Noctilucent Clouds Over Sweden
November 01, 2002
The above panorama, taken on August 4, 2002 over southern Sweden, shows noctilucent clouds hovering over the darkened horizon. At the time the photo was taken, the Sun was approximately 14 degrees below the horizon (-14°). Nighttime or noctilucent clouds occur at an extremely high altitude (about 85 km), and, as their name implies, appear to glow in the night sky. They form in the cold, summer polar mesopause and are likely composed of ice crystals. Noctilucent clouds look similar to cirrus clouds, but optically, are much thinner. In fact they're optically so thin that they can only be seen at twilight time, when the Sun no longer shines on the Earth's surface or the lower troposphere. These conditions are typically met when the Sun is about 5-15 degrees below the horizon. On the scene above, both the Moon and the silvery noctilucent clouds are illuminating the lake in the foreground. The panorama was taken with a digital camera, C-4040 - ISO 400 and 8 second exposure.