By Ramsay McIver

You may have already read comments about this phenomenon, some of you might be wondering “What are they and where and when can they be seen”?

As we approach the end of May, and our evening skies lighten too much for most of us to do serious deep sky astronomy, so begins the Noctilucent Cloud season. Noctilucent Clouds or NLC’s form high up in the mesosphere at an altitude of around 82km.  They are visible in the northern hemisphere at latitudes between 50˚  and 65˚  north, and occur after sunset when the sun is between 6˚  – 16˚  below the horizon and similarly sunrise, that’s roughly 60-90 minutes after sunset and again before sunrise. Last year the first reports of NLC’s in the northern hemisphere were made on the 24th May. At first they appear in the North Western sky and move with the sun as we rotate to the NE. Usually if you can see Capella you are looking in the right direction.

They appear silvery or electric blue in colour and although at first they seem static time-lapse photography will show that they ebb and flow in slow motion across our entire northern sky. According to recent studies these ripples are atmospheric propagation caused by gravity waves and this along with seasonal atmospheric expansion and contraction is what moves these clouds from northern to southern hemispheres. NLC’s were first recorded in 1885 two years after the massive volcanic eruption of Krakatoa in 1883 which is believed to have dramatically increased the amount of dust and moisture in the mesosphere at the time.

The clouds form as microscopic dust particles and ice crystals combine to reflect sunlight. The dust particles come from a variety of sources, meteorite smoke, cosmic dust, volcanic activity, and space rocket fuel adds to the moisture as does certain types of pollutants like methane.

This doesn’t mean to say that between late May and early August they are always present. First the correct conditions must prevail and as usual we are held to ransom by terrestrial clouds which unfortunately always seem to be present.

In 2007 NASA launched a satellite to research and photograph NLC’s from space: the study, called AIM (Aeronomy of Ice in the Mesosphere) was to last two years but in reality ran for significantly longer with the satellite’s batteries finally giving up literally just a few weeks ago. You can learn more about what they discovered here.

So what will the 2023 season bring for NLC’s? NLC’s vary with solar activity and decrease with solar maximums which we approach, but they also increase with volcanic activity. Last year you may recall that there was a huge volcanic eruption in the Pacific when Hunga Tonga literally blew its top, not to mention volcanic activity in Hawaii at Mauna Loa, Iceland and Italy. It is thought that the Tonga eruption alone will increase the amount of moisture and dust in the Mesosphere by a huge 10%. On balance I think there should be more sightings as they have been on the increase for the last decade. Ss for alerts, we will just have to keep looking up and following us as we no longer have AIMS to show us what’s happening.

Visual observation is easiest although observing with binoculars or a small spotting scope will show subtle nuances. If you want to record the clouds they are really very easy to photograph with a mobile phone camera. Last year I took my pictures with a full frame camera at around ISO 6,400 with a 24-70mm lens f 4.0 @ 1/45 sec. Telephotos really take you into the detail of the clouds. They really are quite beautiful and spectacular. This year I’m going to try my luck at timelapse photography.

What do you think you will try?

Noctilucent Clouds from Edinburgh by Ramsay McIver

Image credits: Ramsay McIver 15 July 2021