Friday, 8 June 2007

Noctilucent Clouds

The short summer nights bring few benefits for the amateur astronomer, apart from the warmer temperatures of course.
But one is that this is the best time of year to see Noctilucent Clouds, or NLCs for short. 'Noctilucent' means 'night-shining', and these beautiful high-altitude clouds do indeed 'shine at night', often being at their best around local midnight, which in Ireland, allowing for Summer Time, is usually around 01.20 - 01.40 on your watch. But they can be seen any time from about 00.30 to 02.30, if the sky is dark enough, although very near local midnight the Sun may be just too far below the horizon to illuminate them all fully, especially for those living further South.
They are thought to be caused by ice crystals condensing on meteoric dust, i.e. the very fine dust left behind as meteors burn up on entry high up in the atmosphere, or possibly even just extremely fine particles 'wafting in' from space.
The reason that they can be seen is that they are so high up (about 80-85 km) that the Sun still illuminates them even when it is too far below the local horizon to illuminate ordinary tropospheric clouds. And this is the best time of year to see them because the Sun never dips very far below the N horizon, even at local midnight, giving the best conditions for seeing them. They can only be seen when the Sun is between 6 and 16 degrees below the horizon.
They appear low down near the N horizon, often in the vicinity of Capella, and appear as wispy silvery or sometimes bluish streaks, often parallel to the horizon. Some 'curls' and 'billows' are also occasionally visible. They can be seen anywhere in Ireland or Britain if you have a fairly clear N horizon, but because they occur mainly at latitudes of 60 degrees to 80 degrees, those in the far South don't see them as well or as often.
This year may have greater NLC activity than usual, because they are seen more often around sunspot minimum, so do have a look on clear evenings. They are quite easy to photograph, with exposures of 1" - 4" on 400 ISO film (or 2" to 8" on ISO 200 film, etc); or just experiment with your digital camera and see what you get with each trial. Successive photos over a period of half an hour or so may show changes in structure and motion.
Do not be fooled by ordinary wispy cirrus-type clouds visible late on a summer evening: the sky needs to be dark enough for you to see the first few brightest stars in order for NLCs to be properly visible.

No comments: