Today.Az » Weird / Interesting » Takeoffs and landings cause more precipitation near airports, researchers find
01 July 2011 [13:11] - Today.Az
Researchers have found that areas near commercial airports sometimes experience a small but measurable increase in rain and snow when aircraft take off and land under certain atmospheric conditions.
The new study led by the National Center for Atmospheric Research
(NCAR), is part of ongoing research that focuses on so-called hole punch
and canal clouds that form when planes fly through certain mid-level
clouds, forcing nearby air to rapidly expand and cool. This causes water
droplets to freeze to ice and then turn to snow as they fall toward the
ground, leaving behind odd-shaped gaps in the clouds.
The research team used satellite images and weather forecasting
computer models to examine how often this type of inadvertent cloud
seeding may occur within 62 miles (100 kilometers) of six commercial
airports: London Heathrow, Frankfurt, Charles De Gaulle (Paris),
Seattle-Tacoma, O'Hare (Chicago), and Yellowknife (Northwest
Territories, Canada), as well as Byrd Station in Antarctica. They found
that, depending on the airport and type of plane, the right atmospheric
conditions typically exist up to 6 percent of the time, with somewhat
more frequency in colder climates.
The lead author, NCAR scientist Andrew Heymsfield, says this
phenomenon likely occurs at numerous other airports, especially in mid-
and high-latitude areas during colder months. The key variable is
whether there are cloud layers in the vicinity that contain water
droplets at temperatures far below freezing, which is a common
occurrence.
He adds that more research is needed before scientists can determine
whether the precipitation produced by this effect is significant. The
inadvertent cloud seeding may increase the need to de-ice planes more
often, he adds.
"It appears to be a rather widespread effect for aircraft to
inadvertently cause some measureable amount of rain or snow as they fly
through certain clouds," Heymsfield says. "This is not necessarily
enough precipitation to affect global climate, but it is noticeable
around major airports in the midlatitudes."
The researchers did not estimate the total amount of rain or snow
that would result from such inadvertent cloud seeding. However, they
analyzed radar readings that, in one case, indicated a snowfall rate of
close to an inch an hour after several planes had passed through.
The study is being published this week in the journal Science.
Researchers from NASA Langley Research Center and the University of
Wyoming, Laramie, co-authored the paper. Funding came from the National
Science Foundation, which is NCAR's sponsor, and from NASA.
Solving a cloud mystery
Scientists for decades have speculated about the origins of
mysterious holes and canals in clouds. Heymsfield led a study last year
establishing that the gaps, which sometimes look as though a giant hole
punch was applied to a cloud, are caused when aircraft fly through
midlevel clouds that contain supercooled droplets.
When a turboprop plane flies through such a cloud layer with
temperatures about 5 degrees Fahrenheit or lower (about -15 degrees
Celsius or lower), the tips of its propellers can cause the air to
rapidly expand. As the air expands, it cools and causes the supercooled
droplets to freeze into ice particles that evaporate the droplets and
grow, falling out of the clouds as snow or rain.
Jet aircraft need colder temperatures (below about -4 to -13 degrees
F, or -20 to -25 degrees C) to generate the seeding effect. Air forced
to expand over the wings as the aircraft moves forward cools and freezes
the cloud droplets.
The effect is unrelated to the trails of condensed water vapor known as contrails made by the exhaust of jet engines.
In the new research, the study team used cloud measurements taken by
the NASA CALIPSO satellite to quantify how often such conditions exist
within about 62 miles of several airports located in relatively cloudy
areas. They chose the 62-mile radius because that is approximately the
distance it takes for a commercial aircraft to climb above about 10,000
feet, where many of the supercooled cloud layers are located.
Of the major, mid-latitude airports studied, they found that the
Frankfurt, DeGaulle, and O'Hare airports most frequently experienced the
right conditions for propeller aircraft to generate precipitation. In
each case, the conditions existed more than 5 percent of the time over
the course of a year. The researchers found that the right conditions
existed more than 3 percent of the time for jets at Heathrow, Frankfurt,
and Seattle-Tacoma.
Yellowknife experienced such conditions more often, about 10 percent
of the time for propeller planes and 5 percent for jets, presumably
because of colder cloud conditions at higher latitudes. Byrd often
experienced the very cold conditions that enable jets to cause
inadvertent cloud seeding.
The researchers also found that a diverse range of aircraft can
induce precipitation. By comparing observations of hole-punch and canal
clouds made by a National Oceanic and Atmospheric Administration (NOAA)
satellite with flight path records from the Federal Aviation
Administration, they confirmed that commercial jets (such as Boeing 757s
and the McDonnell Douglas MD-80 series of jets), military aircraft
(B-52s), various regional and private jets, turboprops, and prop/piston
planes all can induce precipitation.
"It appears that virtually any airplane that flies through clouds
containing liquid water at temperatures much below freezing can cause
this effect," Heymsfield says.
Satellite readings analyzed by the team showed that holes and canals
generated by aircraft can occur with some frequency. For example, an
extensive cloud layer over Texas on January 29, 2007, contained 92 such
gaps, some of which persisted for more than four hours and reached
lengths of 60 miles or more.
Heymsfield and his colleagues also used a powerful software tool,
known as the Weather and Research Forecasting model, to learn more about
how the holes form and develop. They found that the hole rapidly
spreads about 30 to 90 minutes after an aircraft passes through. This
would be the peak time for precipitation associated with the
cloud-seeding effect. After about 90 minutes, ice and snow begin to
dissipate.
The University Corporation for Atmospheric Research manages the
National Center for Atmospheric Research under sponsorship by the
National Science Foundation. Any opinions, findings and conclusions, or
recommendations expressed in this publication are those of the author(s)
and do not necessarily reflect the views of the National Science
Foundation. /Science Daily/
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