This image provided by NASA Wednesday Aug. 6, 2008 shows Polar Mesospheric Clouds (also known as noctilucent clouds) are transient, upper atmospheric phenomena observed usually in the summer months at high latitudes (greater than 50 degrees) of both the Northern and Southern Hemispheres. They are bright and cloudlike in appearance while in deep twilight. They are illuminated by sunlight when the lower layers of the atmosphere are in the darkness of the Earth's shadow. This image was acquired at an altitude of just over 200 miles 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 below with some layers of the lower atmosphere already illuminated. The higher, bluish-colored clouds look much like wispy cirrus clouds which can be found in the troposphere as high as 60,000 feet. However noctilucent clouds, as seen here, are observed in the mesosphere at altitudes of 250,000 to 280,000 feet.
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