Image of the day

Captured by
Normand Rivard

Totality!

My Account

New to Astromart?

Register an account...

Need Help?

Where did the noctilucent clouds go?

Started by Inge_S, 07/31/2002 05:28AM
Posted 07/31/2002 05:28AM Opening Post
Here, in the sub-arctic zone, the stars are beginning to re-appear. We have a 6 - 7 long period between sunset and sunrise, but unfortunately the sun does not dive far enough below the horizon to give conditions for deep-sky observation. The northern sky glows brightly, adorned by Capella which always keeps well off the horizon.

At this time of the year, we often see noctilucent clouds in the north from my location. Capella is a useful guide to these clouds, they have about the same distance from the northern horizon. Often, noctilucent clouds have an appearance almost like sunlit cirrus clouds. However, cirrus clouds always will appear dark when Capella is visible. The glow of noctilucent clouds also is peculiar, with a phosphorescent appearance, but not iridiscent (like mother-of-pearl clouds seen in winter). Sometimes very bright, lens-shaped clouds may appear.

The altitude of noctilucent clouds is several times greater than that of cirrus clouds, and they do not seem to be associated with weather. A few years ago, we could see these clouds almost every clear night, but this year I have not seen a single one.

Do you see such clouds from lower latitudes, or is this a pehnomenon associated with our long periods of twilight conditions?

Inge, 59.5 deg N
Posted 07/31/2002 10:00AM #1
Inge
I first learned about noctilucent clouds when I worked at a weather station more than 30 years ago. I have always looked out for them but I have never seen them from various places I have lived in the Southern Hemisphere from 34*S to 41*S. I have spent many summer holidays climbing and hiking in New Zealand's Southern Alps as far south as 45* and still I have never seen them!

Google will turn up very interesting websites if you type in "noctilucent clouds solar cycle". From the following website:
http://www.hamptonu.edu/science/physics/cas/aim/noctclouds.html
I found an interesting graph showing the relationship between the solar cycle and number of noctilucent clouds sighted. Reported by AIM team => Aeronomy of Ice in the Mesosphere.

And on this website
http://www.irf.se/MRIatmos/projects/nlc/
I found this:
"Noctilucent clouds seem to be increasing in occurrence (Gadsden, Journal of Atmospheric and Terrestrial Physics vol 52, p247, 1990) and some scientists believe this may be due to increased greenhouse gases in the atmosphere. The clouds also show an 11-year variation roughly opposite in phase to the Sun's activity cycle, ie thay are less often seen at solar maximum. It is not yet known how closely PMSE are related to noctilucent clouds (sometimes they have been seen together, sometimes not) , nor whether PMSE also vary with the solar cycle nor whether they are increasing on a longer time scale.

At present we are just past (?) the maximum in the solar cycle and this may be part of the reason why there are fewer clouds this year(?).

Perhaps Astromart readers in the deep south of South America may have seen them during the southern hemisphere's summer solstice (December-January).

Another website with interesting information is:
http://www.nlcnet.co.uk/
There are of course heaps of websites with stunning photographs of noctilucent clouds.

Anthony 41*S 175*E