Some of you eagle-eyed skywatchers will have noticed a ghostly glow in the sky on the way back from the pub in recent days. Was it real? Was it just an illusion caused by too much Gin? No, what you saw was real and is called ‘Noctilucence’. It is a spectacular display of very high clouds.
Noctilucence is exhibited by clouds that shine at night (Noctilucent means ‘night shining’ in Latin). Noctilucent Clouds exist in the upper atmosphere (Mesosphere), at a height of around 50 miles. They are composed of ice crystals and can only be seen in astronomical twilight. That means they are best seen in the summer months, but they are too faint to be seen in daylight. They require moisture, dust and very cold temperatures (less than -120 degC) to form. The Mesosphere is at it’s coldest in the summer months, so favours their formation.
The clouds shine on summer nights when the sun is below the horizon at ground level, whilst up at 50 miles the noctilucent clouds are still in the sunlight. The whispy clouds seem to glow in this ghostly manner. They are a comparatively recent discovery and are not fully understood, but they seem to be occurring more often and with increasing brightness.
There have been some fantastic photos posted on the internet in the last week or so, but the best I have seen relevant to us is from Mike Ridley who took this superb composite photo over Durham City from Whinney Hill on 17th/18th June 2019.
Here’s Mike’s link.