Noctilucent Clouds
Article for May/June 2024
If you missed seeing the Aurora Borealis last month, you couldn’t have missed all the photos! The massive sunspot AR3664 sent clouds of plasma, known as coronal mass ejections, towards earth, causing elevated geomagnetic activity which allowed the Aurora to be visible far further south than usual. At its peak, ghostly curtains of grey light could be seen shifting through a glowing sky. Cameras were able to pick up colour that our eyes could not, leading to gorgeous photos of vivid green and purple hues.
The Aurora over Danehill, UK
The sun’s rotation has now moved sunspot AR3664 out of our view. While there is a chance it may return to bless us with more Aurora sightings later in the year, the season is just starting for another glowing nightly sight.
Noctilucent means ‘night shining’. These clouds form at incredible heights in the atmosphere and only become visible on clear summer evenings after sunset. While we sit in shadow, Noctilucent clouds are high enough to catch the last glimmers of the sun’s light, so that they gleam silver and blue through the dark.
They are the highest clouds that form on Earth, at a height of around 80km or 50 miles (most of our clouds form between 2 km and 18 km), in a layer of atmosphere called the Mesosphere. Noctilucent clouds are about three times as high as the Ozone layer and eight times higher than commercial aeroplanes fly. At this height, it is extremely cold; cold enough for tiny ice crystals to form as water vapour combines around the dust left from meteors that have burnt up in Earth’s atmosphere. In summer, the Mesosphere is especially humid, helping to set up the specific conditions required for these clouds to form.
Noctilucent clouds over Stockholm, Sweden, by Kiril Videlov (https://flic.kr/p/o1Lrum)
Though still considered rare, climate change may be making Noctilucent clouds a more common sight. Methane emissions increase the water vapour in the Mesosphere — studies have shown that it is now 40% higher than it was in the 1800s. As a consequence, there are more than twice as many ice crystals in this layer of the atmosphere, and these crystals are much larger, causing Noctilucent clouds to be far more visible than they ever were in the past. In fact, while it is likely they were always there, the first reports of Noctilucent clouds was as recent as 1885.
Unlike the aurora, the conditions for Noctilucent clouds to form are so specific that they can only be predicted a few hours in advance. So, if you’re planning a warm evening of stargazing this summer, or maybe trying to catch the Northern Lights once more, look to the northwest horizon. If you’re lucky, about 90-120 minutes after sunset, you might see these bright clouds glowing. A beautiful warmup for a night of astronomy.
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