Night Sky Almanac by Royal Observatory Greenwich

Author:Royal Observatory Greenwich
Language: eng
Format: epub
Publisher: HarperCollins Publishers
Published: 2020-09-04T00:00:00+00:00

Noctilucent clouds

June sees the beginning of the season in which noctilucent clouds (NLC) are seen in the northern hemisphere. On rare occasions they may become visible at the very end of May, and the season is effectively centred on the summer solstice in June, so they may also be seen in early July. Because of the lack of land masses in the southern hemisphere, reports of NLC there are much rarer. They are most often seen from the research stations on the Antarctic Peninsula but have been observed from New Zealand, Tasmania and Patagonia.

Noctilucent clouds are bright clouds seen in the sky during the middle of the night. They tend to be observed between latitudes of 50° to 65° in both the northern and southern hemispheres and appear in the general direction of the relevant pole. Closer to the pole, the sky does not get dark enough for the clouds to be visible. They are often ‘electric-blue’ in colour, although they tend to take on yellowish tints both early in the night and towards dawn. They are the very highest clouds in the atmosphere, occurring at altitudes of about 82 km. This is towards the upper boundary of the atmospheric layer known as the mesosphere, and they are far above ‘normal’ clouds, which occur in the lowermost layer of the atmosphere (the troposphere), and are generally no higher than 20 km (and often much lower). They are visible because they are so high that they remain illuminated by sunlight, even when the Sun is below the observer’s horizon and the ground and lower clouds are in darkness. For the observer it is astronomical twilight (see here).

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