There’s a Hole in the Cloud!

Unless you are interested in meteorology, you probably have not heard of hole-punch, or fallstreak clouds. Appearing as clouds with sections taken out of them, with a trail of lower cloud, they’re rather noticeable.
Generally speaking, fallstreak hole clouds form when aircraft fly through altocumulus clouds. The wing tips, along with open flaps or propellers, if present, rapidly expand and cool the air they travel through. In altocumulus clouds, this produces a large amount of ice crystals, which grow and takes away much of the water vapor in the cloud. With less water vapor, the cloud evaporates in that section, and the ice crystals fall, leaving a hole in the altocumulus with the ice crystal fallstreak below.
Taking measurements and observations of these clouds is not easy due to the fact that the clouds are relatively rare, and the chances that a ground based observation setup will happen to see one are small. However, a small chance does not mean never, and the image below is a fallstreak cloud seen on April 17th, 2007 in the Chilbolton area of Hampshire in the United Kingdom.
That brings us to today’s article,  “Observations of a glaciating hole-punch cloud” by Chris Westbrook and Owain Davies, published in Weather in 2010. They were lucky enough to observe a fallstreak hole and gain observations about it.
A fallstreak cloud seen in the UK. Taken from Westbrook and Davies, 2010, Figure 3b.
A fallstreak cloud seen in the UK. Taken from Westbrook and Davies, 2010, Figure 3b.
The fallstreak cloud discussed in today’s article was observed using Light Detection and Ranging Systems, or LiDARs. The cloud was also photographed. Measurements from an early-morning radiosonde, or weather balloon, show that the clouds were about -20 Celsius (-4 F), which is considered supercooled for clouds. The fallstreak of ice appeared highly reflective on LiDAR instruments, approximately ten times more reflective than its surroundings.
Most likely, the passage of the aircraft does not form ice crystals from the water vapor drops already in the clouds, but from new droplets. The evidence for this is the concentration of ice crystals in the area of the fallstreak, which was much larger than the surrounding clouds. For a moment as the air rapidly expands with the passage of the aircraft, there is a very localized occurrence of relative humidity levels near 800 per cent. This allows homogeneous crystal formation, or crystal formation without specks of aerosols in the air to start production of ice, which does not normally occur. The rate of ice crystal formation can be calculated with this information, and even if you dilute the result across the 500 meter (1640 ft, or nearly a third of a mile) width of the fallstreak, it is still ten times greater than its surroundings–in accordance with LiDAR observations. Additionally, homogeneously formed ice crystals form simple planes that are highly reflective, as was observed. Therefore this fallstreak was from new droplets; the data does not make sense if it was from the original cloud droplets.
Also of note is that there were columns of rising air on either side of the fallstreak cloud, perhaps due to the sudden influx of ice crystals or the change in moisture. Only so many conclusions can be drawn from one observation, so more research certainly needs to be done, as is often the case in science.
Bibliography
National Weather Service and National Oceanic and Atmospheric Administration. “The Basic Types of Clouds.” Cloudwise. N.p., n.d. Web.
Linked to partway through my post, this webpage shows the basic types of clouds.
Pedgley, David E. “Some thoughts on fallstreak holes.” Weather 63.12 (2008): 356-360.
An article referenced by today’s main article, used for reference and a better understanding of hole-punch clouds. It also has some images of fallstreak clouds that you might want to check out.
Westbrook, Chris, and Owain Davies. “Observations of a glaciating hole‐punch cloud.” Weather 65.7 (2010): 176-180.
Today’s main paper and the source of the image used in this post.

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