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When it comes to talking about the universe, I’m not usually at a loss for words. But every so
often, I see something so awe-inspiring and unusual that words don’t do it justice. And
it seems that when I posted an image of this last year, it looks like I’m not alone here.
Such is the case with noctilucent clouds, a strange and beautiful phenomenon that occurs
high in the Earth’s mesosphere and requires a unique set of conditions to form. In fact,
these cloud formations are so captivating that they’ve inspired communities of watchers,
who share atmospheric data online in the hope of seeing one for themselves.
So, what are noctilucent clouds? What gives them their ghostly luminescence? And what surprising
insights can they offer, not just about Earth’s atmosphere, but perhaps the future of our planet?
I’m Alex McColgan, and you are watching Astrum. Join me today as we view spectacular images of
these eerily beautiful cloud formations, learn how they form, and understand why
they are giving scientists a surprising window into Earth’s changing atmosphere.
Imagine a clear summer night. The Sun has dipped below the horizon, and Capella is shining brightly
to the North. Suddenly, a bright streak erupts low in the northern sky. It might look like shining
silver thread, or maybe an icy blue whirl. Slowly, the streaks get brighter and clearer,
until finally, the whole sky glistens with a patchwork of eerily shining clouds. Night shining
clouds such as this formation photographed over the Baltic Sea are truly spectacular,
and no, they don’t actually emit light. They get their glowing appearance by reflecting
solar radiation during astronomical twilight, when the sun is between 6 and 16 degrees below
Earth’s horizon and remnant light is scattered in the upper atmosphere. Because noctilucent
clouds form at very high altitudes (around 76 to 85 kilometres above the terrestrial surface), they
can reflect the Sun long after an observer has fallen into Earth’s shadow. So, while noctilucent
clouds also form during daylight hours, they are only visible to the unaided eye at night.
To make a noctilucent cloud, you need water, dust particles and incredibly cold temperatures.
Unlike normal clouds, which form in the Earth’s troposphere, where 75% of the atmosphere’s mass
and 99% of its water vapour occurs, noctilucent clouds form high in the upper mesosphere,
which is the atmosphere’s third layer, above the stratosphere.
Just a reminder, the lowest layer of atmosphere on the Earth is the troposphere, the boundary of
which is the tropopause, then the stratosphere, stratopause, mesosphere, mesopause, thermosphere,
thermopause, and then finally the exosphere. For more about all these layers, check out this video.
The mesosphere is a tricky region to study, since its too high for aircraft to fly and
too low for orbital spacecraft due to atmospheric drag. Noctilucent clouds form slightly below the
mesopause, which is the upper boundary between the mesosphere and thermosphere. The mesopause is also
the coldest region of Earth, with temperatures that can plummet below 100 degrees Celsius. (By
comparison, the coldest temperature ever recorded in Antarctica was minus 89.6 degrees Celsius!)
These frigid conditions produce tiny ice crystals less than 100 nanometers in diameter,
which then gather on dust particles. Researchers have learned that the mesosphere must reach
minus 120 degrees Celsius for these ice crystals to form. That’s pretty cold!
But if noctilucent clouds require such cold conditions, why do they occur during summer?
Well, one unusual property of the mesopause is that it is colder in summer than in winter.
Known as the mesopause anomaly, this happens because hot air in the lower troposphere expands,
resulting in upswelling gasses that decompress in the mesosphere, causing adiabatic cooling.
Because noctilucent clouds require very low temperatures, they only appear for 60 to 80
days out of the year, peaking around 20 days after the solstice. If you want to see a night
shining cloud formation, you’ll need a lookout spot between ±50 and ±70 degrees latitude.
Although satellites have spotted many mesospheric clouds north of the 70th parallel, here on Earth,
the polar regions have too much ambient light during summer for optimal viewing. So, if you
happen to live in northern Norway or Alaska, you’ll have to travel south for your NLC watching!
That’s a pretty good survey of what we know, so let’s talk about what we don’t.
Saying the mesosphere is a very dry place would be a bit of an understatement. In fact, it is
one million times dryer than air from the Sahara Desert. Not exactly a place you’d expect to find
clouds! We currently think upswelling air from the troposphere is the likely source of this moisture.
More puzzling, however, is the question of where the dust is coming from. One theory is that it
comes from space in the form of debris. The Earth gets bombarded daily with thousands of meteorites
and other space debris. Could they leave enough dust to create such massive cloud formations?
Or could there be other causes? Interestingly, NLCs were first sighted in 1895, two years after
Krakatoa’s massive volcanic eruption in Indonesia which spewed debris into the upper atmosphere.
It has to be something big like this because otherwise the atmospheric layers don’t mix
very much, and so dust wouldn’t usually make it to the mesosphere. We also know that manmade sources,
such as exhaust from space shuttles, can sometimes trigger noctilucent clouds,
such as a 2014 incident when the SpaceX Falcon 9 caused noctilucent clouds over Orlando, Florida.
In 2009, the United States Naval Research Laboratory successfully created an artificial
NLC using exhaust particles from a suborbital sounding rocket. And in 2018, the University of
Alaska created a noctilucent cloud by releasing water from a suborbital rocket. But without
such proximate causes, the source isn’t easy to pinpoint. Luckily, NASA has launched a satellite,
known as the Aeronomy of Ice in the Mesosphere, or AIM, to answer these and other pressing questions.
It was first launched in 2007 but as of 2022 is still operational.
AIM is powered by two solar panels and is equipped with three payload instruments.
The Cosmic Dust Experiment, which emits pulses to measure speeding dust particles. The Cloud
Imaging and Particle Size, has four cameras that image mesospheric clouds from variable
angles to create a detailed 2D panoramic view. And the Solar Occultation for Ice Experiment,
which measures particles, temperature and atmospheric gasses in order to identify chemicals
and conditions for NLC formation. There have also been several experiments to synchronize AIM’s
observations with those of low-flying aircraft, the first of which was conducted in July 2009.
By synchronizing data between satellites and aircraft, scientists can construct far more
detailed models of mesospheric cloud formation and their features than they could from AIM alone.
For NASA and other atmospheric researchers, answering these questions is no idle matter.
The mesosphere is a remarkably sensitive indicator of changes that are happening elsewhere in the
atmosphere. The same features that make it so unusual, its rarified gasses and sensitivity
to changes far down in the troposphere, make it a useful canary in the coal mine, so to speak.
Decades of NLC study have made it clear that they are becoming more frequent, and climate
researchers are now beginning to link changes in NLC distribution to global climate change.
As the troposphere gets hotter, the mesosphere, vis a vis the mesopause anomaly, may well be
getting cooler. And improving our understanding of the mesosphere has other far-reaching implications
as well. It is a vital part of the atmosphere for reentering spacecraft, and because the mesosphere
is so dry and low-density, studying it could even help us learn about atmospheres on other planets,
such as Mars. By deepening our knowledge of the mesosphere, researchers hope not only to
understand the properties of NLCs but also Earth’s atmosphere as a whole and how it is changing over
time. So, there we have it. Almost everything you would want to know about noctilucent clouds, and
what these eerily beautiful formations can tell us about the inner workings of our atmosphere. I hope
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