Noctilucent clouds

Night-shining skies
by Philip Eden

This summer has provided us with a number of excellent displays of noctilucent clouds. The word "noctilucent" is from the Latin, meaning "shining at night", and sadly there is so much light pollution around these days that these clouds are not seen to their best advantage in urban areas. But if you live in the country the displays, though fairly short-lived, are often very beautiful - ethereal, even.

These clouds are best observed between one and two hours after sunset, preferably when there is little or no other cloud in the sky, and also one to two hours before sunrise although, for obvious reasons, fewer people are around to see them at that hour. At these times, any normal clouds are in the Earth's shadow and appear dark against the twilit sky. Because noctilucent clouds are so high in the atmosphere, they are still catching the rays of the setting sun, and although they are extremely tenuous, they shine with a bluish or silverish colour. They are most likely to be seen towards the northern horizon, although a really good display may extend more than half way to the zenith. The very delicate shield of cloud often displays exceedingly fine ripples, and at other times faint striations.

The origin of these clouds is still a matter of some speculation, but they have been estimated, by reference to the distance the sun is below the horizon, to be between 45 to 90 km above the ground. This is above even the stratosphere, in the upper part of the mesosphere. At that altitude, the wind blows from the east or northeast at 100 to 300 miles per hour, and the clouds can sometimes be seen moving in a southwesterly direction although the apparent motion is barely perceptible because of the clouds' distance above the Earth's surface.

Speculation on the internet is that very fine volcanic dust from an eruption in Russia earlier in the year may be responsible for this summer's displays. However, volcanic debris only occasionally reaches the lower part of the stratosphere, and is extremely unlikely to be found at altitudes above 45km. Furthermore, past studies have found no correlation between the frequency of noctilucent clouds and the frequency of eruptions. The most likely cause is cosmic dust, concentrated near the boundary of the mesosphere and the thermosphere (known as the mesopause) - the wind and temperature structure of this part of the high atmosphere will tend to gather any material in a relatively narrow zone immediately below the mesopause.