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NLC display, June 16/17 evening

Starman

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Starman

Starman

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There was a superb NLC display on the evening of June 16th. Now waiting for the morning of the 17th :)

The top shot shows two planes flying in front of the clouds. It's a high mag shot so there's not much in the way of structure on view. The chimney shot shows a normal cloud in front of the NLC's - the normal cloud appearing dark.

2009-06-16_22h05m_NLC_IMG_8859_1024.jpg

2009-06-16_22h06m_NLC_IMG_8863_1024.jpg

2009-06-16_21h49m_NLC_IMG_8758_1024.jpg

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lukebl

lukebl

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Really sorry if I'm missing something here, but can anyone tell me the difference between NLCs and plain whispy old cirrus?

They all look the same to me in all the photos I've seen. Nice pics, though!

Cheers, Luke

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barkis

barkis

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Really sorry if I'm missing something here, but can anyone tell me the difference between NLCs and plain whispy old cirrus?

They all look the same to me in all the photos I've seen. Nice pics, though!

Cheers, Luke

Hi. Luke.

Here is an exerpt from Wikipedia.

Ron.

Noctilucent clouds, are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator.

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 mi). 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; there is no evidence that they were observed before 1885.

Noctilucent clouds can form only under very restrictive conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. Since their discovery the occurrence of noctilucent clouds has been increasing in frequency, brightness and extent. It is theorised that this increase is connected to climate change.

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Starman

Starman

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NLCs can be identified because they are illuminated when the Sun is between 6 and 16 degrees below the horizon. This is too low for normal tropospheric clouds or aircraft vapour trails which look darker when seen in front of them (the reason for the less than photogenic images is revealed :) ).

They hug the twilight arc and often show the characteristic structure shown in the 'pretty' photo. If the Moon is behind them, its light passes through them without causing much in the way of any brightening of the layer.

The NLC layer is extremely thin and it's not known what really populates it. One model has it being seeded by micro-meteorites (the astro link!) the cores of which then get coated with water ice. The height of the layer can also be used as an indicator of changes in the atmosphere and it's thought that there is a correlation between solar activity and the appearance of NLCs (another astro link!).

Here are some more examples if you're interested...

http://www.digitalsky.org.uk/atmospheric/2008-06-30_02h30m_NLC_IMG_2697.jpg

Notice the foreground clouds in these...

http://www.digitalsky.org.uk/atmospheric/2008-06-30_02h21m_NLC_IMG_2667.jpg

http://www.digitalsky.org.uk/atmospheric/2008-07-04_02h14m_NLC_IMG_2941.jpg

On the edge of the twilight arc...

http://www.digitalsky.org.uk/atmospheric/2008-06-30_NLC-Panorama_IMG_2675_flat2_800h.jpg

http://www.digitalsky.org.uk/atmospheric/2008_07_15_IMG_3144_flat_1600.jpg

From a plane...

http://www.digitalsky.org.uk/atmospheric/2008-07-29_19h47m_UT_IMG_2598a.jpg

Look - no Sun...

http://www.digitalsky.org.uk/atmospheric/2008-06-26_NLC_IMG_2475_1024.jpg

Altitude marked...

http://www.digitalsky.org.uk/atmospheric/2008-07-15_21h56mUT_NLC_IMG_3201_800.jpg

Wavy...

http://www.digitalsky.org.uk/atmospheric/2008-07-04_02h29m_NLC_IMG_2997.jpg

Moving...

http://www.digitalsky.org.uk/atmospheric/IMG_3186-sequence-anim-proc.gif

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lukebl

lukebl

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Thanks for the responses to my seemingly dumb question. It's just that I'd already looked at various reference sites, but still couldn't see why they look any different from thin cirrus.

I fully appreciate that they are at such an altitude that they are illuminated by the sun when it is well below the horizon and no longer illuminating lower clouds, but some of the pics I've seen posted here seem to have only been taken within an hour or so of sunset, when high cirrus is visible too at this time of year.

I guess I'll just have to find some myself and it'll dawn on me! Or maybe I've already been looking at them and dismissed them as cirrus.

L

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Starman

Starman

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It's easy to mistake true cirrus for NLC's and vice versa. I've seen many photos claiming to be be of NLC's which were far too high in the sky and most likely cirrus. Take a pair of bins out and look at them close to the horizon. This may reveal the quite sharp structure that's apparent in the photos.

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markcol

markcol

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Great captures Pete, -judging by your last image it looks like we managed to get the same feature.

(I'm near Freshwater on the Isle of Wight and was taking my shots just before 11 p.m.).

Mark

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Starman

Starman

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They are so high Mark that the same features can be seen over vast distances. If shots were taken at the same time and show background stars, it's possible to triangulate the cloud features and obtain their distance.

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Starman

Starman

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If you Twitter Amanda, I'll be mentioning whether I can see them via that route. My name on Twitter is Avertedvision. If you're not sure if you've got the right thing or not (NLC's not me on Twitter!), a pair of binoculars will normally reveal their true nature. They really do hug the edge of the twilight arc which also helps. I've got a load of other pictures no time to process at the moment :)

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