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Why airplanes make it rain or snow

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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.

This process is known as cloud seeding, and it can be done intentionally to alter weather. However, in this case, airplanes' cloud seeding effect is entirely accidental.

Rainmaking planes

To better understand hole-punch clouds, Heymsfield and colleagues followed the growth of 92 holes and canals — long streaks cut when a plane's path is more horizontal — through satellite data.Some reach lengths of more than 62 miles (100 kilometers) and lasted for four or more hours. Using Federal Aviation Administration tracking information, they found that a full spectrum of aircraft — from jets of all sizes to planes equipped with propellers — cut through the clouds.

They then compared satellite observations with simulations run using a weather model, and found that the airplane's introduction of ice created patterns of air movement upward in the hole and downward at the sides, causing the hole to expand for periods as long as an hour.

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