{"id":44715,"date":"2023-05-17T14:50:48","date_gmt":"2023-05-17T14:50:48","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=44715"},"modified":"2023-05-30T12:13:45","modified_gmt":"2023-05-30T12:13:45","slug":"noctilucent-clouds-on-the-rise","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2023\/05\/17\/noctilucent-clouds-on-the-rise\/","title":{"rendered":"Noctilucent clouds on the rise"},"content":{"rendered":"\n
\"\"
First identified in 1885, the late-night, high, shimmering clouds are now a spectacle more of us will be lucky enough to see <\/figcaption><\/figure>\n\n

Noctilucent clouds on the rise<\/h2>\n\n

Night-shining clouds have fascinated skywatchers for almost 140 years. Rob Banino <\/strong>finds out what they tell us about our changing atmosphere <\/p>\n\n

The Krakatoa eruption of 1883 was one of the deadliest in recorded history. The volcano, which lies on an island in Indonesia\u2019s Sunda Strait, had been threatening to blow for months. It had been sending plumes of ash and steam into the sky since May of that year, but at 1pm on 26 August the pressure beneath its rocky cones finally became too much. <\/p>\n\n

Four increasingly violent explosions over the next 24 hours all but destroyed the island. They killed over 36,000 people and could be heard 3,500 kilometres away in Australia. Twenty-one cubic kilometres of rock and ash was blasted across 800,000 square kilometres and over 80 kilometres up into the air. <\/p>\n\n

So much ash was released into the atmosphere, the region was plunged into darkness for two and a half days. As the ash diffused and drifted around the world, its chemicals absorbed different wavelengths of light, causing spectacular red and orange sunsets and making the Moon glow blue for months. <\/p>\n\n

There was enough ash lingering in the atmosphere a year later to cause summer temperatures in the Northern Hemisphere to drop by an average of 0.4\u00b0C. <\/p>\n\n

Then, on a clear summer\u2019s night in 1885, amateur astronomers in the German town of Bad Kissingen spotted some new, mysterious-looking clouds. It wasn\u2019t so much that they were thinner and wispier than regular clouds, but they were visible after dark. They actually appeared to be shining. <\/p>\n\n

\"\"
Putting on a show over Bruges, Belgium. Night-shining clouds are an elusive sight in the summer months \u2013 but that\u2019s changing <\/figcaption><\/figure><\/div>\n\n

Those mysterious clouds would become known as noctilucent, or night-shining, clouds (NLCs). They can be seen every year between May and August and have become a favourite target for many observers. And since that first sighting, we\u2019ve gradually learned more about them. <\/p>\n\n

We know NLCs can only be seen from latitudes of 45\u201380\u00b0 both north and south, and they form around 75\u201385 kilometres above Earth\u2019s surface, at the top of the mesosphere, the highest part of our atmosphere. (The thermosphere and exosphere are both higher and officially part of Earth\u2019s atmosphere, but the air density is so low that they\u2019re generally considered to be \u2018space\u2019. The ISS, for example, orbits in the thermosphere and most satellites orbit in the exosphere.) <\/p>\n\n

Ingredients for a cloud <\/strong><\/h4>\n\n

For a long time, much about NLCs remained mysterious, even contradictory. For instance, their altitude: clouds need water vapour to form, yet the mesosphere is drier than the Sahara Desert. Then there\u2019s their occurrence only during warm summer months, when the mesosphere is at its coldest. And finally, they appear to shine brightly, often shimmering in silvery blue or sometimes orangey-red, but are only seen after dark. Nothing about NLCs seemed to make sense. <\/p>\n\n

Yet based on when NLCs were first spotted, there was reason to believe they could be connected to the Krakatoa eruption. <\/p>\n\n

\"\"
Frosted flakes: water crystallising around volcanic dust particles is one way NLCs are thought to form <\/figcaption><\/figure><\/div>\n\n

\u201cFor NLCs to exist, you need three things,\u201d says Dr Gregory Brown, an astronomer at the Royal Observatory Greenwich. \u201cThe temperature must be very cold, about \u2013120\u00b0C. You need a condensation centre \u2013 a particle, basically, something for water or ice to condense around and start the process of creating a cloud. In the case of NLCs, it\u2019s thought these condensation centres are probably meteoric dust, although volcanic dust could potentially do the same thing. So that\u2019s one possible connection to Krakatoa.<\/span><\/p>\n\n

\u201cYou need water, because the mesosphere is incredibly dry \u2013 many times drier than the most arid desert on Earth. And one thing the Krakatoa eruption did was release vast amounts of water vapour into the atmosphere. That might have been the thing that triggered the formation of the first NLCs we observed \u2013 not necessarily the condensation centres created by the volcanic dust, but the steam the eruption pushed up into the mesosphere.\u201d<\/span><\/p>\n\n

For a long time that was the working theory for what was causing NLCs: steam from volcanic eruptions condensing around meteoric dust particles in the freezing temperatures of the mesosphere. But the first NLC mystery we managed to solve was why they only appear after dark during the summer. And it\u2019s because they\u2019re so far above Earth\u2019s surface. <\/p>\n\n

\n\n
\"\"<\/figure><\/div>\n\n

\u201cThe Krakatoa eruption released vast amounts of water vapour into the atmosphere. That might be what triggered the formation of the first NLCs\u201d <\/span><\/em><\/strong><\/p>\u2013 Gregory Brown<\/strong><\/em><\/cite><\/blockquote>\n\n

\n\n

NLCS are too tenuous to be visible during the day, but because they\u2019re so high, ice crystals in NLCs can reflect sunlight once the Sun has dropped below the horizon. Essentially they\u2019re giant, high-altitude reflectors sitting in the path of a light source that\u2019s out of sight to those of us on the ground. <\/p>\n\n

\u201cYou can normally see NLCs about an hour after sunset or an hour before sunrise during the middle of the summer, at astronomical twilight,\u201d says Aidan McGivern, a Met Office meteorologist. \u201cThe Sun is sitting just below the horizon, providing enough light from underneath to illuminate these very thin clouds of ice crystals.\u201d <\/p>\n\n

And they are only visible from late May through to mid-August, in the Northern Hemisphere, because that\u2019s when temperatures in the mesosphere are at their coldest. <\/p>\n\n

A spurt of sightings <\/strong><\/h4>\n\n

Since that initial sighting in 1885, we\u2019ve observed and studied NLCs whenever they appeared. We\u2019ve become more familiar with them, through groundand space-based observations, and begun to gain an understanding of them. But just as we were solving the mysteries of where, when and how they form, there was a sudden change: NLCs started appearing more frequently. <\/p>\n\n

Data gathered over a 40-year period from the mid-1960s to the mid-2000s shows evidence for sightings of NLCs becoming more common. And not just because more people are looking for them. Computer simulations modelling the atmosphere over the Northern Hemisphere from 1871 until 2008 found NLCs became significantly more visible during that period.<\/span><\/p>\n\n

\"\"
NLCs are best seen after sunset and before sunrise, when the clouds of the lower atmosphere are in shadow<\/figcaption><\/figure><\/div>\n\n

\u201cIn the 19th century, NLCs were probably visible only once every several decades,\u201d says Prof Franz-Josef L\u00fcbken at the Leibniz Institute of Atmospheric Physics, who led the 2018 computer modelling study. \u201c[But] people living in the mid-to-high latitudes now have a good chance of seeing NLCs several times each summer.\u201d <\/p>\n\n

There\u2019s been no significant increase in volcanic eruptions or, as far as we\u2019re aware, meteoric activity during that period, so what could explain the increase? One theory was that changes in solar activity might be to blame, but the increasing NLC trend doesn\u2019t correlate with the 11-year solar cycle, so that\u2019s effectively been ruled out. That leaves us with only one other possible explanation: a change in Earth\u2019s atmosphere. The sort of change that can be linked to human activity. <\/p>\n\n

\u201cAn increase in carbon dioxide in the atmosphere leads to higher temperatures in the troposphere [the lowest level of Earth\u2019s atmosphere],\u201d says McGivern. \u201cBut because greenhouse gases like carbon dioxide trap heat closer to the surface, the stratosphere and the mesosphere, further up, cool down. When you have lower temperatures in the mesosphere, you\u2019re more likely to get particles of water vapour condensing into ice crystals to form NLCs.\u201d <\/p>\n\n

But carbon dioxide isn\u2019t the only problem. \u201cWhat\u2019s probably a more likely cause for the increased sightings of NLCs is the increasing amount of methane in the atmosphere,\u201d says McGivern.<\/span><\/p>\n\n

\"\"
ESA\u2019s GOSAT satellite revealed the year on year increase in methane concentrations across the world, a likely factor in more frequent NLC displays <\/figcaption><\/figure><\/div>\n\n

Methane, another greenhouse gas that has a more potent effect than carbon dioxide, reacts with ultraviolet radiation in sunlight to form water vapour. And if you combine more water vapour with a colder mesosphere, chances are you get more NLCs. <\/p>\n\n

\u201cAlthough it\u2019s not clear whether this connection does actually exist, there is some evidence to say NLCs might be one of the more obviously visible signs of climate change,\u201d says Brown. \u201c[NLCs] are not damaging in and of themselves, but they show how the atmosphere has subtly, but very importantly, changed due to human emissions.\u201d <\/p>\n\n

Plenty still to learn <\/strong><\/h4>\n\n

There are still more NLC mysteries to solve. For instance, what role space traffic might play in their variability. A 2005 study showed that water vapour from Space Shuttle launches could increase NLCs at higher latitudes, while a study published in 2022 showed a correlation between rocket launches and the frequency of NLCs appearing at latitudes of 56\u201360\u00b0. Then there\u2019s how gravity (or buoyancy) waves, generated by weather systems moving through the troposphere, might affect NLCs, which NASA plans to launch a mission to study later this year (see Cloud Detectives, below).<\/span><\/p>\n\n

\n\n
\"\"<\/figure><\/div>\n\n

\u201cA more likely cause for the increased sightings of NLCs is the increasing amount of methane in the atmosphere\u201d <\/span><\/em><\/strong><\/p>\u2013 Aidan McGivern<\/strong><\/em><\/cite><\/blockquote>\n\n

\n\n

But perhaps the most puzzling question \u2013 for casual observers, at least \u2013 is why these clouds, which form within Earth\u2019s atmosphere, are seemingly of greater interest to astronomers and the space science community than to meteorologists? Fortunately this mystery has a simple solution: by floating near the edge of space, just shy of the K\u00e1rm\u00e1n Line, NLCs are effectively out of range for meteorologists. <\/p>\n\n

\u201cAlthough they are technically clouds and they form in the same way that clouds in the troposphere form, they don\u2019t particularly play a role in our day-to-day weather,\u201d says McGivern. \u201cYou wouldn\u2019t factor them into our weather models, and they\u2019re so thin and high up that they don\u2019t have much of a consequence for our climate.\u201d <\/p>\n\n

\"\"
Catching a fine, structured NLC display tops the wishlist of many summer skywatchers <\/figcaption><\/figure><\/div>\n\n

While scientific opinion seems to agree that climate change is a likely cause of the increase in NLCs we are seeing, this hasn\u2019t yet been established conclusively. After all, scientists have only known about the phenomenon for a little over a century, and have only been able to study the clouds from space since the 1960s and 1970s. Existing data only covers a relatively short span of time \u2013 too short to draw any concrete conclusions \u2013 yet human activity in the form of the Industrial Revolution, the burning of fossil fuels and its effect on the atmosphere, all seem to tally with the changes we\u2019re seeing. <\/p>\n\n

What\u2019s clear is that NLCs still have much to teach us. And as much as they may have changed since we first spotted them in 1885, NLCs\u2019 contradictory nature has remained constant. They\u2019re still most visible after dark and before dawn; they still require the coldest of temperatures that occur during the hottest months of summer; and they still rely on the presence of water in one of the driest parts of Earth\u2019s atmosphere. And, as Brown points out, \u201cThey\u2019re the one type of cloud that astronomers like to see\u201d. <\/p>\n\n

<\/div>
\n

Cloud detectives: AIM and AWE <\/strong><\/h4>\n\n\n\n

Could a new instrument on the ISS solve the mystery of increasing NLCs? <\/strong><\/p>\n\n\n\n

\"\"
From its space station vantage point, AWE will investigate how gravity waves affect space weather <\/figcaption><\/figure><\/div>\n\n\n\n

Various satellites have observed noctilucent clouds (NLCs) over the years, but the first one dedicated to studying them was NASA\u2019s Aeronomy of Ice in the Mesosphere (AIM). With Prof James Russell of Hampton University in Virginia as its principal investigator, and costing $140m, it launched on 25 April 2007 to begin its two-year mission. <\/p>\n\n\n\n

Almost 16 years later, in March 2023, NASA finally ceased AIM\u2019s operations after power loss in its battery left the satellite unable to receive commands. The data AIM gathered was crucial for our understanding of the causes and formation of NLCs, and featured in 379 peer-reviewed scientific papers. <\/p>\n\n\n\n

AIM\u2019s successor, the Atmospheric Waves Experiment (AWE), is scheduled to launch in December 2023. It will perch on the International Space Station and study gravity (or buoyancy) waves, which transport heat and energy produced by powerful weather disturbances in the troposphere up to the higher regions of Earth\u2019s atmosphere. <\/p>\n\n\n\n

\n\n\n\n

NLCs on Mars <\/strong><\/h4>\n\n\n\n

Our robotic eyes on the Red Planet have shown that Earth isn\u2019t the only world that experiences night-shining clouds <\/p>\n\n\n\n

\"\"
Curiosity captured the unusual clouds over Mont Mercou on 19 March 2021 <\/figcaption><\/figure>\n\n\n\n

Thanks to NASA\u2019s Curiosity rover, we can now say we\u2019ve spotted noctilucent clouds on another planet. While exploring the Martian surface in the first few months of 2021, the rover began imaging wispy clouds that scattered light from the setting Sun and shimmered with colour. <\/p>\n\n\n\n

Analysis of the images determined that these clouds were unlike typical Martian clouds, which are composed of water ice and float no higher than 60km up in the atmosphere. <\/p>\n\n\n\n

These new clouds were sitting at higher altitudes, as their shiny appearance against a dark sky indicated that they were catching light from the setting Sun. Such a height would put the clouds in a very cold part of Mars\u2019s atmosphere where it would be cold enough to freeze carbon dioxide into crystals of dry ice that would reflect sunlight. <\/p>\n<\/div><\/section>\n\n

\"\"

<\/figcaption><\/figure><\/div>\n\n

Rob Banino is the managing editor of BBC Science Focus Magazine<\/em>




<\/p>\n\n

PHOTOS: JUHKU\/ISTOCK\/GETTY IMAGES, KURMYSHOV\/ISTOCK\/GETTY IMAGES, TORONTONIAN\/ALAMY STOCK PHOTO, NASA EXPLORERS PROGRAM, NASA\/JPL-CALTECH\/MSSS, GOSAT, RENE LEHISTE\/ITOCK\/GETTY IMAGES<\/p>\n","protected":false},"excerpt":{"rendered":"

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