Clouds with oddly shaped gaps or hole punches, ones that look like UFO's have crashed through them, have bemused public and experts alike, for many decades. While known to occur when turboprop and jet airplanes zip through certain clouds, the cause behind the gaping hole remained an enigma, until now.
Oddly enough, the age-old mystery was solved accidently, thanks to data captured by instruments belonging to a team of scientists from the National Center for Atmospheric Research.
In 2007, the group, which included Dr. Andrew Heymsfield, who ended up spearheading the research, was flying west of the Denver International Airport when they noticed snow falling on just a small patch of the area. While it did not strike them as unusual at that time, when they landed, they observed some interesting anomalies in the data captured by the array of instruments they happened to be carrying.
A ground-based Radar had captured an unusual echo in the area, indicating that the unusually shaped snow had rapidly been formed around there. When they took a look at images captured by the aircraft's camera they noticed a big gaping hole in a group of otherwise dense clouds, in the same vicinity, where the snow had fallen.
The scientists then examined the snowflakes that had precipitated in that area and observed evidence of riming (accumulation of liquid water). There was no evidence of the same in the ice particles that still lay in the cloud - This caused them to conclude that a jet had rammed through the cloud and instantly 'seeded' it - a term used to describe the manipulation of clouds with the help of chemicals, to encourage precipitation - except in this case, it is totally accidental.
All this research has led the scientists to deduce this theory. Some mid-level clouds contain water that is in an unusual super cooled state - at temperatures well below freezing, but still in liquid form. When aircraft whiz through these, they cool them down even further - freezing them. Suddenly, the once liquid cloud now contains ice crystals, which precipitates on the ground, leaving behind a gaping hole in the cloud, and a pile of snow in a fairly narrow patch on the ground below.
To confirm that this is indeed what happens the researchers checked the flight records, which revealed that two turboprops had flown through the area during the time and even more exciting - snow crystals had started falling five minutes after the second one rammed through the cloud. Also, as is typical, the impromptu snowstorm, which had dumped a hefty 2-inches of white powder in 45 minutes, had been confined to a very small area - about 20 miles long and 2.5 miles wide.
However, before you start to warm up your turboprop, bear in mind that not all punctured clouds bring snowstorms. In order do so, they need almost goldilocks-like conditions - Clouds that are too high will not work, since they are frozen, while those that are too low are not cold enough. In addition, turboprops do a better job than jets since they not only fly at the right altitude, but also, propel by generating thrust, which results in pushing large amounts of cold air behind them. Jets engines on the other hand, spew out warm jet exhaust, warming up the clouds instead of cooling them down. So the only thing you can do to get a hole-punch snow day is . . . . . Keep your fingers crossed!
Sources: livescience.com,news.discovery.com,nsf.gov.com, sciencedaily.com