Polar mesospheric clouds can be observed from both the Earth’s
surface and in orbit by astronauts aboard the International
Space Station (ISS). The clouds are also called noctilucent, or
“night-shining,” clouds because they are usually seen at
twilight. When the Sun sets below the horizon and the Earth’s
surface gets dark, these clouds are still briefly illuminated by
sunlight. Occasionally the space station’s high-altitude orbital
track becomes nearly parallel to the Earth’s day/night
terminator for a time, allowing polar mesospheric clouds to be
visible to the crew at times other than the usual twilight. This
unusual astronaut photograph shows polar mesospheric clouds
illuminated by the rising, rather than setting Sun. Low clouds
on the horizon appear yellow and orange, while higher clouds and
aerosols (particles like dust and pollution) are illuminated a
brilliant white. Polar mesospheric clouds appear as light blue
ribbons extending across the top of the image. These clouds
typically occur at high latitud