By Sid Perkins
Under just the right conditions, aircraft can substantially boost precipitation by seeding clouds as they fly through them, a new study reveals.
Water typically freezes at 0° Celsius, but in the absence of dust or other small particles for the water to freeze around, the droplets in clouds can remain liquid at temperatures as low as -40°. In such a supercooled state, even a tiny disturbance can trigger freezing, says Andrew Heymsfield, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo. Now, data suggest that aircraft passing through clouds can produce enough ice crystals to seed the clouds and trigger a small snowstorm, he and his colleagues report in the June Bulletin of the American Meteorological Society.
The team gathered their data during a research flight over northern Colorado in December 2007. While reviewing images taken by aircraft-mounted cameras, the researchers noticed a long, narrow gap in an otherwise solid layer of clouds — a hole that lined up with the flight paths of aircraft approaching Denver International Airport that day. Beneath the gap, a wall of falling snow extended from cloud level down to the ground, Heymsfield notes. Data reveal that snowfall from this unusual storm, which lasted about 45 minutes, blanketed an area 32 kilometers (20 miles) long and 4 kilometers (2.5 miles) wide to a depth of about 5 centimeters (2 inches).