These photographs of polar mesospheric clouds (PMCs) were taken by astronauts as the International Space Station flew over southern Ukraine. Wispy and hard to see, PMCs form 80 to 100 kilometers (50 to 60 miles) above the ground—far above the usual level for clouds—so crews flying at 350 kilometers (220 miles) altitude are ideally placed to observe them.
PMCs usually only appear near the poles, but during the past few decades they have appeared as far south as the American states of Colorado and Virginia. The clouds also have become brighter and thicker. Scientists think the changes are related to long-term climate change, perhaps from increasing water vapor content in the upper atmosphere.
The sunset shot (top) shows the PMCs only as a thin bright line, far above the lowest weather layer of the atmosphere (the troposphere). Looking away from the setting sun in the second image, the PMCs make a spectacular display. They are extensive enough to give a sense of disappearing into the distance, and thick enough to reveal fine details. Some astronauts have called PMCs “one of the most beautiful things you can see from orbit.”
Astronaut photographs ISS040-E-87315 and ISS040-E-87351 was acquired on August 1, 2014, with a Nikon D3S digital camera using an 85 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by the Expedition 40 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Jacobs at NASA-JSC.