by Jon Davies a few miles SW of Glasgow, Scotland on 18th July '06.
shown with permission.
clouds are Earth's highest at 50-53
miles just below the mesopause,
the coldest part of the atmosphere. They are visible in summer at moderate
to high latitudes when the mesopause is at its coldest.
Their very small (1/10,000 mm) ice
crystals scatter the sun's rays still shining at their
altitude long after sunset on the ground. They appear blue because
some of their light is absorbed by ozone lower
in the stratosphere.
NLCs "might" be becoming more frequent. But a factor might
be that more and more sky watchers are looking out for them! More
on NLCs NLC