Twitter was a-buzz yesterday evening (1 Feb 2016) and this morning, as users took snaps of a very rare display of Nacreous Clouds over the UK.
These clouds, also known as Polar Stratospheric Clouds, form high up in our atmosphere (in the stratosphere) between 15 and 25 km in altitude. Normally this part of our atmosphere is very dry but in polar winter temperatures can plunge and conditions can become right for certain clouds to form.
The following pictures were just some of those re-tweeted by @Virtualastro last night; give him a follow to keep up to date.
Nacreous Clouds (named that after the word “nacre”, for mother-of-pearl, due to their iridescent colours) appear when sunlight is scattered through the cloud, and particles within the cloud then produce colours through diffraction processes, making for a beautiful display.
To see a display of Nacreous Clouds you have to head out when the Sun is below the horizon – but only just – during Civil Twilight – when the Sun is between 0 and 6° below the horizon. Look towards the direction of sunset or sunrise (depending on whether you’re out in the evening or morning), you may see these beautiful clouds for yourself.
Find out your civil twilight times at timeanddate.com. For central Scotland (where I am) the best time to see these clouds is between 0730-0810 and 1650-1730. Remember, these clouds are incredibly rare – you can go years without seeing such a display – and there’s no way to predict whether you’ll see them on any given night, but as they happened last night there’s a good chance they might happen again tonight or the following night, 2 or 3 Feb 2016. Get outside and look up!
It’s Noctilucent Cloud season here in the northern hemisphere. These rare clouds – the name means “night-shining” – only appear in the sky at specific times of night and times of year, and the next three months (mid-May till mid-August) are the best time for northern stargazers to spot them (southern stargazers get a chance between mid-November and mid-February).
These high thin clouds are seen in deep twilight, and as the name suggests they look lit up, as indeed they are. When the Sun is between 6 and 16° below the horizon, that is during nautical twilight and much of astronomical twilight, its light can still shine on these high clouds, floating 80km up in the atmosphere. Check out your twilight times and head outside on a clear night to try and spot them.
In addition to these restrictions in time only stargazers between 50° and 65°N can see them. That’s most all of the UK, so we’re ideally placed for hunting NLCs.
No one’s quite sure why they form, and indeed they weren’t seen (or at least weren’t reported) before 1885, and so there is some suggestion that they’re linked with man-made climate change. Whatever the case they are beautiful clouds to see, so take the chance over the next few months.