[BAA 00420] NOCTILUCENT CLOUD SEASON WELL UNDERWAY

[Rick]

BAA electronic bulletin No. 00420 -- http://www.britastro.org/

Beautiful displays of electric blue noctilucent clouds (NLC) have been seen right across Europe for the past two nights - June 16/17 and 17/18, and clearly visible even from the very south of England. Over the past two nights the displays have been extremely conspicuous, with sightings reported from Belgium, Denmark, the Netherlands, France, Germany, Poland and the British Isles. These mysterious clouds are known to flourish during years of solar minimum - and 2009 is such a year.

The BAA's Aurora Section is responsible for handling observations of NLC and observers should consult the Section website at www.britastro.org/aurora for further details.  The following description is taken from this website:

"NLC are seen from Mid-Latitude locations (50-65 degrees) in the twilight arch of long summer nights, from about late May to mid August, with a peak around the first week of July. They are about 80-85km above the Earth's surface and as such the highest clouds ever seen, some ten times higher than cirrus, the highest tropospheric cloud. They are still in sunlight long after sunset. They are often a bluish colour, or intense white, sometimes golden when low near the horizon, and they shine in the sky whereas the lower tropospheric clouds show up dark against them.

The clouds are extremely thin and tenuous, made of tiny ice crystals which brilliantly reflect sunlight. When a sheet of NLC appears edge-on to the observer it appears bright but if NLC is overhead it is usually faint and difficult to distinguish from cirrus or cirrocumulus weakly illuminated by the twilight glow or the moon"

As I write these words at 22.40 UT on 2009 June 17, from my observing site atop the South Downs in West Sussex, NLC are visible below the star Capella, extending from azimuth 330 degrees round to 010 degrees, brightening the sky so it appears that the Sun has only just set.  In fact the Sun is now 14 degrees below the horizon at azimuth 340 degrees.

A band of heavy rain came through about an hour ago, and as the sky cleared to the north the intense blue-white streaks of NLC appeared.  At the moment, although the sky is clear low down in the north there are quite a few bands of dark tropospheric cloud silhouetted against the bright sky behind, which is being lit up by the NLC.  A beautiful sight!

With peak NLC activity generally occurring in early July, observers should be on the look out for further displays of NLC throughout the coming weeks.

Again quoting the BAA's Aurora Section website:

"Data is required on when and where the noctilucent clouds appear, so the more observers there are looking out for NLC, the less likely it will be that occurrences will be missed. It is important, too, to keep a record or negative nights if possible, i.e. clear nights during which, over a complete period of darkness, or near-darkness, no NLC was visible."


Dr John Mason
Press & Publicity Officer
British Astronomical Association

Bulletin transmitted on  Thu Jun 18 00:24:23 BST 2009
(c) 2009 British Astronomical Association

[RobertM]

Hi Rick,

That's a very useful and informative post (not that the others aren't) as I'd like to get to observe some of these elusive clouds.

That's quite an impressively early posting too!

Robert