Cavums are also known as hole-punch clouds or fallstreak holes. These features often appear as large, circular gaps in an otherwise unbroken sheet of altocumulus or cirrocumulus clouds.
What is a cavum?
Cavums form when specific conditions occur within a cloud layer made of supercooled water droplets—water that remains in liquid form even at temperatures below freezing. When something disturbs this delicate balance, such as an airplane passing through, the disturbance can trigger the rapid freezing of the supercooled droplets into ice crystals.
As the ice crystals form, they absorb moisture from the surrounding cloud droplets, causing the droplets to evaporate. This creates a clear patch in the cloud layer that can grow larger over time, forming the characteristic hole of a cavum cloud. The ice crystals may also fall below the main cloud layer, forming streaks known as fallstreaks that give the phenomenon its alternative name.
Comparing canal clouds to cavum clouds
Canal clouds are closely related to cavum clouds, but their shapes differ based on how the disturbance occurs. While cavum clouds typically form circular or oval gaps due to steeper angles of aircraft movement, canal clouds are longer and narrower, reflecting the shallow angle of the plane’s passage. Both phenomena illustrate the delicate balance of temperature, pressure, and moisture in cloud dynamics.
This image captures a group of cavum and canal clouds over the Gulf of Mexico, just off Florida’s west coast, on January 30, 2024. Research published in 2010 from University Corporation for Atmospheric Research (UCAR) and NASA reveals that aircraft passing through clouds can create these formations, with the angle of flight determining their shape. Steeper angles produce circular gaps, while shallow angles create elongated “canal clouds” with trailing streaks of precipitation known as virga.
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The role of human activity in forming fallstreak clouds
Aircraft play a significant role in the formation of cavum clouds. When planes pass through the upper atmosphere, their engines and wings can cause localized cooling and pressure changes that prompt the freezing of supercooled droplets. This makes cavum clouds more likely to form near busy air traffic routes. However, natural disturbances, such as a strong gust of wind or a thermal updraft, can also create these formations.
Winter formation of cavums
Cavum clouds are more likely to form during the winter months, when atmospheric conditions conducive to their creation are more frequent. According to NASA, research indicates that while these conditions occur about 3 to 5 percent of the time globally, the formation of cavums in clouds increases significantly to 10 to 15 percent during winter. The colder temperatures at high altitudes in winter enhance the presence of supercooled water droplets, making it easier for disturbances, such as aircraft, to trigger the freezing and evaporation processes that create cavum clouds.
References
Heymsfield, A. J., Kennedy, P. C., Massie, S., Schmitt, C., Wang, Z., Haimov, S., & Rangno, A. (2010). Aircraft-induced hole punch and canal clouds: Inadvertent cloud seeding. Bulletin of the American Meteorological Society, 91(6), 753-766. DOI: 10.1175/2009BAMS2905.1
