Astronaut Photography from ISS: Unique Observations of the Earth
Part 1: Unique images of Earth not available from any other source.
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Noctilucent Clouds:

The sliver of the setting moon and clouds that shine at night—noctilucent clouds—caught the eye of astronaut Ed Lu aboard the International Space Station as he was flying over Mongolia. Noctilucent clouds are very high clouds that look like cirrus clouds, but are much higher (75-90 km above the Earth’s surface) than clouds that we observe every day. They are so high and so optically thin they can only be observed during twilight hours, when the sun is just below the horizon and only shines on the uppermost atmosphere in summer at latitudes above 50deg.

In this image, the troposphere (lower atmosphere where the Earth’s weather occurs) is the dark region at the bottom. The tropopause, the sharp density transition visualized by the orange-blue boundary is at roughly 18 km altitude. Above this boundary (blue) is the stratosphere. The top of the blue area where the noctilucent clouds occur is at 90-100 km altitude.

Some scientists believe the frequency of occurrence of these clouds has increased during the last century and may be related to global warming.

Why is this image unique?: Humans on board the International Space Station have the opportunity to observe multiple twilights per day while looking at the horizon, and a unique ability to document the occurrence of noctilucent clouds. These images are also unique because they capture the entire profile of the atmosphere in detail.

ISS007-E-10974, 27 July 2003, 400 mm lens, taken over Mongolia
The Gateway to Astronaut Photography of Earth,
Scientific References:
Zahn, U. 2003. Are Noctilucent Clouds Truly a “Miner’s Canary” for Global Change? Eos 84(28).