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. 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. Credit: NASA
