SOME years ago, a meteorologist recorded an experience as follows: "In 1954, I was one of a group of young people studying the growth of shower clouds in the long summer days of central Sweden. One evening our routine was broken by a dinner party; it was nearly midnight - but still only dusk - when we strolled back to our quarters. We were startled to see, low in the northern sky, a skein of diaphanous clouds gleaming with a pale silvery light. They were like a worn cirrus, but with less substance and more remote mysterious luminous night clouds. Henceforth our working day extended into the small hours of the next morning as we sought the clouds again and again, sometimes with quite captivating reward."
What he and his friends had seen was noctiluent cloud. It is quite different in appearance and composition from ordinary cloud, being a silvery white in colour with a bluish tinge, and sometimes seen during the summer months gleaming near the northern horizon after sunset.
These are very rare clouds. The very highest clouds that we normally see in the sky are typically six or seven miles above the ground, at the cruising level of our modern aeroplanes, but noctilucent cloud is known to exist at an altitude of more than 50 miles. Their texture is such that they are not seen during day - because the sun just shines right through them. They are so thin that they are visible only at twilight, when the sun is between six and 16 degrees below the horizon. In such circumstances, the clouds are still sunlit because of their great height, while the atmosphere below them - and any ground based observers - are shrouded in darkness. They can be distinguished from the familiar high cirrus clouds by the fact that they are not tinted by the usual red glow of twilight; they also stand out brightly against the afterglow, in contrast to clouds at lower levels which, being in shadow, appear dark in colour.
Noctilucent clouds were spotted for the first time only a little more than 100 years ago in 1885, and it is a matter of some controversy as to whether their late discovery was because nobody had ever noticed them before, or simply because they were not there to see. Proponents of former theory argue that in 1885 the sky was being very closely watched around the world because of spectacular sunsets associated with the recent eruption of Krakatoa in the East Indies; this may have led to the discovery of something theretofore unnoticed.
Others maintain, however, that noctilucent cloud is a latter day phenomenon. The formation of these clouds depends on the presence of significant amounts of water vapour at high levels in the atmosphere. Water vapour at these heights is known to be a byproduct of the breakdown of methane, and levels of methane in the atmosphere - one of the infamous greenhouse gases - have been steadily increasing since the advent of the industrial era. Some scientists believe, therefore, that the increasing abundance of methane may well be the reason why noctilucent clouds appear to be more common nowadays.