Night-Shining Clouds

Joseph A. Shaw

At the junction between day and night lies twilight—one of the most optically rich and aesthetically pleasing times to observe the sky. In deep twilight, long after the sun has set or before it has risen, a fortunate observer can be treated to a startling visual display when clouds appear suddenly, shining in the dark.


figureNoctilucent clouds observed in the north-northeast sky before sunrise, July 16, 2009, 4:40 a.m. MDT (1040 UTC, 10.4° solar depression angle) from east of Bozeman, Mont., U.S.A. (45.5° N, 111.0° W). Photo information: Nikon D300 at ISO 400, Nikkor 20-mm lens at f/2.8 for 1/2 s.

Noctilucent, or night-shining, clouds are bluish-white, wispy clouds that are seen only at night, particularly during deep twilight. Although noctilucent clouds (NLCs) are often described as appearing after sunset, they can also show up well before sunrise. For example, the photo on the right shows NLCs photographed an hour and a half before sunrise, at 4:40 a.m. Mountain Daylight Time (MDT) (1040 UTC) on July 16, 2009, east of Bozeman, Mont., U.S.A. (45.65° N, 111.01° W), with the sun 10.4° below the horizon.

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