Noctilucent Clouds and How To See Them
By Mary McIntyre FRAS
June and July are difficult months for astronomers in the UK because have so few hours of darkness. Does this mean less sleep deprivation? Not a chance, because it’s Noctilucent Cloud season! Noctilucent clouds (NLCs) are one of the highlights of the summer calendar. The 2018 NLC season was one of the best I’ve ever experienced, and the 2020 season was also very memorable because not only did I see a display where NLCs were visible up to the zenith, but also I had a display alongside Comet NEOWISE; I won’t forget either of those in a hurry!
What are NLCs?
Noctilucent clouds (NLCs) are also known as “night shining clouds” and if you’ve ever observed them you will know they certainly live up to their name. Visible during deep twilight, they have a beautiful blueish-white ethereal glow. They are very different from the tropospheric clouds that we see during the day. Primarily, it’s due to their altitude. Cumulus clouds are some of the lowest clouds we see and they reside around 2km above sea level. Cirrus clouds, the thin clouds associated with ice halos, sundogs and other atmospheric optical effects, are the highest tropospheric clouds we see (excluding towering thunder clouds) and they reside at an attitude of around 6 km.
Where are They?
To find NLCs, we need to go much higher. The boundary of the troposphere is 10km above sea level. Above that we have the stratosphere; a layer that extends to an altitude of 50km, and it’s unique to Earth. This is where UV radiation from the Sun breaks down oxygen into ozone to form the ozone layer. It’s also the home of a kind of polar stratospheric clouds called nacreous clouds, but they are rarely seen from the UK. Above the stratosphere, we have the mesosphere and that extends to 85km; above that is the thermosphere. The boundary between the mesosphere and the thermosphere is where you’ll find NLCs. Aurora occurs just 15km above this, so NLCs really are on the boundary of space! NLCs are also seen in the tenuous atmosphere on Mars, where they reach a whopping altitude of 100km. This makes them the highest clouds recorded on any planet in the solar system.
Formation and Visibility
NLCs are polar mesospheric clouds, so they cluster around the Polar Regions. They are made of water ice, but here lies another huge difference between NLCs and tropospheric clouds; that is the size of the ice crystals. Cirrus cloud ice crystals that cause halos have a diameter of around 100 microns (0.1mm), so they’re pretty small. However, NLC ice crystals have a diameter of just 0.1 microns (0.0001mm) so they’re absolutely minute! This is why NLCs are not visible during the day. The lower levels of the atmosphere need to be in shadow and NLCs still illuminated from below by the Sun in order for them to be seen.
For NLC ice crystals to form, a source of water is needed. The mesosphere is very dry so NLCs may not form every day. The mesosphere must also be below -123 degrees Celsius for ice crystal formation. The temperature of the mesosphere is colder when the troposphere is warmer, so NLCs can only form during the summer months. For us in the northern hemisphere, that tends to be from mid-May through to mid-August with the peak activity in June and July. In the southern hemisphere, NLC season is mid-November to mid-February.
The ice crystals also need a nuclei to trigger crystal formation. NLCs had never been reported prior to 1885. Historically, excellent meteorological observations had been reported so it’s unlikely they would have been missed. Two years prior to the first reported display there had been a major eruption on Mount Krakatoa so it was thought volcanic ash particles were the catalyst. It’s now thought micrometeorite dust is responsible.
Link to Global Warming and Solar
At one time NLC displays were rare, but they are now observed much more frequently. One theory is that climate change may be responsible. Human activity has pushed up the emissions of greenhouse gasses and that has lead to an increase in temperature within the troposphere. As stated above, an increase in temperature at that level causes a decrease in temperature up in the mesosphere. It has also been observed that NLC displays are brighter and more numerous when the Sun is at the minimum phase of the 11 year solar cycle. When the Sun is more active there is more UV radiation hitting the atmosphere. As well as breaking down oxygen into ozone, UV will also break down water molecules in the higher levels. Because the mesosphere is already quite dry and tenuous to begin with, UV radiation will remove all of the water and therefore no ice crystals can form. Solar scientists believe we are currently in a grand minimum so this may be another reason why we are seeing them more frequently.
How to see them
NLCs may be visible when the Sun is between 6 and 16 degrees below the horizon, because this is when the lower levels of the atmosphere are in shadow. NLCs are so high that they remain illuminated by the Sun so they appear to glow against the twilight sky. Northern Hemisphere NLCs are visible from between around 45 and 60 degrees latitude. Any further north the Sun doesn’t get low enough below the horizon for the lower atmospheric levels to be in shadow. Because they’re polar mesospheric clouds, they cluster around the poles. During the northern hemisphere NLC season, they cluster around the north polar regions. This means they are usually very low to the northern horizon from mid-latitudes, so only the largest accumulations of clouds will be visible from further south.
NLCs become visible around 60 – 90 minutes after sunset and between 60 – 90 minutes before sunrise, but near to the solstice they may remain visible all night long. After sunset they tend to be visible towards the north west and before sunrise more towards the north east. The amount of NLC visible in each display van vary enormously! Sometimes there is just a hint of white glow low in the north, other times they can stretch out over a hundred degrees across the northern horizon and reach all the way up to the zenith. The two panoramic images below show how different they can appear along the horizon.
They are usually silvery blue in colour (although they have been reported to appear as red and green) and can take on a huge variety of structures and patterns. Sometimes they are little more than a white glow, other times they have intricate structures that look like reflected ripples of water. They behave very differently from tropospheric clouds and they are absolutely stunning through binoculars. They are one of my favourite things to observe and they make a stunning timelapse subject.
There is still so much we don’t know about NLCs so they are very difficult to forecast. As they accumulate and rotate around the pole, they form a characteristic shape called the ~NLC Daisy” because it can resemble petals on a flower. There is a satellite that observes and photographs this accumulation each day and if you visit the Space Weather website - spaceweather.com – you can find the most recent images on their front page. This gets updated daily and if you see any accumulation pointing towards the UK, then you’re in with a chance of seeing them. Remember, the accumulation may not look like it’s anywhere near us, but these clouds are so high that they will be visible from the UK. The photo below shows the daisy on 22nd June 2020, the morning that I saw NLCs reaching all the way to the zenith from Oxfordshire. (Source: spaceweather.com)
There are several webcams across Europe and the UK that are pointing north (links are below). If NLC is visible from a location that is an hour ahead of us, then there is a chance that we may see them from the UK a bit later. I find myself glued to these cameras during NLC season!
There is also a great community of NLC spotters on Twitter. If you follow @NLCalerts and @NLCnet and keep an eye on the #NLCnow hashtag, you will see observers reporting when displays are happening. If you see NLC yourself, don’t forget to report it to these outlets with your location to help spread the word of an active display. Also, don't forget to report your observations to the BAA Aurora/NLC section!
NLCs may be visible at very antisocial hours, but I think they are truly one of the most beautiful sights to behold and definitely worth staying up late or getting up early for!
To see more of my NLC photos, check out my NLC Flickr album: https://flic.kr/s/aHsmNjiAGt
To see my NLC timelapse video playlist on You Tube: https://youtube.com/playlist?list=PLE_LPip90NvY7PGahGXpQ5SxwJOiu_DmA
NB: I originally wrote this in-depth article for my blog page 7 years ago. Since then I’ve updated it every year and added new images. This year I’ve re-written the information completely and broken it down into subheadings so hopefully that makes it easier to read.
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