They are illuminated by sunlight when the lower layers of the atmosphere are in the darkness of Earth’s shadow.
This astronaut photograph of polar mesospheric clouds was acquired at an altitude of just over 200 miles (about 321 kilometers) in the pre-dawn hours of July 22, 2008, as the International Space Station was passing over western Mongolia in central Asia.
The dark horizon of the Earth appears at the bottom of the image, with some layers of the lower atmosphere already illuminated by the rising Sun.
The higher, bluish-colored clouds look much like wispy cirrus clouds, which can be found as high as 60,000 feet (18 kilometers) in the atmosphere.
However, noctilucent clouds, as seen here, are observed in the mesosphere at altitudes of 47 to 53 miles (about 76 to 85 kilometers) at the edge of outer space.
