Friday, July 2, 2010

Study Shows Aircraft Cause Precipitation, Hole-Punch Clouds

As turboprop and jet aircraft climb or descend under certain

atmospheric conditions, they can inadvertently seed mid-level clouds and

cause narrow bands of snow or rain to develop and fall to the ground,

new research finds. Through this seeding process, they leave behind

odd-shaped holes or channels in the clouds, which have long fascinated

the public.

The key ingredient for developing these holes in the clouds: water

droplets at subfreezing temperatures, below about 5 degrees Fahrenheit

(-15 degrees Celsius). As air is cooled behind aircraft propellers or

over jet wings, the water droplets freeze and drop toward Earth.

“Any time aircraft fly through these specific conditions, they are

altering the clouds in a way that can result in enhanced precipitation

nearby,” says Andrew Heymsfield, a scientist with the National Center

for Atmospheric Research (NCAR) and lead author of a new study into the

phenomenon. “Just by flying an airplane through these clouds, you could

produce as much precipitation as with seeding materials along the same

path in the cloud.”

Precipitation from planes may be particularly common in regions such as

the Pacific Northwest and western Europe because of the frequent

occurrence of cloud layers with supercooled droplets, Heymsfield says.

The study, which addresses longstanding questions about unusual cloud

formations known as hole-punch and canal clouds, is being published this

month in the Bulletin of the American Meteorological Society. It was

funded by the National Science Foundation, NCAR’s sponsor. In addition

to NCAR, the research team included scientists from Colorado State

University and the University of Wyoming, as well as a retired cloud


-----Punching holes in clouds-----

Across the world, sightings of blue-sky holes piercing a cloud layer

have triggered bemusement and speculation. A front-page feature on

Yahoo! carried the headline “A Halo over Moscow” after photos emerged of

just such a hole in October 2009.

As far back as the 1940s, scientists have wondered about the causes of

these clouds with gaps seemingly made by a giant hole punch. Researchers

have proposed a number of possible aviation-related causes, from

acoustic shock waves produced by jets, to local warming of the air along

a jet’s path, to the formation of ice along jet contrails. Indeed, the

earliest observations implicated jet aircraft, but not propeller

aircraft, as producing the holes.

Researchers in the 1980s observed that propeller aircraft could

transform supercooled droplets into ice crystals, and experiments were

launched in the 1990s to characterize the phenomenon.

But scientists had not previously observed snow as it fell to the ground

as a result of aircraft until Heymsfield and his colleagues happened to

fly through some falling snow west of Denver International Airport with

an array of instruments. While the research team did not notice anything

unusual at the time of their 2007 flight, a subsequent review of data

from a ground-based radar in the area revealed an unusual echo,

indicating that the band of precipitation had evolved quickly and was

unusually shaped.

“It became apparent that the echo had evolved in a unique way, but I had

no satisfactory explanation,” says Patrick Kennedy, a Colorado State

University radar engineer who spotted the unusual readings and helped

write the study.

-----Piecing together clues-----

Heymsfield and Kennedy went back through data from their aircraft’s

forward- and downward-viewing camera. They noticed a hole in an

otherwise-solid deck of altocumulus clouds in the forward imagery, as

well as a burst of snow that extended to the ground.

Since the hole was oriented in the same direction as the standard flight

tracks of commercial aircraft in the region, Heymsfield surmised that a

plane flying through the cloud might have somehow caused ice particles

to form and “snow out” along its path, leaving a canal-shaped hole-punch

cloud behind.

A subsequent review of flight track records from the Federal Aviation

Administration revealed that turboprop planes operated by two airlines

flew close to the hole-punch location, following a standard flight path

that produced the subsequent band of snow. Snow crystals began falling

about five minutes after the second aircraft flew through the cloud. The

snowfall, in a band about 20 miles long and 2.5 miles wide, continued

for about 45 minutes, resulting in about two inches of snow on the ground.

The researchers also examined data from onboard spectrometers that

profiled the snowflakes within the band of snow beneath the hole punch.

These plate-shaped crystals showed evidence of riming (accumulation of

liquid water), whereas ice particles elsewhere in the cloud showed

little or no riming.

“This tells us that the aircraft literally ‘seeded’ the cloud just by

flying through it,” Heymsfield says.

The cloud layers outside Denver contained supercooled droplet--particles

of water that remain liquid even at temperatures as low as -35 degrees

Fahrenheit (about -34 degrees C). When a turboprop plane flies through

such a cloud layer, 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 and fall out of the clouds as snow

or rain.

The research team conducted additional studies into the cooling over the

wings of jet aircraft, thereby accounting for earlier observations of

the impact of jets. 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.

“This apparently happens frequently, embedded in the cloud layers,”

Heymsfield says. “You wouldn’t necessarily see it from satellite or from

the ground. I had no idea this was happening. I was sitting in back of

the plane. And then this data set just fell in our laps. It was a lucky


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 or the Department of Energy.

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