Explanatory remarks

(Section 2.6.3.3)

Noctilucent clouds occur near the altitude of the mesopause. These “night-shining” clouds are only visible in the twilight or night-time sky for a few months in summertime. They are only visible to a ground observer at middle to high latitudes, approximately 50°−65° north and south. They are rarely seen from latitudes less than 45°, and the summer daylight or bright twilight prevents observation at latitudes greater than 75°. In the northern hemisphere, the observing season is typically mid-May through to mid-August in the northern hemisphere, and mid-November through to mid-February in the southern hemisphere. The incidence of noctilucent clouds tends to peak just prior to and for a few weeks after the summer solstice in each hemisphere.

It is thought that noctilucent clouds are the “ragged edge” of a much more pervasive layer of polar mesospheric clouds. Although noctilucent clouds cannot be observed from the Earth’s surface at polar latitudes, polar mesospheric clouds can be observed from satellites and manned spacecraft in orbit around the Earth. They are observed edge-on against the dark sky background, with their brightness and frequency increasing with latitude towards the poles.

Noctilucent clouds are seen against the background of a dark sky when the Sun is well below the horizon, but while these high-altitude clouds are still illuminated by sunlight. They become visible at about the same time as the first magnitude stars appear. The observer should have an unobstructed view of the horizon towards the twilight sector of the sky. Generally noctilucent clouds will be seen close to the horizon, typically extending up to about 15°−20° above the horizon along the twilight arch. They can extend to higher elevations, sometimes to beyond the zenith, especially at the beginning of displays and near thr end. Around local midnight, when the cloud is poorly illuminated, noctilucent clouds will diminish in brightness and recede closer to the northern horizon in the northern hemisphere, and the southern horizon in the southern hemisphere.