The excess water vapour puzzle
CIRRUS CLOUDS are wispy sheets of ice crystals six to ten miles above the Earth's surface.
Clouds and particles in the atmosphere play a significant role in regulating the Earth's temperature because they help determine how much of the sun's heat and energy is reflected back into space and they trap outgoing radiation from the Earth's surface.
Regulating distribution
Cirrus clouds are also important in regulating the distribution of water vapour, the most important greenhouse gas, in the upper troposphere. Now, University of Washington scientists are puzzled by the existence of water vapour at concentrations as much as twice what they should be in and around cirrus clouds high in the atmosphere, a finding that could alter some conclusions about climate change.
A group of European and U.S. scientists is advocating a broad research effort to solve the puzzle and understand just what is occurring in cirrus clouds. "Based on our current knowledge, it shouldn't exist," said Marcia Baker, a University of Washington professor of Earth and space sciences. Cirrus clouds form in the upper troposphere and modulate the exchange of water between the troposphere and the stratosphere.
Vapour in the upper troposphere can rise into the stratosphere but tiny ice crystals can fallback toward the surface. Outside the clouds, there are water vapour and minute atmospheric particles called aerosols, but no ice crystals.
The aerosol factor
Scientists have come to expect that new ice crystals will begin to form in aerosols when vapour levels rise to the point at which they are 60 per cent above equilibrium with the surrounding air, according to a University of Washington press release.
Yet measurements have shown that vapour levels can reach 90 per cent to 100 per cent above equilibrium without forming new ice particles. Inside the clouds, it is expected that vapour levels above equilibrium cannot be maintained, yet evidence shows that often vapour levels are as much as 30 per cent above equilibrium in large areas of clouds.
It is possible the aerosols might have as-yet undiscovered properties that prevent crystals from forming , Baker said.
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