Fragile, ethereal and mysterious, they're being seen more in the last couple of decades. Could these wraiths be a portent of doom?
|Image of Noctilucent clouds over Uppsala, Sweden: Wikipedia|
Noctilucent clouds are thin, filament-shaped clouds that form in deep twilight. They look very beautiful, light against a deep blue background. But, yes, it looks like they have a dark side.
They form in the coldest parts of the atmosphere, at an altitude of about 83,000 metres called the mesopause where the temperature is around -140°C.
They're normally restricted to a circular area around the North pole, specifically between 55 and 65 degrees. What's more, they tend to happen only in Summer. They have been seen in the south, but very rarely.
They were first reported in the late 1880s. Probably because this was soon after the eruption of Krakatoa, it was initially thought they were associated with volcanic activity, or with dust injected into the upper atmosphere by meteors.
But there are a lot about noctilucent clouds we don't understand. Scientists are mostly curious about why they are increasing.
Certainly, dust trails left by burning meteors might have something to do with their formation, but we've always had meteors. Why the required conditions for noctilucent clouds are becoming more common is more interesting question.
There are two current hypotheses, and both are associated with climate change.
The first is that increasing greenhouse gases allow high frequency light to the ground but don't allow the reflected low frequency light out again. This insulating layer not only means the surface is warmer, it also means the higher atmosphere gets colder, which aids the clouds' formation.
The second hypothesis is that their increased formation is associated with more methane in the upper atmosphere. This methane is from more intensive farming.
Either way, not great.
Further reading here.
They're also reported (for the Northern Hemisphere, at least) in spaceweather.com