An analysis of data from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite indicates that there is a wet layer about 70 to 80 km up. It is generally believed that water vapour in the atmosphere is trapped below 12 km while the region between 50 and 90 km (the mesosphere) is almost dry. According to Louis Frank at the University of Iowa, USA, small comets that encroach upon the outer reaches of the atmosphere, release water continuously that ends up in the mesosphere (Science , Vol 277, NO 5329).
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