Making Sense of Holes in the Clouds

Making Sense of Holes in the Clouds

Cavum, also called hole-punch clouds and fallstreak holes, look so odd that people sometimes argue they are signatures of flying saucers or other unidentified anomalous phenomena. Seen from below, they can look like a large circle or ellipse has been cut neatly from the clouds, with feathery wisps left in the middle of the hole.

They are equally impressive when seen from above. This image shows a cluster of cavum over the Gulf of Mexico off of Florida’s west coast on January 30, 2024. It was captured by the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s Terra satellite.

Otherworldly explanations are not required to explain the eye-catching cloud formation. While scientists have periodically mentioned the phenomena in scientific journals and speculated about their cause since the 1940s, a pair of studies published in 2010 and 2011, led by University Corporation for Atmospheric Research (UCAR) scientists, laid out an explanation that put other theories to rest. They are caused by airplanes moving through banks of altocumulus clouds.

These mid-level clouds are composed of liquid water droplets that are supercooled; that is, the droplets remain liquid even when temperatures are below the typical freezing point of water (32 degrees Fahrenheit, or 0 degrees Celsius). Supercooling happens when water droplets are exceptionally pure and lack small particles, such as dust, fungal spores, pollen, or bacteria, around which ice crystals typically form.

Supercooling may sound exotic, but it occurs routinely in Earth’s atmosphere. Altocumulus clouds, which cover about 8 percent of Earth’s surface at any given time, are mostly composed of liquid water droplets supercooled to a temperature of about -15°C.

But even supercooled clouds have their limits. As air moves around the wings and past the propellers of airplanes, a process known as adiabatic expansion cools the water by an additional 20°C or more and can push liquid water droplets to the point of freezing without the help of airborne particles. Ice crystals beget more ice crystals as the liquid droplets continue to freeze. The ice crystals eventually grow heavy enough that they begin to fall out of the sky, leaving a void in the cloud layer. The falling ice crystals are often visible in the center of the holes as wispy trails of precipitation that never reach the ground—features called virga.

Unlike previous attempts to explain the phenomena, the UCAR researchers, with colleagues from several other institutions, including NASA’s Langley Research Center, made use of a combination of aircraft flight data, satellite observations, and weather models to explain how the clouds form and to track how long they lasted. When planes passed through clouds at a fairly sharp angle, the researchers found that small, circular cavum appeared. If they passed through the clouds at a shallow angle, longer “canal clouds” with lengthy virga trails, like the one shown above, became visible.

Other factors that can affect the length of these clouds include the thickness of the cloud layer, the air temperature, and the degree of horizontal wind shear, the researchers reported. Their analysis showed that a full spectrum of aircraft types including large passenger jets, regional jets, private jets, military jets, and turboprops can produce cavum and canal clouds. With more than 1,000 flights arriving at Miami International Airport each day, there are many opportunities for planes to encounter the atmospheric conditions needed to produce cavum.

NASA Earth Observatory image by Michala Garrison, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Adam Voiland.

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