Called 'noctilucent' clouds, which literally means 'night-shining' in Latin, they are normally spotted in polar regions during the summer months. 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 an 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.
Apparently 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.