Mission to target highest clouds BBC NEWS - By Jonathan Amos, BBC News science reporter, in Baltimore - May 26, 2006 - A Nasa satellite mission will be launched this year to study the highest and most mysterious clouds on Earth. Noctilucent, or "night-shining", clouds appear as thin bands in twilight skies, some 80km (50miles) above the surface. Recent records suggest they have become brighter, more frequent and are being seen at lower latitudes than usual. Scientists cannot say for sure but they suspect human activity may be altering the conditions in the mesosphere that drive the clouds' formation. "Noctilucent clouds were first seen in 1885 by a British amateur astronomer, Robert Leslie," explains James Russell from Hampton University, Virginia, US. "They're very beautiful. They have distinctive features - bands, and ripples we call billows - and form right on the edge of space."... AIM's three instruments will investigate the recipe needed to make the clouds - cold temperatures, the presence of water vapour, and small dust particles around which the water can condense and freeze out to create ice crystals. Scientists think most of the dust comes not from below but from above - from space. It is extremely hard for dust in the lower atmosphere to be pulled so high, while meteoritic dust is known to be settling onto the planet all the time as rocky space debris falls to Earth. If this has stayed reasonably stable over time, then the explanation for the observed changes in the occurrence and properties of the clouds will have to be sought in the satellite's temperature and water/ice data. The mesosphere is already very cold, down to about -125C and more, but researchers say it is getting even colder. Although the extra carbon dioxide (CO2) put into the atmosphere by human activities has warmed the air near the Earth's surface, it is thought to have had the opposite effect in the middle and upper atmosphere by radiating heat more efficiently into space. "In addition to CO2, methane has been increasing in the atmosphere," added Dr Russell. "Once methane makes it into the high atmosphere, the sun breaks down the molecule and forms water - so, that's another source for water vapour in addition to the water vapour coming from below. "These are all likely causes for the changes we are seeing. Our mission will collect the data that can be put into the models to help us get to sound conclusions about what is really going on." ... http://news.bbc.co.uk/2/hi/science/nature/5021778.stm