Observing Noctilucent Clouds
These beautiful clouds are seen from Mid-Latitude locations (50-65 degrees) in the twilight arch of long summer nights, from about late May to mid August, with a peak around the first week of July. They are about 80-85km above the Earth’s surface and as such the highest clouds ever seen, some ten times higher than cirrus, the highest tropospheric cloud. They are still in sunlight long after sunset. They are often a bluish colour, or intense white, sometimes golden when low near the horizon, and they shine in the sky whereas the lower tropospheric clouds show up dark against them.
The clouds are extremely thin and tenuous, made of tiny ice crystals which brilliantly reflect sunlight. When a sheet of NLC appears edge-on to the observer it appears bright but if NLC is overhead it is usually faint and difficult to distinguish from cirrus or cirrocumulus weakly illuminated by the twilight glow or the moon.
The fact that these clouds appear at all is something of a mystery. Evidently water vapour has convected from the lower atmosphere. The air pressure at the height of the Mesopause, about 80 km, is only one ten-thousandth of the pressure at sea level, and the temperature is about -130C.
Data is required on when and where the noctilucent clouds appear. They are seen perhaps 3 or 4 times a year from any one locality but this is mainly because in our Western European climate most places are oftener cloudy than clear. So the more observers there are looking out for NLC, the less likely it will be that occurrences will be missed. It is important, too, to keep a record or negative nights if possible, i.e. clear nights during which, over a complete period of darkness, or near-darkness, no NLC was visible.