An epic display of glowing clouds occurred overnight on 17-18th June 2019, which was reported widely across Europe. The clouds were bright enough for casual members of the public to notice and wonder what they were. A scientific explanation is below.
Noctilucent clouds exist at around 83 km above the Earth in the mesosphere which is the third layer above the stratosphere and troposphere. Typical clouds only reach up to the tropopause where the temperature increases with height and the stratosphere begins, at around 12 km. Noctilucent is Latin for ‘night glowing’ because they appear to be illuminated at night. This is because they are so high that the sun is still able to shine on them overnight at the peak of summer where the sun doesn’t go far below the horizon. They are also called polar mesospheric clouds because the geometry of the sun at the poles make them much more common there. They consist of ice crystals which grow on meteorite dust and have become increasingly common since being first spotted (and written about) in the 1800’s. It is suggested that global warming has a role to play here, not just that more humans exist to observe them. This could take a few mechanisms but one is that storms have more energy in a warm world, allowing their updrafts to push through the barrier of the tropopause (overshooting tops) which pushes moisture into the stratosphere and possibly large scale atmospheric circulation changes in the upper layers which we still have a lot left to understand.