bill at wstoddard.com
Fri Dec 19 06:50:56 PST 2003
Posted on Tue, Jul. 23, 2002
9/11 offered rare chance to study contrail-free sky
By Dan McKinney
Special to the Mercury News
Last September, out of tragic circumstances, atmospheric scientists were
given a once-in-a-lifetime opportunity.
For several days after the attacks on the World Trade Center, all
commercial aviation was grounded -- an event unprecedented in the
industry's history. The episode offered scientists a rare glimpse at North
America without contrails -- and a chance to compare those atmospheric
conditions to more routine ones.
In the late afternoon of Sept. 11, the only three contrails visible from
space, according to satellite images, were those coming from Air Force
One and its two fighter jet escorts on their way to Washington. Scientists
closely monitored the atmosphere during that time because it was devoid
of all but a few aircraft.
``The 9/11 study helped because we were able to identify contrails
developing without interference from others,'' said Patrick Minnis, a senior
research scientist at the NASA Langley Research Center in Hampton, Va.
The Sept. 11 aviation shutdown gave scientists some of their first hard
evidence of regional climate shifts. One researcher now believes contrails
contribute to an overall cooling during the day and warming at night -- a
finding that may sound beneficial, but actually points to worrisome
artificial climate change, scientists say.
David Travis, a climatologist at the University of Wisconsin-Whitewater,
presented a paper at a recent American Meteorological Society
conference showing a widening of day-to-night temperature ranges in
the days following Sept. 11. His research includes findings that challenge
prevailing views of contrails and climate.
Without contrails, Travis was able to measure temperatures across the
continent and compare them to similar measurements taken before Sept.
11. He found that the difference between the warmest temperature during
the day and the coldest temperature at night grew 3 degrees on average,
and as much as 5 degrees in parts of the country where contrails were
likely to form.
Net cooling effect
This would mean contrails shield the Earth from the sun during the day,
causing lower temperatures, and warm it at night, trapping the Earth's
heat like a blanket. Because there are generally more flights and more
trails during the day than at night, Travis speculates contrails have a net
cooling effect on the Earth. His conclusion is a radical departure from the
conventional theory that contrails contribute to warming.
``Contrails can cool or warm the Earth. The key is, there are more contrails
at day than at night,'' Travis said. ``Because of that, the daytime effect is
likely to dominate,'' meaning contrails do more cooling than heating.
Travis said his background as a climatologist helped him arrive at that
answer. Most previous knowledge of contrail effects had been gleaned
from complex meteorological theories. But Travis compared hard data from
the Sept. 11 shutdown to previous climate history and based his
conclusion on what the evidence told him.
``Most of what my colleagues have done is theoretical modeling done with
computers,'' he said. ``I'm an empirical researcher looking at data.''
Travis said a major shift in how scientists think about contrails' effect on
climate may be in the making. ``Now we see that a contrail effect can be
measured in the climate record,'' he said. Travis' finding will be published
in an upcoming issue of the journal Nature.
``The big implication is that this is the first direct evidence that the global
warming debate is complicated by jet contrails,'' Travis said.
So if contrails are off-setting warming, is that a good thing? Travis is
quick to point out that contrails only affect regional climates.
``We don't want people to think that it's a great thing that contrails are out
there,'' he said. ``Because any artificial temperature change isn't good for
The plane-free skies after the terrorist attacks also allowed scientists to
study another contrail-related issue, the connections between the vapor
trails and a type of naturally occurring high cloud cover known as cirrus clouds.
Like cirrus clouds
Researchers have found that contrails behave exactly like cirrus clouds.
Cirrus clouds are excellent insulators, and they may contribute
significantly to global warming. Some scientists believe contrails could
encourage their formation, and the terrorist attacks gave researchers a
chance to see how many cirrus clouds would naturally be present without
The mysterious cloud species is to atmospheric scientists what dark
matter is to astrophysicists -- mostly unknown. Wispy thin cirrus clouds
travel at 30,000 to 60,000 feet above ground, and, unlike cumulus clouds,
their lower, puffy, popcorn-shaped cousins, cirrus clouds can be nearly
``The way that the climate community has been trying to think about this
is by separating natural cloudiness from human-induced perturbations,''
said Ronald Cohen, a specialist in atmospheric chemistry at the University
Cohen spent almost 10 years studying the effects of aviation on the
atmosphere. The links between cirrus cloud formation and contrails are
still elusive, he said, but theories addressing aviation's role are being
investigated. A recent NASA study suggests that the small particles found
in airplane emissions could serve as nuclei, attracting water vapor to
themselves and eventually forming artificial cirrus clouds.
A paper Minnis presented in early May at an American Meteorological
Society meeting makes the connection between the high wispy clouds
and contrails more clear. By watching the few, isolated contrails present
during the Sept. 11 aviation shutdown, Minnis was able to precisely
measure their growth as they transformed into cirrus clouds.
``During regular air traffic, we will see linear contrails crossing each other,
often in the midst of thin cirrus clouds,'' he said. ``The thin cirrus clouds
could be the old contrails formed a few hours earlier. Surely, that
happens. We just need to learn how often it happens.''
Other regions that are regularly covered by cirrus clouds were
unobscured in post-Sept. 11 skies, Minnis found. ``Large areas that
normally have cirrus clouds were clear because there were no contrails,''
Contact Dan McKinney at science at sjmercury.co
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