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 from the International Space Station at an altitude of just over 320 km (200 mi) in the pre-dawn hours. Image courtesy of NASA.