Contrails form when water vapour in the exhaust from jet-engines freezes
high in the troposphere where airliners cruise. Being efficient at scattering
light, contrail ice crystals have a greater 'greenhouse' effect than the
carbon dioxide gas also produced by jet engines, says Myhre.
Measuring contrails' impact is not an exact science. Depending on a plane's
altitude, and the temperature and humidity of the atmosphere, contrails can
vary enormously in their thickness and duration, and therefore in their
reflecting or insulating power. Most last minutes or hours, some last days. During the day, persistent contrails trap slightly more heat than they
reflect back into space - at night they continue to trap heat. In areas with
dense air traffic, such as Europe and North America, contrails could be
warming the atmosphere by up to 0.1ŗC, Minnis and colleagues found in 19992.
Although warming by contrails is minor - about 75 times less than man-made
CO2 - "the future climatic impact of aircraft is very
important", warns Myhre - with a predicted fivefold increase in air
traffic in the next 50 years. Myhre and Stordal combined satellite images with data on the journey length
and fuel consumption of air traffic. Comparing this with models of how
contrails scatter light, they estimated how much heat contrails trap or
Like others before them, the duo found a net warming effect. But taking
previous measurements of the reflecting properties of ice crystals in icy
cirrus clouds into account, they found that when light hits contrails at low
angles - like at dawn and dusk - they in fact reflect light, causing a cooling
effect. The researchers plan to keep watching contrails as jet technology
and flight patterns are set to change. Although cleaner burning,
next-generation jet engines are expected cruise at higher altitudes and
to make more contrails.
Myhre, G. & Stordal, F., On the
tradeoff of the solar and thermal infrared radiative impact of
Research Letters, 28, 3119 -
Minnis, P., Schumann, U., Doelling, D. R,
Gierrens, K. M. & Fahey, D. W., Global distribution of contrail
radiative forcing. Geophysical
Research Letters, 26, 1853 -
© Nature News Service / Macmillan Magazines Ltd 2001
"Jet trails above fueling weather
changes below, researchers say."
By Ronald Kotulak, Tribune
Published August 8, 2002
Fluffy rows of cirrus clouds created by the increasing number of jet contrails
can sometimes fill half the sky and may significantly affect ground
temperatures, according to a report in Thursday's issue of the British science
Using a unique opportunity to study the effect of contrails on temperature
after all commercial planes were grounded for three days after the Sept. 11
terrorist attacks, a team of researchers found that clouds spawned by jets
lower peak daytime temperatures and raise the lowest nighttime readings.
Such temperature changes could have a profound impact,
affecting growing patterns of some major crops and increasing the likelihood
of larger insect populations, according to the researchers.
It may also be the clearest indication that human activity can alter the
"We know for a fact that because of the contrails there are less clear
days in the U.S. than there have ever been," said David Travis, a
University of Wisconsin climatologist who headed the study. "We believe
that human activity has much greater potential to change regional climate than
it does to change global climate."
The range between the daytime high and nighttime low temperature is called the
diurnal temperature, and it has decreased by 3 degrees on average around the
nation, and by 5 degrees in the Midwest, Northeast and Northwest where air
traffic is the heaviest, Travis said.
But whether the contrail effect changes the average 24-hour temperature is
still under study. Based on three decades of records from 4,000 weather
stations, scientists are trying to determine if the cooling during the day or
the heating at night has a greater influence on the average daily temperature.
So far the highs and lows seem to average out.
During the three-day grounding, the diurnal temperature increased by 3 to 5
degrees, the only time it has made such a significant change in the last 30
years, Travis said.
Satellite images of the U.S. showed that the sky was clearer than usual during
the three days.
"During the three-day period when there were no commercial flights and
only a few contrails from military planes, we got this sudden increase in the
temperature range due to the fact that we suddenly had clearer skies across
the country," Travis said.
The diurnal temperature range rose sharply on Sept. 11 and fell again on Sept.
14 when commercial flights resumed, he said.
The diurnal temperature range for Sept. 11-13 for the last 30 years across the
U.S., prior to 2001, was 35 degrees. But for the same period last year after
the terrorist attacks, it rose to 38 degrees.
Cirrus clouds affect temperature by reflecting some sunlight back into space
during the day and by reducing the amount of infrared heat escaping from the
earth at night, Travis explained. Scientists have long suspected that contrail
clouds affect ground temperatures but there was little data comparing
contrail-free skies with today's jet traffic. Satellite images of the Earth
for Sept. 11 to 13 provided the first view of the skies nearly free of
contrails in 50 years.
"This result, if it is corroborated by additional studies, represents the
first large-scale evidence of the effect of contrails on climate," said
Patrick Minnis, a senior research scientist at NASA's Langley Research Center
in Hampton, Va.
Minnis used satellite images to study the individual growth of contrail clouds
during the airport shutdown, a task that was previously impossible because the
numerous contrails that crossed paths blurred the findings.
Studying satellite images of lone contrails from military craft on Sept. 12,
Minnis was able to determine that the cirrus clouds formed from these
contrails lasted an average of six to eight hours.
Six to eight contrails can grow to form a cloud cover the size of
Massachusetts, he said. These clouds now make up 4 to 6 percent of the cloud
cover on any given day, a huge increase from the 1 to 2 percent previously
On a typical day, an estimated 13,000 planes crisscross the nation at
altitudes greater than 25,000 feet, the elevation at which cirrus clouds begin
to form, Minnis said. As a result, contrails are more common between airports
than they are around airports.
Cirrus clouds form in the upper atmosphere when there is sufficient moisture
and temperatures of 30 to 40 degrees below zero.
Contrails are formed from jet engine exhaust when particles from burnt fuel
and water vapor, which is flash-frozen into tiny droplets, serve as nuclei to
which moisture already in the atmosphere can cling and grow into clouds.
Jets frequently fly at altitudes where there is not enough moisture to form
clouds naturally, but there is enough moisture to sustain cloud growth once
the initial cloud is formed from a contrail, Minnis said.
Unlike the warming from greenhouse gases, which are thought to have a global
impact, contrail clouds act regionally over areas with the most flights,
Travis said. How contrail clouds fit into the global warming scenario is
unclear, but their ability to moderate extreme temperatures will complicate
Also complicating the global warming issue is a recent report by the General
Accounting Office estimating that jet engine emissions may account for about 3
percent of greenhouse gases.
"Our winters have not been as dramatically cold in recent years as they
have been historically," Travis said. "Some of what has been
attributed to global warming may be due to contrails that throw down a blanket
of clouds to warm the nights."
Reducing the extremes of day and night temperatures could have a subtle impact
on ecosystems, Travis said. It may, for instance, increase insect populations
that thrive better in moderate temperatures.
Some crops could also be affected. Cranberries and citrus orchards need a
certain number of cold nights and warm days to ripen properly, Travis said.
Travis said he has mixed feelings about contrails. On the one hand they have
been a prime focus of his research interest for the last 10 years. On the
other hand they almost spoiled a recent vacation in the Pacific Northwest.
"There were so many contrails in the sky we weren't having any nice
days," he said. "The contrails were making the days kind of gloomy
and overcast. There aren't as many sunny days as we had when we were
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