On Sat, 11 Dec 2004 19:01:04 -0500, "Morgans"
wrote in ::

I was unable to find where the link talked about that...


Absence of contrails increases diurnal temperature range

Clouds formed by the water vapor in the exhaust from jet planes have a
small but significant effect on daily temperatures, a new study
confirms. The grounding of commercial flights for three days after
last September's terrorist attacks in the United States gave David
Travis at the University of Wisconsin-Whitewater and colleagues a
chance they never thought they'd have: to study the true impact that
contrails from jet engines have on our climate1

Despite a wealth of experiments, it had been virtually impossible to
gauge the effect of contrails because air traffic, particularly over
regions such as Europe and North America, never stopped. Until 11
September 2001, that is. Contrails left high in the atmosphere spread
out into cirrus-like clouds under the right atmospheric conditions.
Natural cirrus clouds - thin layers of wispy water vapor that often
resemble fish scales - trap heat being reflected from the ground and,
to a lesser extent, reflect some of the Sun's rays.

Travis's team compared the average daily high and low temperatures
over North America from 11 to 14 September 2001, with climatic records
from 1977 to 2000, matching the weather over those three days with
similar weather in September from historical records.
They found that the difference between daily high and nightly low
temperatures in the absence of contrails was more than 1 oC greater
than in the presence of contrails. Comparing the three-day grounding
period with the three days immediately before and after, the impact
was even larger - about 1.8 oC.

The researchers suggest that in regions with crowded skies, contrails
work just like artificial cirrus clouds, preventing days from getting
too hot by reflecting the Sun's rays, and keeping nights warmer by
trapping the Earth's heat. Averaged over the globe, which is largely
free of air traffic, the effect is negligible. "But locally, contrails
are equally as significant as greenhouse gases," says Carleton.

The discovery is important, "especially when you consider that air
traffic is expected to increase at about five per cent a year". But
making use of the information by incorporating it into climate models,
for example, will be difficult. Little is known about what conditions
lead to contrail formation, how long they last, and whether they
affect more than just temperature.

References

1Travis, D. J., Carleton, A. M & Lauritsen, R. G. Contrails reduce
daily temperature range. Nature, 418, 601, (2002).