Flights increase rain and snow near airports
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. 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.
The release notes that 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.
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 29 January 2007, contained 92 such gaps, some of which persisted for more than four hours and reached lengths of sixty 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 thirty to ninety minutes after an aircraft passes through. This would be the peak time for precipitation associated with the cloud-seeding effect. After about ninety minutes, ice and snow begin to dissipate.
— Read more in Andrew J. Heymsfield et al., “Formation and Spread of Aircraft-Induced Holes in Clouds,” Science 333, no. 6038 (1 July 2011): 77-81 (DOI: 10.1126/science.1202851)