“Andrew Heymsfield of the National Center for
Atmospheric Research and colleagues flew a research
plane through the snow produced by a hole-punch cloud
west of Denver International Airport in 2007. The plane
was loaded with instruments for studying how ice forms
in clouds. Radar from the ground picked up a strange
echo in their wake, indicating oddly-shaped snowflakes.
“ “We didn’t know it, but we went
right through this precipitation feature that was spotted
from the ground,” Heymsfield said. Video from
the flight showed a hole in a patch of altocumulus clouds
(see below), and two inches of snowfall directly below
the hole.
“Heymsfield and colleagues examined flight records
for the nearby airport and linked the hole and the snow
to two commercial turboprop aircraft that had taken
off about an hour before. The movement of supercooled
water droplets over the planes’ propellers cooled
them enough to make them freeze and plunge to Earth
as snowflakes, Heymsfield said. Normally, snowflakes
or raindrops need a speck of dust or another imperfection
to form around. But supercooled water can freeze instantly,
if it cools quickly to around minus 35 degrees Fahrenheit.”
