Are contrails detectable with radar?

Propeller and jet planes create ice crystals, snowflakes and raindrops in freezing clouds

Cloud hole over Texas

Boulder (USA) - Air traffic causes more rainfall around airports. US researchers found the cause of this in holes and canals that tear planes in clouds with a high proportion of supercooled, liquid water. They tracked this hitherto unexplained weather phenomenon with the help of satellite images and take-off and landing lists. According to the scientists, this affects the local weather, but effects on the earth's climate are unlikely. They published their weather study in the journal "Science".

"An airplane propeller pushes the air backwards and lets it cool down by up to 30 degrees," says Andrew Heymsfield of the National Center for Atmospheric Research (NCAR) in Boulder. The flow conditions behind an aircraft are responsible for this. The air pressure drops locally for a short time. The result: the liquid water contained in clouds freezes into small ice crystals. And these then serve as condensation nuclei for larger crystals, snowflakes and raindrops. The different speeds of air currents around the wings of jet planes also showed a similar effect in the analysis.

Heymsfield and his colleagues were able to observe these effects of air traffic directly on satellite images - including of the clouds around the airports in Frankfurt, London and Chicago. Depending on the flight path, the planes left holes or kilometer-long channels in a previously loose cloud cover. A cloud hole lasted about four hours over an airport in Texas and reached an extent of up to 100 kilometers. The water previously contained in the cloud fell to the ground as rain due to the additional cooling.

According to the researchers' estimates, this effect occurs on average for a good hour every day at all larger airports and is particularly pronounced in the vicinity of the polar regions. Heymsfield is currently investigating the relevance of this man-made rainfall in the tropical regions on the Virgin Islands in the Caribbean.

Today it is already known that the contrails of jet planes lead to stronger cloud formation worldwide and have a verifiable effect on the earth's climate. However, the researchers see no connection with the climate for the cloud holes, which are delimited locally around the airports. However, especially in polar regions, scientific studies of clouds, which are used for improved climate models, should be carried out at greater distances from airports. Otherwise there would be a risk that they would be falsified by the cloud holes caused by airplanes.