Airborne planes can increase precipitation from clouds by around 10-fold, and under certain conditions can increase rain and snow storms over airports as planes pass through clouds during take-off or landing.
Finnish researchers have confirmed this phenomenon, and captured its underlying features, using 11 years of weather radar, satellite and LiDAR (light detection and ranging) data.
A radar station at Kerava, Finland, recorded a trail of heightened precipitation – the yellow streak to the left – on the aircraft approach path to Helsinki-Vantaa airport.
The findings are reported in a paper published in the Journal of Geophysical Research: Atmospheres.
Lead author Dmitri Moisseev, from the University of Helsinki and Finnish Meteorological Institute, detected on radar scans unusually long, straight patterns of intense rainfall that veered toward the Helsinki-Vantaa airport against a backdrop of lighter rain or snow.
Curiously, their shapes looked like the inverse of cloud formations known as hole-punch, canal clouds or fall-streak holes, formed when aircrafts create ice particles while flying through supercooled liquid clouds.
Clouds are formed by condensation of tiny water droplets and ice crystals. Pure, suspended water droplets can stay liquid to –40 degrees Celsius, so when they condense they form clouds much colder than the normal freezing point of zero degrees. These supercooled liquid clouds are common in low- to mid-level cloud layers.
Planes generate air pressure changes that drop the temperature below –40, causing a chain reaction of ice crystal formation from supercooled water droplets surrounding the aircraft’s flight path.
The extra ice crystals increase the rate at which they collide to form larger snowflakes, thus intensifying snowfall, the authors explain.{%recommended 2023%}
What is interesting is that this doesn’t only occur when the plane flies through a cloud. The researchers detected the effect starting well above the clouds that were already raining or snowing, at temperatures that facilitate ice crystal formation.
They report data collected from 23 observations over six days of intensified local precipitation, ranging from 30 to 60 minutes spanning several kilometres, and matched their features with planes flying in and out of the airport.
Their analysis showed that the crystals fall from an upper liquid cloud layer that is not visible via radar, triggering increased rain or snow from clouds below.
“This process appears to be responsible for the abrupt intensification of snowfall by a factor of six to 14,” the authors write.
The effect is created not by fuel emissions, but rather the plane’s wings. “The interesting thing about this feature is that it is caused by aircraft, but it is not caused by pollution,” explains Moisseev.
“Even if there would be absolutely ecological planes, which don’t have any combustion, no fuel or anything, it would still happen.”
The natural formation of ice crystals by the physical passage of planes can offer useful insights for studying the formation of rain and snow, he adds.
These observations could help meteorologists “nowcast” rain and snow conditions two to six hours in advance, which is critical for airport controllers.
