Megacryometeors are enormous chunks of ice that fall from the sky on clear, cloudless days, sometimes weighing tens of kilograms and occasionally smashing through roofs or cratering the ground. They are not hail, not ice shed from aircraft, and not debris from comets. They appear to form in the upper atmosphere through processes researchers still do not fully understand, and their frequency seems to have risen sharply since the mid-twentieth century. The phenomenon sits at an odd intersection of atmospheric science, climate research, and natural hazard assessment, and it has attracted serious scientific attention only since the early 2000s.
What Counts as a Megacryometeor
The term was coined by Jesús MartÃnez-FrÃas, a planetary geologist at Spain’s Center for Astrobiology, to distinguish these large ice falls from ordinary hailstones and from ice that breaks off aircraft. To qualify, an ice mass generally needs to weigh at least half a kilogram, though many documented specimens have been far larger. The defining feature is not just size but context: megacryometeors fall from skies with no cumulonimbus clouds, no thunderstorms, and no obvious source of frozen water overhead. One well-documented case occurred in March 2007, when an ice chunk weighing about 10 kilograms fell from a clear sky and crashed through the roof of an industrial storage building in Mejorada del Campo, a town roughly 20 kilometers east of Madrid.1PubMed Central. Monitoring the fall of large atmospheric ice conglomerations: a multianalytical approach to the study of the Mejorada del Campo megacryometeor The ice punched through the roofing from about 15 meters above ground level. Nobody was injured, but the incident illustrated just how destructive these falls can be.
Reports of anomalous ice falls are actually centuries old. Records from as far back as the 1800s describe large ice masses falling from blue skies. For a long time these accounts were dismissed as folklore, misidentified hail, or hoaxes. What changed was systematic collection and laboratory analysis of the ice itself, beginning in earnest around 2000. When researchers started subjecting recovered megacryometeors to chemical, isotopic, and microbiological testing, the results pointed clearly to an atmospheric origin rather than an extraterrestrial one or a human-made source like an airplane lavatory leak.
How Megacryometeors Differ from Hail
The confusion with hail is understandable, but the two phenomena are fundamentally different. Hailstones form inside powerful cumulonimbus clouds, where strong updrafts carry water droplets to altitudes where they freeze, accumulate additional ice layers, and eventually fall once they are too heavy for the updraft to support. Hail requires a thunderstorm. Even the largest recorded hailstones, which can exceed a kilogram in exceptional supercell storms, form under violently turbulent cloud conditions that are obvious to anyone on the ground.
Megacryometeors, by contrast, fall from cloudless or lightly clouded skies with no convective storm activity. Their internal structure also differs. Hailstones typically show concentric shells of clear and opaque ice, reflecting the multiple trips up and down through the cloud they took during formation. Megacryometeors tend to have a more irregular, sometimes layered but not concentrically layered structure, suggesting a different growth process. Their isotopic signatures, when analyzed, match atmospheric water at very high altitudes rather than the turbulently mixed water you would expect inside a storm cloud.
Size is another separator, at least statistically. While rare hailstones can reach grapefruit or softball dimensions, most megacryometeors recovered are much larger. Documented specimens have ranged from under a kilogram to over 200 kilograms in extreme cases. At those masses, they are not plausible products of any known hail-formation mechanism.
Where They Have Fallen
Megacryometeor events are not confined to any one region. Between 2001 and April 2006, researchers documented 46 ice-fall events across the globe. Confirmed impacts have occurred in Argentina, Australia, Canada, Colombia, India, Japan, Mexico, New Zealand, Portugal, Spain, Sweden, the Netherlands, the United Kingdom, and the United States.2AMBIO A Journal of the Human Environment. Megacryometeors: Distribution on Earth and Current Research Effects have ranged from megacryometeors crashing through roofs to producing small impact craters in the ground. One crater was documented in La Milana, Soria, Spain; others were observed in Surrey in the United Kingdom and Oakland, California.3AMBIO A Journal of the Human Environment. Megacryometeors: Distribution on Earth and Current Research
The geographic spread matters because it rules out a purely local explanation. If megacryometeors were caused by something specific to one latitude band, one climate zone, or one set of air traffic corridors, you would not see them on every inhabited continent. Their occurrence in countries with very different climates, from tropical Colombia to subarctic Sweden, points to a mechanism tied to the global atmosphere rather than regional weather patterns.
There is also a temporal pattern. Research into the historical rate of these events suggests that, after 1950, the number of recorded ice-fall events increased sharply.4AMBIO A Journal of the Human Environment. Megacryometeors: Distribution on Earth and Current Research Whether this reflects a genuine increase in frequency or simply better reporting is a key open question, and it is one of the reasons the phenomenon has attracted attention from climate scientists.
Leading Theories on How They Form
The short answer is that nobody is entirely sure. The leading hypothesis, championed by MartÃnez-FrÃas and collaborators, is that megacryometeors form in the upper troposphere or lower stratosphere, where temperatures are well below freezing and water vapor can nucleate onto particles and grow into large ice masses over extended periods. Unlike hail, which forms and falls within minutes inside a turbulent cloud, a megacryometeor may grow slowly, accumulating ice over a longer timescale before some perturbation dislodges it or causes the air mass supporting it to weaken.
Isotopic analysis of recovered specimens supports this picture. The ratios of oxygen and hydrogen isotopes in the ice are consistent with atmospheric water that has been through normal precipitation cycling at high altitude, not with water from an industrial source, an aircraft’s plumbing, or an extraterrestrial body. The ice is chemically ordinary. It is atmospheric water that has frozen in an extraordinary way.
One proposed mechanism involves unusual thermal or dynamic conditions in the tropopause, the boundary between the troposphere and stratosphere. This region has been measured as cooling in recent decades even as the lower atmosphere warms, a signature of greenhouse gas accumulation. If cooling at these altitudes creates conditions more favorable for ice aggregation, or if changes in wind patterns at these heights allow ice masses to grow undisturbed for longer, that could explain both why megacryometeors form and why they appear to be more frequent now than in the past. This idea is plausible but far from proven, because the upper atmosphere is extremely difficult to observe in the kind of detail that would let researchers catch a megacryometeor actually forming.
Ruling Out Aircraft Ice
Whenever a large chunk of ice falls from a clear sky, the first suspicion is usually that it came from an airplane. Commercial aircraft can accumulate ice on their fuselages, and occasionally that ice breaks off during descent. Lavatory and galley water systems can also leak and freeze at cruising altitude, producing so-called “blue ice” when treated wastewater is involved. These explanations are tempting because aircraft are the most visible human presence at high altitude.
Researchers have developed several ways to distinguish aircraft ice from genuine megacryometeors. The most decisive is chemical analysis. Aircraft-origin ice may contain traces of de-icing fluids like propylene glycol, residues from lavatory chemicals, or coloring agents. Blue ice is often literally blue or green due to the disinfectants used in aircraft lavatories. Megacryometeors, when tested, are chemically clean: they contain the same dissolved minerals and trapped air you would expect in natural atmospheric ice.
Flight path records are another tool. For each ice-fall event, investigators can check whether any aircraft was overhead at the time. In many documented megacryometeor cases, no aircraft was anywhere near the location during the relevant window. The Mejorada del Campo event in 2007 was subjected to precisely this kind of multidisciplinary analysis, combining chemical testing with meteorological and flight-path data, to confirm its atmospheric rather than aircraft origin.5PubMed Central. Monitoring the fall of large atmospheric ice conglomerations: a multianalytical approach to the study of the Mejorada del Campo megacryometeor
That said, aircraft ice does fall with some regularity, and not every large ice chunk that hits the ground is a megacryometeor. The analytical work needed to classify a given event is time-consuming and expensive. Many ice falls, especially those that happen in remote areas or are not immediately reported, are never investigated at all. This means the 46 documented events from 2001 to 2006 are probably a fraction of the true number, with an unknown share of unreported falls and an unknown number of aircraft-ice incidents miscategorized in either direction.
The Climate Connection
The apparent increase in megacryometeor events after the mid-twentieth century has led researchers to ask whether climate change plays a role. This is one of the more speculative aspects of the field, but the reasoning has a straightforward logic. If the dynamics of the upper troposphere and lower stratosphere are changing due to greenhouse gas accumulation, and if those changes include cooling at the tropopause, shifts in jet stream behavior, and altered humidity profiles at high altitude, then the conditions under which large ice masses form and persist could be changing too.
The tropopause cooling hypothesis is the most discussed. The lower stratosphere has cooled over recent decades, a trend well established by satellite and radiosonde data. Meanwhile the troposphere below it has warmed. The resulting steepening of the temperature gradient at the tropopause could conceivably affect ice formation processes in ways that are poorly understood. If this gradient change favors more sustained ice aggregation at altitude, more megacryometeors could be the result.
Skeptics rightly point out that the increase in reports could also reflect better global communication, greater scientific awareness of the phenomenon, or simply more people with cameras and the internet to report what they see. Before 2000, an ice chunk crashing into a field in rural India or Argentina would not have made it into any scientific database. The signal is difficult to separate from the noise of improved reporting. Researchers working on the topic acknowledge this but argue that the isotopic and chemical consistency of recovered specimens across widely separated events points to a real, recurring atmospheric process rather than a series of coincidences and misidentifications.
How Dangerous Are They
A 10-kilogram block of ice falling at terminal velocity is a serious physical hazard. Terminal velocity for an irregularly shaped ice mass of that size is roughly comparable to a dense object dropped from an extreme height: fast enough to punch through a standard roof, shatter on pavement with considerable force, or cause severe injury or death if it struck a person. The Mejorada del Campo specimen went through an industrial roof. Other documented events have cratered soil and damaged vehicles.
No confirmed fatalities from megacryometeors have been recorded in the modern scientific literature, but this is partly a matter of probability. The events are rare, and most of Earth’s surface is uninhabited. The risk to any given person in any given year is vanishingly small. Still, the phenomenon has been discussed in the context of natural hazard assessment, particularly because it is unpredictable. Unlike hail, which comes with observable storm systems and weather warnings, a megacryometeor arrives from a clear sky with no advance notice. There is no forecasting system for these events, and given how poorly understood the formation mechanism is, building one is not on the horizon.
For practical purposes, the hazard is comparable to other low-probability, high-consequence events like being struck by a small meteorite. The difference is that megacryometeors appear to be more frequent than meteorite strikes on inhabited areas, and their frequency may be increasing. Whether this warrants any kind of policy response is an open question. No government agency currently tracks megacryometeor events systematically, and the research community studying them remains small.
What Happens to the Evidence
One of the persistent challenges in megacryometeor research is that ice melts. Unlike a meteorite, which can sit in a museum indefinitely, a megacryometeor begins to disappear the moment it lands. If nobody is around to see it fall, or if it lands in a warm environment, the evidence can be gone within hours. Even when a fall is witnessed, getting the ice into a freezer and then to a laboratory before it degrades takes coordination and luck.
Researchers have tried to establish rapid-response protocols. When a suspected event is reported, the ideal procedure involves collecting as much of the intact ice as possible, sealing it in clean containers, freezing it, and transporting it to a lab equipped for isotopic and chemical analysis. In practice, this often depends on local authorities or bystanders recognizing the significance of a large chunk of ice sitting in a smashed roof or a backyard crater. Many events are never sampled. The ones that are tend to come from urban or suburban areas where witnesses are nearby and reporting chains to scientific teams exist.
The fragility of the evidence means that the catalog of well-characterized megacryometeors is much smaller than the catalog of reported events. A fall can be reported based on a photograph, witness testimony, or the damage it caused, but without a preserved and analyzed sample, it cannot be confirmed as a genuine atmospheric megacryometeor rather than aircraft ice or an unusually large hailstone from an unobserved storm.
Ice From Other Skies
The question of large ice masses falling from atmospheres is not unique to Earth. Planetary scientists studying Saturn’s moon Titan have modeled the behavior of Titan’s hydrocarbon lakes and atmosphere and found that ethane ice precipitation may occur under certain conditions. Titan’s rain-filled ephemeral lakes can experience temperature fluctuations large enough to trigger stratification and ethane ice formation, despite the moon’s surface temperature varying by less than 5 kelvin on typical seasonal and diurnal cycles.6PubMed Central. Stratification Dynamics of Titan’s Lakes via Methane Evaporation The mechanism is completely different from what produces megacryometeors on Earth, involving methane evaporation and hydrocarbon chemistry rather than water ice, but the broader principle that planetary atmospheres can produce surprising ice phenomena under unusual thermal conditions has resonance across the field.
On Earth, the study of megacryometeors remains a niche pursuit. Fewer than a dozen research groups worldwide have published on the topic. Funding is limited, partly because the events are rare enough not to register as a major public safety concern and partly because the mechanism is so poorly constrained that it is difficult to design experiments to study it. Most progress has come from opportunistic analysis of recovered specimens combined with atmospheric modeling. The field is in an early stage, roughly where meteorite science was before standardized collection networks existed. Whether megacryometeors become a bigger area of research may depend on whether the apparent uptick in events continues, and whether any of those events causes the kind of dramatic harm that forces the question onto policymakers’ desks.