NLCs – Summer’s Mysterious Clouds
During the summer months astronomy can be quite a frustrating pastime. Apart from the usual disappointment of cloudy skies that we suffer throughout the year, it is dark for only a few hours each night.

On the edge of space
An interesting alternative to the stars and planets is the search for noctilucent clouds (NLCs), sometimes known as polar mesospheric clouds. The clouds we usually see in the sky reside in the troposphere and therefore do not exceed a height of 8 kilometres. NLCs form in the mesosphere and are typically found at an altitude of 8 to 85 kilometres.

We know that tropospheric clouds are made up of water or ice, or a mixture of both. But little is known about NLCs clouds. The temperature in the mesosphere is around Minus 150 degrees Celsius. That there is sufficient moisture at this height is one of the great mysteries of NLCs. But it is believed that whatever moisture is available at these great heights condenses as ice crystals around dust particles.

   The silvery hue of noctilucent cloud (Ken Osborne)

Uncertain source
The source of the dust is unclear. It may have been transported from lower in the atmosphere or it may be meteoric in origin. Either way, these clouds become visible when the sun has set and lies between 6 and 16 degrees below the horizon. When the sun is higher in the sky these clouds fail to scatter more than one percent of the sunlight that falls upon them, hence they remain invisible.

Appearance
In appearance they appear as white veils, bands, waves or whirls, not altogether dissimilar from daytime cirrus cloud. At twilight the two cloud types may be confused although cirrus is likely to show some reddish or pink coloration, whilst NLCs appear white or blue. Any confusion should clear as the skies darken and the cirrus disappears from view.

            Looking northwards after sunset, noctilucent clouds may appear (James Weightman)

Viewing
Look towards the north after sunset. Between 2200 and 0200 any cirrus cloud should begin to fade leaving behind any noctilucent cloud. But don’t expect instant results. You may go the whole summer without a single viewing. Persistence usually pays off however. So remember to send me any photographs you might take. (Send them as jpegs - landscape format only - to richard.angwin@bbc.co.uk)

Sightings of NLC are of great scientific interest and details of how to record observations can be found at www.nlcnet.co.uk and forwarded to BAA Aurora Section in Edinburgh.