ice crystals and scientists are baffled as to how these form in a arid layer that is several million times drier than the Sahara Desert. But their prevalence in the summer months might be one clue.
'Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere,' Gary Thomas from the University of Colorado said. The water droplets also need dust particles to stick to to create the ice crystals.
This could explain why the phenomenon was first recorded in 1885, two years after the Krakatoa eruption, when several tonnes of carbon dioxide, ash and dust were emitted into the atmosphere. However scientists do not know why the clouds have become more common since then rather than fading away. They have speculated that debris from space may be the answer.
Professor Nick Mitchell, director at the Centre for Space, Atmospheric and Oceanic Science at the University of Bath, said: 'These are beautiful and extremely special cloud formations that are a wonderful sight in our skyline. (Daily Mail)
