Why airplanes make it rain or snow

Airplanes may alter the weather around airports to a small degree by punching holes in clouds and even causing snow or rain, a new study indicates.

Looking at weather data from seven airports located in mid- to high-latitude areas, the researchers found that landing and departing flights had as much as a 6-percent chance of inadvertently "seeding" the clouds they punched through and causing precipitation.

This inadvertent rainmaking by air traffic probably isn't important on a global scale, but it could mean more rain or snow in regions around airports, particularly at higher latitudes, the authors write in the July 1 issue of the journal Science. [Holey Clouds: Gallery of Formations Cut by Airplanes]

How they do it

For decades, large holes in clouds baffled observers, and, at least in the case of one suspiciously saucer-shaped indentation over Moscow, even led to theories of UFO visitation. In research published in 2010, Andrew Heymsfield, a senior scientist at National Center for Atmospheric Research in Boulder, Colo., and his team offered an explanation for the strange clouds by linking the cooling effect of airplanes to these mysterious gaps.

Under normal conditions, clouds at temperatures between 0 and minus 40 degrees Celsius (32 and minus 40 degrees Fahrenheit) contain suspended, super-cooled droplets of liquid water.

An airplane, powered by jet engines or propellers, "seeds" clouds like these by expanding and cooling the air that flows under its wings or through its propellers. This cooling creates ice, which attracts the super-cooled water droplets. Together, these grow heavier and create snow or rain, which may plummet to the ground or evaporate aloft. In the hole-punch clouds, this appears as the signature wisps of ice crystals or snow within or below them.

This hole-creating process occurs in liquid clouds below about minus 10C (14F) for propeller aircraft and minus 20C (minus 4F) for jets, according to Heymsfield.

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