English: Polar mesospheric clouds (PMC) are featured in this image photographed by an Expedition 40 crew member on the International Space Station when the space station was flying over southern Ukraine. Wispy and hard to see, PMC form far above the usual level for clouds, 80-100 kilometers above the ground, so that crews flying at approximately 350 kilometers are ideally placed to observe them. PMC usually only appear near the poles, but during the past few decades they have appeared as far south as Colorado and Virginia in the USA. They also appear brighter and thicker. Scientists think that these changes are related to long-term climate change, perhaps from increasing water vapor content in the upper atmosphere.