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Fallstreak hole

A fallstreak hole, also known as a hole punch cloud or cavum, is a large circular or elliptical gap that appears in layers of mid- to high-level clouds, such as altocumulus or cirrocumulus.^[1]^[2] These formations typically occur when the passage of an aircraft or other disturbances through clouds containing supercooled liquid water droplets—water that remains liquid despite temperatures below 0°C, often around -15°C—causes adiabatic cooling, triggering the droplets to freeze into ice crystals.^[3]^[2]^[4] The ice crystals then grow by absorbing surrounding moisture and fall out of the cloud layer as virga (evaporating precipitation), creating and expanding the visible hole.^[1]^[5] The phenomenon has been observed and documented for over 50 years, initially puzzling scientists until research in the early 2000s confirmed the role of aviation.^[3] Studies using flight data, satellite imagery, and numerical models, such as those conducted by the University Corporation for Atmospheric Research (UCAR), demonstrated that various aircraft types—including jets and turboprops—can induce these holes, with sharper descent angles producing compact circular gaps and shallower angles forming elongated "canal clouds" with trailing virga.^[2]^[5] Altocumulus clouds, which cover about 8% of Earth's surface and consist primarily of supercooled droplets, are the most common setting for fallstreak holes, particularly near major airports with high flight traffic, such as those in Miami where over 1,000 flights occur daily.^[2] While globally the impact is negligible, regionally fallstreak holes can lead to minor increases in precipitation near busy air routes due to the ice particles' fallout.^[3] The holes often expand over time through mechanisms like gravity waves, which propagate outward and sustain vertical motions for up to an hour or more after formation.^[3]^[6] Recognized in the World Meteorological Organization's cloud atlas as a supplementary feature, these atmospheric curiosities highlight the unintended interactions between human activity and weather processes.^[2]

Definition and Characteristics

Physical Description

A fallstreak hole, officially termed a cavum by the World Meteorological Organization (WMO), is a well-defined, generally circular or elliptical gap that forms within a thin layer of supercooled water droplet clouds.^[4] These gaps typically measure several kilometers in diameter and result from the localized fallout of ice crystals that nucleate and precipitate from the supercooled droplets, creating a clear central void while leaving the surrounding cloud layer largely intact.^[7] Unlike general cloud dissipation or tears, a fallstreak hole represents a distinct clearing caused by the phase transition of water from liquid to solid, without disrupting the overall cloud structure.^[8] The WMO adopted the nomenclature cavum in the 2017 edition of the International Cloud Atlas to formally recognize this supplementary cloud feature, previously known colloquially as a fallstreak hole, hole-punch cloud, or skypunch. Structurally, the hole often exhibits a crisp, rounded edge when viewed from below, though it may appear oval or elongated from an angle, and is frequently accompanied by trailing virga—wisps of falling ice crystals that evaporate before reaching the ground.^[4] This virga extends downward from the central void, enhancing the hole's distinctive appearance as a localized aperture in an otherwise uniform cloud deck.^[7] Fallstreak holes commonly occur in mid-level cloud types such as altocumulus or cirrocumulus, where conditions support the persistence of supercooled droplets.^[4] The surrounding cloud remains stable, underscoring the phenomenon's nature as a precise, self-contained perturbation rather than widespread evaporation or mechanical rupture.^[8]

Visual Appearance

A fallstreak hole typically appears as a sharply defined, circular or elliptical gap in a layer of mid- or high-level clouds, such as cirrocumulus or altocumulus, creating a striking contrast against the surrounding uniform cloud deck.^[7]^[1] The hole's edges are often crisp and well-demarcated, giving the impression of a neatly punched opening in the sky, which can measure from hundreds of meters to several kilometers in diameter.^[9] In photographs and observer accounts, these formations stand out due to their geometric precision, sometimes resembling a portal or void amid the otherwise continuous cloud layer.^[10] Variations in shape occur depending on atmospheric conditions; while perfectly round holes are common in calm air, wind shear can distort them into elliptical or even irregular forms, particularly in the early stages of development.^[11] Holes often expand outward over time, growing at rates of approximately 0.4 to 0.6 meters per second and reaching diameters of up to 4 kilometers within 90 minutes, as observed in both real-world cases and simulations.^[9] In some instances, elongated variants known as canal clouds form, appearing as linear channels rather than discrete openings.^[11] Accompanying the hole is frequently a wispy trail of virga—falling ice crystals that may evaporate before reaching the ground—extending downward from the center, producing feathery or brush-like streaks that enhance the "fallstreak" visual effect.^[7]^[10] These filaments can appear radial or dangling, adding a dynamic, ethereal quality to the scene, while the hole's periphery may exhibit slightly denser cloudiness.^[1] For recognition, fallstreak holes differ from lenticular clouds, which form smooth, lens-shaped caps over mountains rather than voids, and from contrails, which are linear and trail behind aircraft without creating expansive gaps.^[11]

Formation Mechanism

Natural Processes

Fallstreak holes form naturally in supercooled clouds, where water droplets exist at temperatures below 0°C without freezing due to the absence of sufficient nucleation sites. Spontaneous ice nucleation occurs when random perturbations, such as atmospheric turbulence or wind shear, disturb the droplets, causing a subset to freeze into ice crystals.^[12]^[1] This process typically takes place in mid-level clouds like altocumulus or cirrocumulus.^[7] Once initiated, the ice crystals grow rapidly through the Bergeron process, in which water vapor from the surrounding supercooled droplets deposits onto the crystals due to the lower saturation vapor pressure over ice compared to liquid water.^[13] The enlarging crystals become heavy enough to fall through the cloud layer, creating a visible hole. This triggers a chain reaction as the falling crystals and downdrafts further perturb nearby droplets, leading to additional freezing and expansion of the cleared area.^[1]^[7] The initial freezing spreads over several minutes, with the full hole developing in 10 to 30 minutes under favorable conditions, though the feature can persist for up to an hour or more as ice crystals continue to fall.^[14]^[15] The cleared region may slowly refill as moist air mixes in from surrounding areas.^[1] Natural fallstreak holes are less common than those induced by aircraft, occurring primarily in unstable mid-level atmospheres where supercooled conditions prevail without human interference.^[12]^[11]

Aircraft-Induced Triggers

Fallstreak holes are frequently triggered by aircraft passing through supercooled clouds, where the mechanical disturbances from wings or propellers induce rapid ice formation. The primary mechanism involves adiabatic cooling caused by the expansion of air in the aircraft's wingtip vortices or propeller slipstreams, which lowers the local temperature sufficiently to cause homogeneous nucleation of ice crystals in supercooled liquid water droplets. This process is most effective in altocumulus clouds located along common flight paths, typically at altitudes between 5,000 and 25,000 feet, where temperatures range from -8°C to -40°C depending on the aircraft type—propeller-driven aircraft can initiate freezing at warmer thresholds (-8°C to -15°C), while jet wings require colder conditions (below -20°C) and can produce cooling of up to 20°C.^[16]^[3] The phenomenon was first observed in the 1940s, with early reports of unusual cloud holes appearing shortly after military aircraft flights during World War II, though definitive linkage to aviation occurred in 1948 through meteorological analyses. Observations increased significantly in the post-WWII era as commercial air traffic expanded, leading to the common designation of these features as "hole punch clouds" due to their association with inadvertent cloud seeding by passing planes. This anthropogenic trigger contrasts with rarer natural formations and has been documented extensively near busy airports, where suitable supercooled altocumulus layers are prevalent.^[16]^[17] Once initiated, the hole propagates as the newly formed ice crystals fall under gravity, sublimating or evaporating surrounding supercooled droplets through the Wegener-Bergeron-Findeisen process, in which water vapor diffuses from liquid droplets to ice crystals, causing further freezing and enlarging the void. This fallout creates a fallstreak of virga trailing from the hole and can extend the feature along the aircraft's path, forming elongated canals if the plane passes at a shallow angle. Aircraft-induced cases account for the vast majority of documented fallstreak holes, particularly in regions with high flight density, and the initial voids often become visible within minutes of the aircraft's passage.^[3]^[2]

Atmospheric Requirements

Cloud Types Involved

Fallstreak holes form primarily within altocumulus clouds, which occur at altitudes between approximately 2 and 7 kilometers and consist of layered formations with small, rounded elements, or cirrocumulus clouds, found above 6 kilometers as high-altitude patches of small cloudlets, and less commonly in stratocumulus clouds.^[7]^[18]^[19] These clouds must feature uniform, thin decks less than 100 meters thick with high concentrations of supercooled water droplets, allowing the holes to disrupt the layer's overall homogeneity when triggered.^[18] These conditions provide the necessary stability and droplet uniformity for the phenomenon.^[18] Fallstreak holes do not occur in stratus or cumulus clouds, which lack the requisite thin, uniform structure, nor in warm clouds above freezing temperatures where supercooled droplets are absent, or in thick nimbostratus layers that prevent clear gap formation.^[18]

Temperature and Humidity Factors

Fallstreak holes form in layers of supercooled liquid water droplets, which remain unfrozen despite temperatures between 0°C and -40°C, allowing the persistence of metastable water in the cloud.^[20] Nucleation of ice crystals, essential for hole development, is most efficient in the narrower range of -10°C to -20°C, where heterogeneous freezing processes activate readily upon perturbation.^[2] Observations confirm this in mid-level clouds at temperatures around -15°C to -30°C, where supercooled droplets are prevalent before rapid glaciation occurs.^[21] High relative humidity exceeding 90% with respect to liquid water is required within the cloud layer to sustain the supercooled droplets, often approaching 100% saturation for droplet growth and stability.^[21] Below the cloud base, conditions become subsaturated with respect to water, promoting the evaporation of falling virga trails and preventing replenishment of the hole.^[16] This humidity gradient ensures the localized depletion of liquid water propagates downward without diffusing back into the surrounding layer. Stable atmospheric conditions with weak vertical wind shear are crucial to preserve the cloud layer's integrity and allow the hole to expand without premature dissipation.^[21] Weak shear limits mixing with drier air and maintains the horizontal spread of ice crystals.^[22] These phenomena are most prevalent in mid-latitudes between 30° and 60° N/S, where supercooled cloud layers occur frequently, particularly during cooler seasons when lower temperatures favor supercooling.^[18] In these regions, atmospheric profiles often support the necessary thermodynamic balance, leading to higher incidence rates compared to tropical or polar zones.^[23]

Observations and Examples

Historical Sightings

Fallstreak holes, also known as hole-punch clouds, have been observed since at least the early 20th century, with the earliest documented photographs appearing in British meteorologist C.J.P. Cave's 1926 book Clouds, which illustrated unusual gaps in cloud layers without attributing them to any specific cause.^[24] These pre-aviation records suggest the phenomenon may have occurred naturally in altocumulus clouds, predating widespread aircraft activity, though formal descriptions remained scarce until the 1940s. In 1940, German meteorologist Schumacher provided one of the first detailed accounts of holes in an altocumulus layer in the journal Zeitschrift für angewandte Meteorologie, describing them as circular clearings potentially linked to atmospheric instabilities.^[25] As commercial aviation expanded in the post-World War II era, reports from pilots and ground observers increased, with sightings noted along flight corridors in the late 1940s and 1950s. For instance, in 1948, R.M. Poulter photographed a "canal of blue sky" in clouds over the UK, attributing the elongated gap and accompanying feathery cirrus to heat from aircraft exhaust in Weather magazine. By 1954, J.M. Stuart documented precipitation streamers, or virga, falling from a distrail—a linear hole—produced by an aircraft in high altocumulus clouds, as reported in Meteorological Magazine, marking an early link to aviation-induced triggers. These observations coincided with the rise of propeller-driven aircraft, which were suspected of disturbing supercooled water droplets in cloud layers. The 1960s brought more systematic attention, with the first formal meteorological literature entry on hole-punch clouds appearing in a 1966 Weatherwise article by V.J. Simon, who described a circular gap in altocumulus over Schenectady, New York, explicitly implicating a passing jet aircraft as the cause.^[16] This study, referenced in later American Meteorological Society publications, solidified the connection to early jet aviation. Globally, European sightings proliferated in the 1970s; for example, the UK Met Office archived reports of unusual cloud gaps during this decade, including potential aircraft-related events over southern England. American researcher Andrew J. Heymsfield further advanced understanding through 1975 studies in the Journal of the Atmospheric Sciences, analyzing fallstreak formations in cirriform clouds via aircraft observations, though these focused more on natural processes than specific sightings. Early misconceptions about fallstreak holes were common, with their striking circular shapes in uniform cloud decks often misattributed to unidentified flying objects (UFOs) or even explosive devices during the Cold War era.^[2] Such interpretations persisted until the 1980s, when accumulating meteorological evidence from radar and in-situ measurements confirmed aircraft perturbations as the primary trigger, dispelling extraterrestrial or military theories in peer-reviewed literature.^[26]

Modern Documented Cases

In January 2007, NASA's Terra satellite, equipped with the Moderate Resolution Imaging Spectroradiometer (MODIS), captured a remarkable series of multiple fallstreak holes over the southern United States, spanning Louisiana, Texas, Oklahoma, and Arkansas.^[27] These circular gaps, up to several kilometers wide, formed in altocumulus clouds due to disturbances from passing aircraft that triggered the freezing of supercooled water droplets, leading to ice crystal fallout and visible holes.^[28] The event highlighted the role of air traffic in generating these phenomena near busy flight corridors. A notable example from the 2010s occurred in November 2014 over eastern Australia, where photographers documented a large fallstreak hole in altocumulus clouds near Melbourne, accompanied by prominent virga trails of falling ice crystals.^[29] The image went viral on social media, drawing widespread public attention and speculation, with meteorologists attributing it to aircraft passing through the cloud layer at Sydney's nearby airspace.^[30] Similarly, in 2018, residents near Kauai, Hawaii, reported and photographed a fallstreak hole linked to military aircraft activity, which spread rapidly online and emphasized the phenomenon's occurrence in tropical regions under specific humidity conditions.^[31] In the United States, a February 2021 outbreak over the Southeast, including Georgia, produced similar formations documented via ground observations and apps.^[32] Advancements in technology have enhanced documentation of fallstreak holes in the 2020s, with NOAA's GOES satellites providing real-time visible and infrared imagery of events, such as clusters over Florida in 2024.^[33] Drones have occasionally captured close-up views near airports, revealing the ice crystal trails in high resolution. The rise of social media has significantly boosted public interest and reporting of fallstreak holes since the 2010s, with platforms like Twitter and Reddit hosting thousands of shared photos annually, often misidentified as UFOs or anomalies.^[29] In 2025, sightings continued to be reported in Europe, including a fallstreak hole over Suffolk, UK, in July, captured by weather enthusiasts and shared widely online.^[34]

Scientific Context

Research Developments

Early research on fallstreak holes, also known as cavum clouds, began in the 1970s with laboratory experiments demonstrating ice nucleation and multiplication in supercooled clouds. In 1974, J. Hallett and S. C. Mossop conducted experiments showing that rime splintering—where small ice particles break off from graupel in supercooled conditions between -3°C and -8°C—can rapidly increase ice crystal concentrations, facilitating the Bergeron process that depletes surrounding liquid droplets. Concurrently, A. J. Heymsfield's 1975 observations and modeling of cirrus uncinus cells revealed fallstreaks forming from convective instabilities in supercooled layers, with holes appearing due to localized dehydration and particle fallout. Field observations in the 1990s further confirmed the dominance of the Bergeron process in natural fallstreak hole formation, particularly in altocumulus layers. Studies during this period, including airborne measurements over maritime regions, documented how adiabatic cooling from aircraft or natural waves triggers heterogeneous ice nucleation, leading to rapid droplet evaporation and hole development through ice crystal growth. These observations shifted understanding from purely theoretical mechanisms to empirical evidence of ice-liquid interactions in mid-level clouds. In 2017, the World Meteorological Organization updated its International Cloud Atlas to formally recognize cavum as a supplementary cloud feature, describing it as a well-defined circular or linear hole in thin supercooled water droplet layers, often with virga below.^[4] This classification drew on extensive observational data and modeling from institutions like the National Center for Atmospheric Research (NCAR), which analyzed aircraft-induced perturbations, and the European Centre for Medium-Range Weather Forecasts (ECMWF), contributing to refined global cloud representations.^[35] Advancements in the 2020s have incorporated numerical simulations to model aircraft wakes and hole dynamics more precisely. Computational fluid dynamics (CFD) approaches have simulated wake turbulence inducing pressure drops that nucleate ice in supercooled altocumulus, replicating observed hole initiation and expansion. These developments have addressed key gaps by transitioning from anecdotal reports to quantitative datasets, enabling better predictions of fallstreak hole occurrence. Recent analyses also highlight climate change implications, as warming temperatures may reduce the frequency of supercooled liquid clouds.^[36] Fallstreak holes share atmospheric conditions with contrails, particularly in regions of high humidity and low temperatures where supersaturation relative to ice occurs, but they differ fundamentally in formation mechanisms. Persistent contrails, formed from aircraft exhaust freezing into ice crystals at altitudes above 8 km and temperatures below -40°C, can spread into cirrus-like sheets without creating distinct punch-out gaps, whereas fallstreak holes arise from the perturbation of supercooled liquid droplets in mid-level clouds, leading to localized freezing and evaporation that excavate circular voids. Some hybrid phenomena, termed "contrail holes," have been observed where aircraft-induced fallstreaks interact with existing contrail structures, enhancing ice crystal production in the vicinity.^[22]^[16]^[18] Fallstreak holes are related to billow clouds and undulatus formations through shared instabilities in the atmosphere, such as Kelvin-Helmholtz instabilities (KHI) driven by wind shear between air layers, but they are distinguished by the involvement of phase change processes. Billow clouds, often appearing as wave-like rolls in altocumulus or stratocumulus decks, result primarily from mechanical shear without requiring supercooling or droplet freezing, whereas fallstreak holes depend on the rapid nucleation of ice in supercooled water, creating fallstreaks rather than mere undulations. Undulatus patterns, characterized by elongated wave crests, similarly arise from gravity or shear waves but lack the evaporative clearing and virga trails central to fallstreak holes.^[22]^[37] Virga, the trails of precipitation that evaporate before reaching the ground, often accompany fallstreak holes as the falling ice crystals sublimate in drier sub-cloud air, linking both to mixed-phase cloud dynamics where liquid and ice coexist. However, standalone virga can form from non-supercooled clouds or warmer precipitation without the characteristic hole, as it does not require the localized triggering of freezing that excavates the cloud layer in fallstreaks. This connection highlights broader processes in mixed-phase clouds, where about 8% of Earth's mid-level cloud cover involves supercooled droplets susceptible to such instabilities.^[22]^[38] In the wider context of aviation meteorology, fallstreak holes exemplify supercooling hazards in altocumulus clouds, where aircraft passage can inadvertently seed ice formation, posing risks akin to aircraft icing by promoting rapid droplet freezing on surfaces. These phenomena are studied for their implications in cloud seeding and turbulence forecasting, as the same supercooled conditions that produce fallstreaks can lead to airframe icing during descent or climb through affected layers.^[16]^[28]

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