Mid- and high-level cloud layers sometimes undergo a strange phenomenon known as hole punch clouds or fallstreak holes.
What Is a Fallstreak Hole?
Fallstreak holes are huge circular or elliptical gaps that can appear in cirrocumulus or altocumulus clouds. When seen from below, they can look like a large cavity that has been neatly carved from the clouds, with feathery wisps left at the center of the hole.
Also known as cavum or hole-punch clouds, these formations look so strange that most people think of them as flying saucers or other unidentified anomalous phenomena. However, they are only triggered by jet aircraft and can happen frequently throughout the year. In fact, the best place to observe these holes is at the airport.
How Do Fallstreaks Form?
High to mid-level clouds, like altocumulus, are usually made of tiny water droplets which are "supercooled." This means that the droplets remain liquid even if temperatures fall below the normal freezing point of water which is 32 degrees Fahrenheit (0 degrees Celsius). Altocumulus clouds, which cover around 8% of Earth's surface at any given time, are often made of liquid water droplets that are supercooled up to 5 degrees Fahrenheit (-15 degrees Celsius).
Supercooling occurs when water droplets are pure and lack tiny particles, like dust, pollen, bacteria, or fungal spores. It is in these particles where ice crystals usually form. Supercooling may sound exotic, but it actually happens routinely in the Earth's atmosphere.
Supercooled clouds also have their limitations. A process called adiabatic expansion happens as air moves around the wings and past the propellers of airplanes. This cools the water by an additional 68 degrees Fahrenheit (20 degrees Celsius) or more and pushes liquid water droplets to the point that they freeze even without the help of airborne particles.
As the liquid water droplets continue to freeze, ice crystals become more ice crystals. Eventually, they grow heavy enough that they start to fall out of the sky, leaving a void in the cloud layer as a result.
These cloud formations have been known for about some time, but scientists are still puzzled as to why the process of freezing starts in one area of a cloud layer and how it grows in size. To answer this question, a group of scientists conducted a study that was published in July 2011.
Led by Andy Heymsfield from the National Center for Atmospheric Research in Boulder, Colorado, a team of researchers used the GOES satellite to track 92 fallstreak holes over Texas for more than four hours in January 2007.
Within an hour, the holes grew a considerable amount up to 6 to 31 miles (10 to 50 kilometers) and then began to shrink slowly. Heymsfield proposed that a side effect was to blame in creating the ice that caused the holes to grow.
To test this assumption, the researchers ran a detailed computer model of the internal mechanisms of a cloud and then introduced a line of ice crystals like those produced by an aircraft. The simulation revealed that the hole grew slowly if the heating effect of creating ice was turned off, and the hole did not grow at all if the effects of evaporation were removed. The result of the study suggests that the freezing process is set off by aircraft that fly through the clouds.
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