There are two things you usually need for stargazing: dark skies and clear skies. But if you live in the northern hemisphere like me, this isn鈥檛 the time of year for dark skies. And if you live in the north of the UK, as I do, clear skies are never a guarantee. This is why this week I will be talking about a specific type of cloud you can look for during long summer nights.
Noctilucent clouds, also known as polar mesospheric clouds, are a rare kind of cloud that shine a silvery blue colour against the darker night sky. This is where their name, which means 鈥渘ight shining鈥�, comes from.
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This beautiful phenomenon is created by ice crystals that form on dust particles in thin clouds. The particles are thought to come from tiny bits of space dust in the mesosphere, the third highest layer of the atmosphere. The clouds can come in many shapes 鈥� thin strings, thicker bands, waves or whirls. They only form during summer, because this is when the mesosphere is coldest. The fact they sit so high up in the sky, at about from the surface of the planet, means these clouds can be lit by the sun long after it has set.
The best time to see noctilucent clouds is between an hour and a half to 2 hours after sunset, and the closer you live to the pole the better your chances. In the northern hemisphere, they start to be visible around the end of May, and it is possible to see them until the end of August. If you live in the southern hemisphere, they tend to appear between November and February.
You don鈥檛 need any special equipment, just a bit of patience and a clear view of the northern horizon. Noctilucent clouds will appear in the west at first, then move north before eventually being visible in the east, just before sunrise. They move in this way because they are usually forming at the poles.
If you look just after sunset, you might first see a golden streak or two near the horizon before the pale, silvery blue shine of the noctilucent clouds appears higher up in the sky. These wispy, electric blue clouds will move around, creating patterns like the aurora, but at a much slower pace. If you use binoculars, you will see the patterns in more detail.
Like the aurora, there are and apps, such as CloudSpotter, that you can on to increase your chances of spotting these rare clouds. Lots of noctilucent cloud spotters use social media to post when they see them 鈥� there are even X accounts for
If you don鈥檛 see them the first time you look, there will be plenty more opportunities. Events like rocket launches can increase your chances, as they emit dust and gas that ice crystals can then latch onto, and a in 2018 found that increasing greenhouse gases in the atmosphere is also making sightings more common.
Abigail Beall is a features editor at New Scientist and author of The Art of Urban Astronomy. Follow her @abbybeall
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