Washington: Jet aircraft make odd-shaped holes in clouds and cause rain or snow as they climb or descend in the skies under certain atmospheric conditions, according to a new American study.
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.
Lead author Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR), said: "Any time aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby.
"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."
While researchers have known that propeller aircraft could transform supercooled droplets into ice crystals, 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.
Although 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," said Patrick Kennedy, a Colorado State University radar engineer who spotted the unusual readings and helped write the study.
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 said.
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 super cooled 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.
Heymsfield said: "This apparently happens frequently, embedded in the cloud layers.
"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 break."
The study appears in the Bulletin of the American Meteorological Society.