Recent climate assessments have stressed the importance of anthropogenic changes on direct radiative forcing (e.g. Intergovernmental Panel on Climate Change (IPCC) 1999; Royal Commission on Environmental Pollution 2002). One specific area of interest currently is the effect of aircraft-induced cirrus cloud, referred to as contrails. The assumption that there may be a significant effect stems from the fact that cirrus cloud changes both the planet's albedo and its infrared emissivity, and so has a mod ulating effect on climate (Changnon 1981; Murcray 1970), and also that air traffic is increasing greatly year-on-year (IPCC 1999). Contrails are visible line clouds that form behind aircraft that are flying in sufficiently cold air. Contrail formation is caused by the increase in relative humidity that occurs in the engine exhaust plume as a result of mixing warm and moist exhaust gases with cold and less humid ambient air (Appleman 1953; Scorer 1955). Several recent studies reported formation and visib ility as predicted by thermodynamic theory for a variety of aircraft and ambient conditions (Busen and Schumann 1995; Schumann 1996; Jensen ef al. 1998). Contrails can spread out into cirrus-like clouds under the right atmospheric conditions and may incr ease total cloud and cirrus cloud amounts, changing the earth's radiation balance. The structure and evolution of contrails vary considerably with atmospheric conditions. Figures 1-3 show some examples.
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