EO: Night-Shining Clouds are Getting Brighter

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EO: Night-Shining Clouds are Getting Brighter

Post by bystander » Thu Jan 27, 2011 11:16 pm

Night-Shining Clouds are Getting Brighter
NASA Earth Observatory | Holli Riebeek | 2011 Jan 27
After the Sun sets on a summer evening and the sky fades to black, you may be lucky enough to see thin, wavy clouds illuminating the night, such as these seen over Billund, Denmark, on July 15, 2010. Noctilucent or polar mesospheric clouds, form at very high altitudes—between 80 and 85 kilometers (50–53 miles)—which positions them to reflect light long after the Sun has dropped below the horizon. These “night-shining” clouds are rare—rare enough that Matthew DeLand, who has been studying them for 11 years, has only seen them once in person. But the chances of seeing these elusive clouds are increasing.

DeLand, an atmospheric scientist with Science Systems and Applications Inc. and NASA's Goddard Space Flight Center, has found that polar mesospheric clouds are forming more frequently and becoming brighter. He has been observing the clouds in data from Solar Backscatter Ultraviolet instruments that have been flown on satellites since 1978. The graph above shows how the brightness of the clouds has changed in the Northern Hemisphere. For reasons no one fully understands, the brightness wiggles up and down in step with solar activity, with fewer clouds forming when the Sun is most active. The biggest variability is in the far north. Underlying the changes caused by the Sun, however, is a trend toward brighter clouds. The upward trend in brightness, says DeLand, reveals subtle changes in the atmosphere that may be linked to greenhouse gases.

Polar mesospheric clouds are extremely sensitive to changes in atmospheric water vapor and temperature. The clouds form only when temperatures drop below -130 degrees Celsius (-200 Fahrenheit), when the scant amount of water high in the atmosphere freezes into ice clouds. This happens most often in far northern and southern latitudes (above 50 degrees) in the summer when, counter-intuitively, the mesosphere is coldest.

Changes in temperature or humidity in the mesosphere make the clouds brighter and more frequent. Colder temperatures allow more water to freeze, while an increase in water vapor allows more ice clouds to form. Increased water vapor also leads to the formation of larger ice particles that reflect more light.

The fact that polar mesospheric clouds are getting brighter suggests that the mesosphere is getting colder and more humid, says DeLand. Increasing greenhouse gases in the atmosphere could account for both phenomena. In the mesosphere, carbon dioxide radiates heat into space, causing cooling. More methane, on the other hand, puts more water vapor into the atmosphere because sunlight breaks methane into water molecules at high altitudes.

So far, it’s not clear which factor—water vapor or cooling—is causing polar mesospheric clouds to change. It’s likely that both are contributing, says DeLand, but the question is the focus of current research.
Mysterious Night-Shining Clouds Getting Brighter
Our Amazing Planet | 2011 Jan 27
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EO: A Day of Night-Shining Clouds

Post by bystander » Thu Jan 27, 2011 11:58 pm

A Day of Night-Shining Clouds
NASA Earth Observatory | Holli Riebeek | 2011 Jan 27
Scientists have a good reason to track noctilucent or polar mesospheric clouds: they are a pretty good gauge of even the tiniest changes in the atmosphere. These “night-shining clouds,” as they are sometimes called, are thin, wavy ice clouds that form at very high altitudes and reflect sunlight long after the Sun has dropped below the horizon.

To look for changes in the atmosphere, scientist Matthew DeLand (of Science Systems and Applications Inc. and NASA’s Goddard Space Flight Center) has been monitoring polar mesospheric clouds with instruments that were actually designed to study ozone, including the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite. OMI provides more detailed and frequent observations than previous instruments, giving DeLand a way to refine his previous measurements of a long-term trend towards more and brighter noctilucent clouds linked to rising greenhouse gases.

These images show OMI measurements of polar mesospheric clouds on July 10, 2007. The clouds, detectable because they are the only things that reflect light in this part of the atmosphere, are shown in white and pink. The Aura satellite travels in a polar orbit, circling from south to north as the Earth turns beneath it. As a result, the satellite gets several opportunities to image the poles every day. This series of images shows the clouds over six consecutive orbits between 7:16 and 15:52 Universal Time. Throughout the day, a wide area of polar mesospheric clouds developed over northern Greenland and Canada, peaking around 10:30 UTC (the third orbit).

Deland first noticed the trend toward more and brighter clouds in measurements from the Solar Backscatter Ultraviolet (SBUV) instruments, which see 23,000 square kilometers (8,800 square miles) at once in a single box (pixel). By contrast, OMI sees 490 square kilometers (189 square miles) in a pixel, making its measurements 47 times more detailed. Because it naturally takes fewer clouds to fill a single OMI pixel, the measurements will provide a useful tool for understanding why SBUV is seeing more and brighter clouds.

The fact that OMI sees the clouds multiple times throughout the day is also significant because it allows DeLand to see how the clouds change within the day. This information is important because it helps him combine data from many sensors to determine how the clouds are changing from decade to decade. SBUV instruments have flown on seven different satellites over the past 32 years, and they have each passed over the poles at slightly different times of the day. If one of the instruments was consistently measuring the clouds at a time when the clouds naturally peak, it could create a false trend, making it appear that polar mesospheric clouds were getting brighter or more abundant.

With OMI showing how the clouds change throughout the day, DeLand now has an index to help correct the SBUV measurement trends to account for the time of day. The correction allows him to develop a more accurate view of the long-term trend. Even with the corrections, the trend indicates that the atmosphere has been responding to increased greenhouse gases over the past 30 years.
Direct observations of PMC local time variations by Aura OMI - MT DeLand et al Polar mesospheric clouds (PMCs) observed by the Ozone Monitoring Instrument (OMI) on Aura - MT DeLand et al Latitude-dependent long-term variations in polar mesospheric clouds from SBUV version 3 PMC data - MT DeLand et al
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UT: Coming Soon – Night Shining Noctilucent Clouds

Post by bystander » Tue May 31, 2011 3:33 am

Coming Soon – Night Shining Noctilucent Clouds
Universe Today | Adrian West | 2011 May 24
Soon you may see an eerie spectacle on clear summer nights if you are located at latitudes between 50° and 70° north and south of the equator: Noctilucent Clouds.

These ghostly apparitions are a delight to see and are quite rare. It is incredibly difficult to predict exactly when they will appear, but we do know they should begin to appear soon.

The season for Noctilucent Clouds (Noctilucent = Latin for “Night Shining”) starts early June and continues into late July. They are seen just after dusk, or before dawn and an apparition can last around an hour.

These mysterious clouds, with their bizarre tenuous wispy shapes reminiscent of ripples in sand or the changing surface of a pool of water, spread like a glowing web across the northern sky. Colours can range from brilliant whites, with tinges of blue, pink and orange.

Formed by tiny ice crystals, they are the highest clouds in the Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 miles) almost at the edge of space.

They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon, while the lower layers of the atmosphere are in the Earth’s shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon, only being recorded for about 120 years.

Noctilucent clouds can only form under very restrictive conditions, and their occurrence can be used as a guide to changes in the upper atmosphere. Since their relatively recent classification, the occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent.

There is evidence that the relatively recent appearance of noctilucent clouds and their gradual increase, may be linked to climate change. Another recent theory is that some of these bright displays come from particulates and water vapour in the atmosphere left over from Space Shuttle launches.

How can you see them? Over the next couple of months look north during dusk and dawn and try and spot this mysterious and elusive phenomenon. They are best seen when the sun is between 6 and 16 degrees below the horizon, and seem to occur more frequently in the Northern hemisphere than the Southern.

Good luck!
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Re: UT: Coming Soon – Night Shining Noctilucent Clouds

Post by owlice » Tue May 31, 2011 2:02 pm

"Night Shining Noctilucent Clouds"

As opposed to day shining noctilucent clouds?? Give that author a Repeat Redundancy Award and Prize!
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Re: UT: Coming Soon – Night Shining Noctilucent Clouds

Post by bystander » Tue May 31, 2011 2:10 pm

No, as opposed to dull daytime noctilucent clouds.
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AIM: Clouds, Clouds, Burning Bright

Post by bystander » Wed Aug 10, 2011 6:20 am

Clouds, Clouds, Burning Bright
NASA | Aeronomy of Ice in the Mesosphere (AIM) | 2011 Apr 18
High up in the sky near the poles some 50 miles above the ground, silvery blue clouds sometimes appear, shining brightly in the night. First noticed in 1885, these clouds are known as noctilucent, or "night shining," clouds. Their discovery spawned over a century of research into what conditions causes them to form and vary – questions that still tantalize scientists to this day. Since 2007, a NASA mission called Aeronomy of Ice in the Mesosphere (AIM) has shown that the cloud formation is changing year to year, a process they believe is intimately tied to the weather and climate of the whole globe.

"The formation of the clouds requires both water and incredibly low temperatures," says Charles Jackman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., who is NASA's project scientist for AIM. "The temperatures turn out to be one of the prime driving factors for when the clouds appear."

So the appearance of the noctilucent clouds, also known as polar mesospheric clouds or PMCs since they occur in a layer of the atmosphere called the mesosphere, can provide information about the temperature and other characteristics of the atmosphere. This in turn, helps researchers understand more about Earth's low altitude weather systems, and they've discovered that events in one hemisphere can have a sizable effect in another.

Since these mysterious clouds were first spotted, researchers have learned much about them. They light up because they're so high that they reflect sunlight from over the horizon. They are formed of ice water crystals most likely created on meteoric dust. And they are exclusively a summertime phenomenon.

"The question people usually ask is why do clouds which require such cold temperatures form in the summer?" says James Russell, an atmospheric scientist at Hampton University in Hampton, Va., who is the Principal Investigator for AIM. "It's because of the dynamics of the atmosphere. You actually get the coldest temperatures of the year near the poles in summer at that height in the mesosphere."

As summer warmth heats up air near the ground, the air rises. As it rises, it also expands since atmospheric pressure decreases with height. Scientists have long known that such expansion cools things down – just think of how the spray out of an aerosol can feels cold – and this, coupled with dynamics in the atmosphere that drives the cold air even higher, brings temperatures in the mesosphere down past a freezing -210º F (-134 ºC).
Image
Noctilucent clouds streaming across the sky in Utrecht, The Netherlands
on June 16, 2009. (Credit: Robert Wielinga)

In the Northern hemisphere, the mesosphere reaches these temperatures consistently by the middle of May. Since AIM has been collecting data, the onset of the Northern season has never varied by more than a week or so. But the southern hemisphere turns out to be highly variable. Indeed, the 2010 season started nearly a month later than the 2009 season.

Atmospheric scientist Bodil Karlsson, a member of the AIM team, has been analyzing why the start of the southern noctilucent cloud season can vary so dramatically. Karlsson is a researcher at Stockholm University in Sweden, though until recently she worked as a post-doctoral researcher at the University of Colorado. A change in when some pretty clouds show up may not seem like much all by itself, but it's a tool for mapping the goings-on in the atmosphere, says Karlsson.

"Since the clouds are so sensitive to the atmospheric temperatures," says Karlsson. "They can act as a proxy for information about the wind circulation that causes these temperatures. They can tell us that the circulation exists first of all, and tell us something about the strength of the circulation."

She says the onset of the clouds is timed to something called the southern stratospheric vortex – a winter wind pattern that circles above the pole. In 2010, that vortex lingered well into the southern summer season, keeping the lower air cold and interfering with cloud formation. This part of the equation is fairly straightforward and Karlsson has recently submitted a paper on the subject to the Journal of Geophysical Research. But this is not yet the complete answer to what drives the appearance of these brightly lit clouds.

AIM researchers also believe there is a connection between seemingly disparate atmospheric patterns in the north and south. The upwelling of polar air each summer that contributes to noctilucent cloud formation is part of a larger circulation loop that travels between the two poles. So wind activity some 13,000 miles (20,920 km) away in the northern hemisphere appears to be influencing the southern circulation.

The first hints that wind in the north and south poles were coupled came in 2002 and 2003 when researchers noticed that despite a very calm lower weather system near the southern poles in the summer, the higher altitudes showed variability. Something else must be driving that change.

Now, AIM's detailed images of the clouds have enabled researchers to look at even day-to-day variability. They've spotted a 3 to10 day time lag between low-lying weather events in the north – an area that, since it is fairly mountainous, is prone to more complex wind patterns – and weather events in the mesosphere in the south. On the flip side, the lower atmosphere at the southern poles has little variability, and so the upper atmosphere where the clouds form at the northern poles stays fairly constant. Thus, there's a consistent start to the cloud season each year.

"The real importance of all of that," says Hampton's Russell, "is not only that events down where we live can affect the clouds 50 miles (80 km) above, but that the total atmosphere from one pole to the next is rather tightly connected."

Hammering out the exact mechanisms of that connection will, of course, take more analysis. The noctilucent cloud season will also surely be affected by the change in heat output from the sun during the upcoming solar maximum. Researchers hope to use the clouds to understand how the sun's cycle affects the Earth's atmosphere and the interaction between natural- and humankind-caused changes.

"These are the highest clouds in Earth's atmosphere, formed in the coldest place in Earth's atmosphere," says Goddard's Jackman. "Although the clouds occur only in the polar summer, they help us to understand more about the whole globe."

Aim Mission Page

The Mystery and Beauty of Noctilucent Clouds
Discovery News Photos | Mark Thompson | 2011 June 14

Noctilucent Clouds and A Bright Northern Star
Universe Today | Adrian West | 2011 June 30
Image
Noctilucent Cloud with bright star Capella over West Berkshire UK (Credit: Adrian West)

Noctilucent Clouds are finally here! Well, at least they were for me at about 3:00am on the 29th of June.

I have heard that there have been some sightings, but for me, this mornings display heralds the new NLC season – a month later than usual?

Conditions were amazingly warm, and the night was still and magical as I looked northwards from my home in West Berkshire UK. I couldn’t help but notice a burning bright star almost due North and quite low, Capella in the constellation of Auriga! This is when I spotted the first faint wisps of noctilucent cloud.

Capella isn’t always in the North, but it is this time of year and it usually makes a guest appearance during morning noctilucent cloud displays.

Noctilucent clouds are very rare and tenuous clouds on the edge of space and occur at altitudes of around 76 to 85 kilometers (47 to 53 miles).

They are only seen when conditions are just right (still not fully understood) after sunset or before sunrise. They are illuminated by the sun, which is still way below the horizon from the observers location. Due to their very delicate nature, noctilucent clouds can only be seen at these times. More info on what NLC’s are, can be found here.

Will you see any NLC’s?

Stunning Noctilucent Clouds Shine Brightly in the UK
Universe Today | Nancy Atkinson | 2011 July 01
Mysterious “night shining” or Noctilucent Clouds are beautiful to behold, and here are some gorgeous examples what skywatchers in the UK have been experiencing. Stu Atkinson took this stunning panoramic view from Kendal Castle. (Click link for access to a larger version). NLCs are usually seen during the summertime, appearing at sunset. They are thin, wavy ice clouds that form at very high altitudes and reflect sunlight long after the Sun has dropped below the horizon. Scientists don’t know exactly why they form, and they seem to be appearing more and more in recent times. See more from Stu Atkinson at his Cumbrian Sky website.

Science writer Will Gater also had a great view of NLCs this morning. See more of his images and animations at his website post.

APOD: Noctilucent Clouds Over Edmonton (2011 Jul 20)
http://asterisk.apod.com/viewtopic.php?t=24480
http://asterisk.apod.com/viewtopic.php?t=19746
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Season for Mysterious 'Night-Shining' Clouds Is Here

Post by bystander » Wed Aug 10, 2011 6:39 am

Season for Mysterious 'Night-Shining' Clouds Is Here
Our Amazing Planet | Joe Rao | 2011 July 22
Every summer since the late 19th century, Earth's polar skies have lit up with eerie blue-white glowing clouds, slowly twisting and undulating in the twilight sky.

These mystifying clouds are referred to as "night-shining" clouds, or noctilucent clouds.

Such clouds form in an upper layer of the Earth's atmosphere called the mesosphere during the summer and can be seen from the high latitudes on Earth. [See images of these mysterious clouds.]

Volcano drew attention

A series of massive eruptions from the Krakatau volcano (also spelled Krakatoa) in late August 1883 may have serendipitously helped to draw attention to the phenomenon of noctilucent clouds.

Dust and ash injected high into the atmosphere from the Indonesian volcano caused spectacular and colorful sunsets worldwide for several years.

On the evening of June 8, 1885, T. W. Backhouse was admiring one such beautiful sunset from Kissingen, Germany, when he noticed something rather strange: as darkness deepened and the ruddy glows faded, he noticed wispy bluish-white filaments seemingly glowing in the north and northwest sky. At that time, scientists dismissed this effect as some curious manifestation caused by the volcanic ash.

But after a few more years, the ash settled and the vivid sunsets induced by Krakatoa faded.

And yet the noctilucent clouds persisted.

Interestingly, there is some debate that Backhouse possibly was not the first to describe them, since in a report dated from 1854, Thomas Romney Robinson, situated at Armagh, Ireland, communicated his personal observation of the " . . . phosphorescent properties of ordinary clouds." So it might be that Robinson was making a reference to noctilucent clouds 31 years before Backhouse.

What causes them

Noctilucent clouds can form only under very restrictive conditions. They are the highest clouds in our atmosphere, located in that layer known as the mesosphere at altitudes between 47 to 53 miles (76 to 85 kilometers). They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow.

Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to — dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds. Ordinary clouds, which generally appear at altitudes of up to 50,000 feet, get their dust from sources like desert wind storms.

But it's all but impossible to push wind-blown dust all the way up into the mesosphere. So scientists speculate that the dust associated with noctilucent clouds originates from outer space. Every day, our Earth encounters countless millions of meteoroids that have been shed by comets. While some of this material rams into our atmosphere in a flash to produce the effect of a shooting star, other tiny particles remain aloft. As for the source of the water vapor necessary to produce clouds at such extreme altitudes, upwelling winds during the summertime are capable of carrying water droplets from the moist lower atmosphere toward the mesosphere.

That's why noctilucent clouds only appear during the warm summer months. The clouds consist of tiny ice crystals about the size of the particles in cigarette smoke.

How to see them

In order for you to have a good chance to see noctilucent clouds, four criteria must be met:
  1. The sky must be free of tropospheric ("ordinary") clouds.
  2. The region of the atmosphere where they form must be sunlit. This means that the sun must be no more than 16 degrees below the horizon.
  3. The background sky must be adequately dark enough for the clouds to stand out. This final requirement means that the sun must be at least 6 degrees below the horizon, what astronomers refer to as the end of civil twilight.
  4. Your viewing location should be at a latitude north of 45 degrees (about the latitude of Minneapolis, Milan, Italy, and Budapest, Hungary, although as you will soon see, the clouds have been sighted at more southerly latitudes in recent years.

Timetable for viewing noctilucent clouds

In the table (right), we indicate the "observing windows of opportunity" for making a possible sighting of noctilucent clouds for different dates and at different latitudes.



We indicate the number of minutes after local sunset that it becomes dark enough to sight the clouds (first number) and when the sun has dropped to 16-degrees below the horizon (second number), when the clouds are no longer being illuminated by reflected sunlight and hence can no longer be seen. (Note that for 55 and 60 degrees latitude, the words "All Night" are provided in place of the second number. That’s because at these high latitudes, twilight lasts all night long in the summer and the sky never gets completely dark. So from these locations, it is possible to see noctilucent clouds all through the night!)

Example: On Aug. 1 at 45 degrees latitude north (the approximate latitude of Minneapolis-St. Paul or Bangor, Maine), you can look for noctilucent clouds from 33 to 108 minutes after local sunset. Your viewing window of opportunity is 75 minutes.

For latitudes down to 40 degrees north (where the clouds have occasionally been sighted in recent years), try looking from about 30 to 90 minutes after local sunset.

More viewing tips

Interestingly, while reports of noctilucent clouds from Europe and Russia date back to the late 19th century, the first observation from North America did not come until 1933, probably because most were not specifically looking for them, or if they did see them, they didn't realize what they were looking at.

From North American observations of the clouds over the past three-quarters of a century, we have been able to deduce some interesting facts, namely that the earliest and latest sightings were (respectively) Apr. 1 and Sep. 28. Peak activity comes around July 20 — about one month after the summer solstice. Ninety two percent of the displays are observed during the months of June, July and August and 82 percent are observed after the summer solstice. Before the solstice, the clouds tend to be faint and cover small areas of the sky, whereas after the solstice they are usually brighter and more extensive.

In general, it would seem that the best times to look for them are during July and August.

As to what you're looking for: gossamer, electric-blue clouds, resembling luminous tendrils, spreading across the northern to northwestern sky and slowly twisting and rippling in the twilight.

Case for global warming?

Over the last few decades, the occurrence of noctilucent clouds seems to have been increasing in frequency, brightness and extent.

A century ago, for instance, the clouds were confined to latitudes above 50 degrees north; you had to go to places such as the United Kingdom, Scandinavia and Russia to see them. But in recent years, they've been glimpsed as far south as Colorado, Utah and Virginia.

It is theorized that this increase is connected to climate change. Gary Thomas, a professor at the University of Colorado has noted that "extreme cold is required to form ice in a dry environment like the mesosphere." Ironically, global warming helps. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high levels of our atmosphere.

Studies from above

Satellites that have been launched to help study these clouds include Sweden's Odin and NASA's AIM (Aeronomy of Ice in the Mesosphere).

Last September, the United States Naval Research Laboratory (NRL) and the United States Department of Defense Space Test Program (STP) conducted the Charged Aerosol Release Experiment (CARE) using exhaust particles from a Black Brant XII suborbital sounding rocket launched from NASA's Wallops Flight Facility to create an artificial noctilucent cloud.

The rocket's exhaust plume was widely observed and reported from New Jersey to Massachusetts.

Recent evidence indicates that at least some noctilucent clouds result from freezing water exhaust from space shuttles. In fact, the clouds have been observed and photographed by astronauts from orbiting shuttles; from space they are called polar mesospheric clouds.
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UT: A Noctilucent Masterpiece

Post by bystander » Wed Aug 10, 2011 6:47 am

A Noctilucent Masterpiece
Universe Today | Jason Major | 2011 Aug 08
Night-shining “noctilucent” clouds create a magical glow in the night skies over Reykjavíc, Iceland in this beautiful photo by Örvar Atli Þorgeirsson, taken on August 6. In the foreground is “The Sun Voyager” (Sólfar), an iconic steel sculpture located on the city waterfront representing a Viking ship.

Örvar did not set out to photograph this rare atmospheric phenomenon but had instead intended to shoot aurora triggered by recent solar outbursts.

“The forecast on the 6th of August was predicting extreme aurora activity,” Örvar says in his Flickr description. “Even though it was very early August and the night would not get fully dark I went out as the aurora can be seen in deep twilight conditions. I saw the aurora for 1 – 2 minutes that night. I did not get a good picture of it though. Instead we witnessed this even rarer phenomenon called noctilucent clouds.”

Noctilucent clouds are extremely high-level clouds made located in the mesosphere, around 76 to 85 kilometers (47 to 53 miles) high… nearly at the very edge of space. (Most commercial airplanes fly between 6 and 7 miles high.) They are high enough to reflect sunlight coming from beyond the horizon long after night has fallen over the land below. They usually appear as a wispy web of blue, white, purple and orange tendrils stretched across the sky.
  • “These clouds where extremely beautiful to look at and reminded me of the aurora but where much more stationary and had this beautiful blue color.”
    — Örvar Atli Þorgeirsson
Noctilucent clouds are mainly visible at latitudes between 50º – 70º north and south during the months of June and July. This means Reykjavíc, located right in the middle, can get great views. (Of course it helps to have a talented photographer like Örvar to capture them so nicely!)

Oddly enough noctilucent clouds are a relatively recent phenomenon, only having been recorded for about 120 years. They have been connected with space shuttle passages through the upper atmosphere, and it’s even been suggested that they may be associated with the 1908 Tunguska impact.
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Science@NASA: Meteor Smoke Makes Strange Clouds

Post by bystander » Wed Aug 08, 2012 9:42 pm

Meteor Smoke Makes Strange Clouds
NASA AIM | NASA Science News | Dr. Tony Phillips | 2012 Aug 07
Click to play embedded YouTube video.
[img3="This graphic shows how methane, a greenhouse gas, boosts the abundance of water at the top of Earth's atmosphere. This water freezes around "meteor smoke" to form icy noctilucent clouds. Graphic courtesy of Prof. James Russell of Hampton University "]http://science.nasa.gov/media/medialibr ... _strip.jpg[/img3]
Anyone who's ever seen a noctilucent cloud or “NLC” would agree: They look alien. The electric-blue ripples and pale tendrils of NLCs reaching across the night sky resemble something from another world.

Researchers say that's not far off. A key ingredient for the mysterious clouds comes from outer space.

"We've detected bits of 'meteor smoke' imbedded in noctilucent clouds," reports James Russell of Hampton University, principal investigator of NASA's AIM mission to study the phenomenon. "This discovery supports the theory that meteor dust is the nucleating agent around which NLCs form."

Noctilucent clouds are a mystery dating back to the late 19th century. Northern sky watchers first noticed them in 1885 about two years after the eruption of Krakatoa. Ash from the Indonesian volcano caused such splendid sunsets that evening sky watching became a worldwide past time. One observer in particular, a German named T.W. Backhouse who is often credited with the discovery of NLCs, noticed something odd. He stayed outside longer than most people, long enough for the twilight to fully darken, and on some nights he saw wispy filaments glowing electric blue against the black sky. Scientists of the day figured they were some manifestation of volcanic dust.

Eventually Krakatoa’s ash settled and the sunsets faded, but strangely the noctilucent clouds didn’t go away. They’re still present today, stronger than ever. Researchers aren’t sure what role Krakatoa’s ash played in those early sightings. One thing is clear, however: The dust behind the clouds we see now is space dust.

Mark Hervig of the company GATS, Inc, led the team that found the extraterrestrial connection.

"Using AIM's Solar Occultation for Ice Experiment (SOFIE), we found that about 3% of each ice crystal in a noctilucent cloud is meteoritic," says Hervig.

The inner solar system is littered with meteoroids of all shapes and sizes--from asteroid-sized chunks of rock to microscopic specks of dust. Every day Earth scoops up tons of the material, mostly the small stuff. When meteoroids hit our atmosphere and burn up, they leave behind a haze of tiny particles suspended 70 km to 100 km above Earth's surface.

It's no coincidence that NLCs form 83 km high, squarely inside the meteor smoke zone.

Specks of meteor smoke act as gathering points where water molecules can assemble themselves into ice crystals. The process is called "nucleation."

Nucleation happens all the time in the lower atmosphere. In ordinary clouds, airborne specks of dust and even living microbes can serve as nucleation sites. Tiny ice crystals, drops of water, and snowflakes grow around these particles, falling to Earth if and when they become heavy enough.

Nucleating agents are especially important in the ethereal realm of NLCs. The clouds form at the edge of space where the air pressure is little more than vacuum. The odds of two water molecules meeting is slim, and of sticking together slimmer still.

Meteor smoke helps beat the odds. According AIM data, ice crystals can grow around meteoritic dust to sizes ranging from 20 to 70 nanometers. For comparison, cirrus clouds in the lower atmosphere where water is abundant contain crystals 10 to 100 times larger.

The small size of the ice crystals explains the clouds' blue color. Small particles tend to scatter short wavelengths of light (blue) more strongly than long wavelengths (red). So when a beam of sunlight hits an NLC, blue is the color that gets scattered down to Earth.

Meteor smoke explains much about NLCs, but a key mystery remains: Why are the clouds brightening and spreading?

In the 19th century, NLCs were confined to high latitudes—places like Canada and Scandinavia. In recent times, however, they have been spotted as far south as Colorado, Utah and Nebraska. The reason, Russell believes, is climate change. One of the greenhouse gases that has become more abundant in Earth's atmosphere since the 19th century is methane. It comes from landfills, natural gas and petroleum systems, agricultural activities, and coal mining.

It turns out that methane boosts NLCs.

Russell explains: "When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor. This extra water vapor is then available to grow ice crystals for NLCs."

If this idea is correct, noctilucent clouds are a sort of "canary in a coal mine" for one of the most important greenhouse gases.

And that, says Russell, is a great reason to study them. "Noctilucent clouds might look alien, but they're telling us something very important about our own planet."

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