Denver International Airport snow can be caused by planes inadvertently seeding clouds?

Why is snow so common at Denver International Airport? A new study led by Andrew Heymsfield, a cloud physicist at Boulder's National Center for Atmospheric Research, and published in the journal Science suggests that planes can inadvertently seed clouds under the right conditions, essentially producing the sort of conditions that can delay takeoffs and landings.

Look below to get the skinny in a video featuring Heymsfield himself -- be prepared, it's science-y -- as well as to read a synopsis of the study courtesy of NCAR.

Flights can inadvertently generate rain and snow near airports

BOULDER -- Researchers have found that areas near commercial airports can sometimes experience a small but measurable increase in rain and snow when aircraft take off and land under certain atmospheric conditions.





"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 likely to be 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 an enhanced 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.

Andy Heymsfield describes research that links aircraft of all kinds with a slight increase in rain or snow near airports--when the conditions are just right.

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.


The effect is unrelated to the trails of condensed water vapor known as contrails made by the exhaust of jet engines. Contrails occur at colder temperatures of -40 degrees F (-40 degrees C) or below.

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% of the time over the course of a year. The researchers found that the right conditions existed more than 3% of the time for jets at Heathrow, Frankfurt, and Seattle-Tacoma.

Yellowknife experienced such conditions more often, about 10% of the time for propeller planes and 5% 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.

More from our Videos archive: "Denver International Airport and Close Encounters: The freaky conspiracy link (VIDEO)."



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