It is tempting to believe that Milton was referring to noctilucent clouds when he wrote:
Was I deceived, or did a sable cloud
Turn forth her silver lining on the night?
But it is unlikely to have been so. Noctilucent clouds were spotted for the first time in 1885, more than 200 years after Milton penned these lines. 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.
The highest clouds normally visible are rarely more than five miles above the ground. But noctilucent clouds are an exception; they occur in a narrow zone about 50 miles up, at the very coldest level of the atmosphere at what we call the mesopause, and they are also very rare. Their texture is such that they are not seen during the 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.
Noctilucent clouds generally appear low in the northern sky, and have a silvery white appearance, often with a bluish tinge. They can be distinguished from ordinary high 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 the familiar high cirrus clouds which appear dark. They are a summer phenomenon, seen in the northern hemisphere mainly in July and August, and most frequently between the latitudes of 50 and 65 degrees north.
Some scientists believe that noctilucent cloud was spotted for the first time in 1885 because the sky was being very closely watched around the world that year for spectacular sunsets associated with the recent eruption of Krakatoa in the East Indies; this may have led to the discovery of something until then 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 by-product of the breakdown of methane, and levels of methane in the atmosphere have been steadily increasing since the advent of the industrial era. The increasing abundance of methane may well be the reason why noctilucent clouds appear to be more common nowadays.