Noctilucent Clouds and How To See Them
By Mary McIntyre FRAS
www.marymcintyreastronomy.co.uk
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
Activity
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.
Forecasting
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
NLC
webcam networks:
http://ukazy.astro.cz/nlc-
https://www.iap-kborn.de/
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.
Other Sources:
The Cloud Book – How to Understand the Skies by Richard
Hamblyn
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