Observations made using NASA's Aeronomy of Ice in the Mesosphere (AIM) spacecraft have revealed that this year, the noctilucent cloud
season in the Antarctic began earlier than usual - an observation that some scientists believe is a sign of the impact the inexorable build-up of greenhouse gases is having on Earth's climate.
Around the summer solstice is usually the best time for observing noctilucent clouds
but they can often be seen all through the summer months.
and --, 2003b: Atomic oxygen depletion in the vicinity of noctilucent clouds
Professor Gary Thomas, from the University of Colorado in Denver, USA, who studies noctilucent clouds
, said, 'Extreme cold is required to form ice in a dry environment like the mesosphere.
The commonly held view is that when this vapour reaches the freezing mesopause region, ice crystals form and turn into noctilucent clouds
Greenhouse gases will enhance noctilucent clouds
in two ways.
The rare noctilucent clouds
are formed from tiny ice crystals and are known for their thin streaky appearance, usually bluish or silvery in colour.
Jay Brausch is a dedicated amateur observer of the aurora borealis and noctilucent clouds
residing at Glen Ullin, North Dakota, USA, 46[degrees]48'N, 101[degrees]46'W.
Radar has been used to examine icy dust that hovers some 90 km above the Earth and can grow to become the ice particles inside noctilucent clouds
, according to Scott Bailey of Virginia Tech in Blacksburg.
YOUR correspondent suggests (Daily Post, July 20) that noctilucent clouds
(NLCs) over N Wales are a sign of 'man-made' global warming.
form at altitudes as high as 50 miles and are made from ice crystals that glow when sunlight shines through them.
have puzzled and awed cloud watchers since they were discovered a century ago.