Polar noctilucent clouds. Astronaut photograph of polar noctilucent, or mesospheric, clouds. Noctilucent (night shining) clouds are formed 76 to 85 kilometres above the Earth's surface, near the mesosphere-thermosphere boundary of the atmosphere, a region known as the mesopause. At these altitudes, water vapour can freeze into clouds of ice crystals. When the Sun is below the horizon such that the ground is in darkness, these high clouds may still be illuminated, lending them their ethereal, 'night shining' qualities. Here, the stratosphere can also be seen (pale orange band). Photographed from the International Space Station (ISS) when it was over the Pacific Ocean south of French Polynesia.