Noctilucent Clouds

Have you ever spotted some dark cloud silhouettes on a backdrop of a brightly lit, twilight sky – somewhat resemblant of sand dunes made of electric blue cloud?

credit: Hayden Goodfellow
credit: Hayden Goodfellow

 

Then you’ve witnessed the picturesque phenomenon called Noctilucent Clouds (NLCs) due to their “night shining” – or, as they are sometimes called, Polar Mesospheric Clouds (PMCs) due to their location in the atmosphere. They are located ~83km above the Earth’s surface (just a few km short of the mesopause, the coldest part of the atmosphere), which is why they are so brightly illuminated by the waning Sun as it dips below the horizon and fails to illuminate the regular clouds in the troposphere. Long story cut short in simple terms: these are magnificent clouds of ice, located very, very high up…

NLC diagram
The prime time for NLC appearances here in the northern hemisphere is June to August, as this is when the Sun dips below the horizon at just the right angle, providing light to be scattered off the ice particles.
  1. The mesosphere is indeed a very cold place, providing the temperatures needed for the ice crystals to combat the low pressure of the mesopause in order to form – we’re looking at anything below -123°C! These temperatures occur from May – June.
  2. The water vapour needs some sort of nuclei to settle on (i.e. what scientists call ‘nucleation‘), and dust particles are the prime suspect. In particular, extraterrestrial dust particles – e.g from meteors – have been speculated about, or perhaps volcanic ash dust. This springs back to the first NLCs recorded in June 1885 by T. Backhouse, following the 1883 Krakatoa eruption.
  3. Being ice crystals, they also need a source of water vapour – obviously. That, or another potential source of water is methane, which reacts with chemical elements in the stratosphere to produce water molecules.

The increasing supply of methane in the stratosphere is a major reason for scientists’ suspicion of the evermore frequent NLC phenomena to be indicative of global warming. Indeed, no NLCs were seen (or at least no records have been found) before the late 19th century, before the peak of the industrial revolution.

credit: Hayden Goodfellow
credit: Hayden Goodfellow

Why am I writing about this?

Primarily, because it’s darn interesting. How something so beautiful could indeed be a warning sign of global warming.

Secondly, because a talk on these was given by Ken Kennedy at the CAPS conference I attended, which marked a very interesting part of the conference for me.

Thirdly, because a friend of mine has created a spectacular time-lapse of the NLCs seen on the night of 11/12th July 2014 at St. Andrew’s, Scotland (whose 56° latitude sits snugly in the middle of the perfect viewing range of 50-63°latitude). Have a watch of Hayden’s video here:

and here’s a good place on the net of the web to find out more: http://freespace.virgin.net/eclipsing.binary/whatarenlc.html

or here: http://ed-co.net/nlcnet/

 

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