Planes flying through clouds can increase rain or snowfall by as much as a factor of ten, scientists in Finland have discovered. This intensification of precipitation is not caused by emissions, say the researchers, but by ice crystals created as the aircraft’s wings pass though cloud layers above active rain or snow.
Researchers at the University of Helsinki first noticed odd patterns in the data from their weather radar, which monitors clouds and precipitation in the region. “On some days we observed unnatural looking features in the radar observations,” explains Dimitri Moisseev, head of the university’s Radar Meteorology Group. “These features looked like tracks of airplanes approaching or departing from Helsinki-Vantaa airport. So the next natural step was to figure out whether our hypothesis that these radar features are caused by airplanes was correct and to understand the physics behind this phenomenon.”
The tracks showed up as straight patches of intense precipitation that were up to 10 times stronger than the background rain or snow. Looking back at historic data, the team identified 17 days that featured large numbers of these events between 2008 and 2018. Using flightpath data they were able to link most of these tracks to specific aircraft that were arriving or departing from Helsinki-Vantaa airport.
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The researchers wondered if the physics was similar to hole-punch clouds, which form when planes pass through clouds of supercooled liquid. Although the temperatures within these clouds are below 0 °C, they are generally not cold enough to trigger homogeneous freezing – when a water droplet freezes without a solid particle, such as a speck of dust, to act as a nucleus for ice formation.
But aircraft travelling through the cloud can cause the droplets to freeze. As the tips of the plane wing pass through the cloud, the air behind them expands and causes a local drop in pressure and a fall in temperature of around 20 °C. Ice crystals forms through homogeneous freezing if the temperature drops lower than about –40 °C, which leads to further cooling and additional freezing. When the heavy ice crystals drop out of the cloud they leave a clear patch of sky in the middle to form the “hole-punch”, with the ice crystals normally evaporating before they hit the ground.
To work out what was going on with the tracks of intense precipitation, Moisseev and his colleagues turned to lidar, radar and satellite data. This gave them information on the past events they had identified, such as cloud height, temperature, particle size and precipitation rate.
They discovered that the planes were not flying through the precipitating clouds, but through supercooled cloud layers above them. Their data showed that, like with hole-punch clouds, the adiabatic cooling created by the aircraft wings was also causing homogeneous freezing. But the resulting ice crystals were dropping from the upper cloud layer into the cloud layer below, from which the rain and snow were falling.
“The aircraft produce ice particles that seed this lower cloud, which leads to collisions between newly formed ice crystals by aircraft and ice crystals in the lower cloud,” Moisseev told Physics World. “These collisions create larger, heavier [droplets], that fall faster. The result is that we get heavier snowfall or rainfall.”
However, says Moisseev, this phenomenon is unlikely to have much impact on the wider weather, and won’t be moticed on the ground among natural variations in snow- and rain-storms. But he says that the work does help advance our “understanding of ice and mixed-phase clouds and precipitation formation in such clouds”, in particular how precipitation can work in mixed-phase clouds and what processes could be responsible for the intensification of rain and snow in natural conditions. “I consider it like a great ‘laboratory’ experiment in cloud physics,” Moisseev says.
Full details are reported in Journal of Geophysical Research: Atmospheres.
