A number of
researchers in recent
years have reported perplexing findings of water vapor 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. Now a group of
European and "Based on our
current
knowledge, it shouldn't exist," said Marcia Baker, a Part of the problem
is that many
atmospheric scientists have dismissed the findings as erroneous because
the
current understanding of atmospheric conditions and cirrus clouds would
make
the water vapor anomaly impossible, Baker said. Yet a number of pieces
of
evidence published in peer-reviewed journals and presented at
scientific
meetings during the last six years have supported the finding. 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.
Cirrus clouds also are important in regulating the distribution of
water vapor,
the most important greenhouse gas, in the upper troposphere. "We have thought our
models
of the formation and evolution of cirrus clouds are generally adequate
in how
they portray the role of cirrus clouds in regulating water vapor, but
if the
recent findings are accurate and high humidity is are widespread, our
assumptions could need significant adjustment," Baker said. "The point is to
bring this
to the more general science audience as a broad puzzle, but also to lay
the
groundwork for research to solve the puzzle," she said. Cirrus clouds form
in the upper
troposphere and modulate the exchange of water between the troposphere
and the stratosphere.
Vapor in the upper troposphere can rise into the stratosphere but tiny
ice
crystals can fall back toward the surface. Outside the clouds,
there are
water vapor and minute atmospheric particles called aerosols, but no
ice
crystals. Scientists have come to expect that new ice crystals will
begin to
form in aerosols when vapor levels rise to the point at which they are
60
percent above equilibrium with the surrounding air. Yet measurements
have shown
that vapor levels can reach 90 percent to 100 percent above equilibrium
without
forming new ice particles. Inside the clouds,
it is expected
that vapor levels above equilibrium cannot be maintained, yet evidence
shows
that often vapor levels are as much as 30 percent above equilibrium in
large
areas of clouds. Scientists have
speculated about
what causes these anomalies. It is possible the aerosols might have
as-yet
undiscovered properties that prevent crystals from forming in some
conditions,
or there could be some kind of coating on the aerosols that prevents
ice from
forming, Baker said. There also could be some undiscovered property of
ice
crystals that prevents them from growing in certain conditions. "There could be a
different
phase of ice at the temperatures and pressures in cirrus clouds that
has a
higher equilibrium for vapor," Baker said. "These are the kinds of
questions for which we are trying to find answers."
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