Ice: The Solid State of Water

This article delves into the specifics of water ice, a ubiquitous yet often underestimated substance. For a broader understanding of “ices” as they pertain to the vastness of the planetary sciences , one might consult Volatile (astrogeology) . If, however, your interests lie in American law enforcement, you’re looking for United States Immigration and Customs Enforcement . For other interpretations of this deceptively simple word, the Ice (disambiguation) page awaits.

Ice

An ice block

Physical properties

• Density (ρ): 0.9167 [^1] –0.9168 [^2] g/cm³
• Refractive index (n): 1.309

Chemical properties

• Chemical formula: H₂O

Mechanical properties

• Young’s modulus (E): 3400 to 37,500 kg-force /cm³ [^2]
• Tensile strength (σt): 5 to 18 kg-force/cm² [^2]
• Compressive strength (σc): 24 to 60 kg-force/cm² [^2]
• Poisson’s ratio (ν): 0.36±0.13 [^2]

Thermal properties

• Thermal conductivity (k): 0.0053(1 + 0.0015 θ) cal/(cm s K), θ = temperature in °C [^2]
• Linear thermal expansion coefficient (α): 5.5×10⁻⁵ [^2]
• Specific heat capacity (c): 0.5057 − 0.001863 θ cal/(g K), θ = absolute value of temperature in °C [^2]

Electrical properties

• Dielectric constant (εr): ~95 [^3]

Note: The properties of ice, much like human patience, vary substantially with temperature, purity, and other factors.

Ice, in its most fundamental definition, is simply water that has succumbed to the cold, transforming from a liquid into a solid state. This transformation typically occurs at or below the rather arbitrary thresholds of 0 °C , 32 °F , or 273.15 K . It’s not merely a terrestrial phenomenon; ice manifests naturally across our own Earth , on distant planets, within the enigmatic depths of Oort cloud objects, and even as diffuse interstellar ice between stars. Given its naturally occurring crystalline inorganic solid form and ordered structure, ice is, by definition, a mineral . Its appearance is as varied as the motivations behind a bad decision: it can be perfectly transparent, or, depending on the unwelcome presence of impurities like soil particles or trapped air bubbles, it can present as a more or less opaque bluish-white.

On Earth, the vast majority of ice adopts a rather predictable hexagonal crystalline structure , dutifully labelled as ice I h (pronounced, for those who care, “ice one h”). However, the universe, in its infinite capacity for over-complication, allows for at least nineteen distinct phases or packing geometries , depending on the precise interplay of temperature and pressure. The most common phase transition to ice I h occurs when liquid water is simply cooled below 0 °C (273.15 K , 32 °F ) under standard atmospheric pressure . Yet, if water is cooled with unseemly haste—a process known as quenching —it can bypass the crystalline order and form up to three types of amorphous ice. Intriguingly, interstellar ice is predominantly low-density amorphous ice (LDA), suggesting that this less structured form is, in fact, the most abundant type of water ice in the entire cosmos. And for those who appreciate the truly bizarre, when ice is cooled slowly to below −253.15 °C (20 K , −423.67 °F ), correlated proton tunnelling can occur, giving rise to peculiar macroscopic quantum phenomena . Because, why not?

Ice blankets vast portions of the Earth’s surface, particularly in the unforgiving polar regions and above the snow line , where it accumulates from snowflakes to forge colossal glaciers and monolithic ice sheets . As snowflakes and solid chunks of hail , ice is a common form of atmospheric precipitation , and it can also be deposited directly from water vapor to form delicate frost . The transition from solid ice back to liquid water is known as melting, while the direct leap from ice to water vapor, bypassing the liquid phase entirely, is sublimation . These fundamental processes are not mere curiosities; they are integral, playing a key, if often overlooked, role in Earth’s intricate water cycle and the overarching climate . In recent decades, however, the sheer volume of ice on Earth has been in a noticeable decline, a predictable consequence of ongoing climate change . The most significant reductions have been observed in the Arctic and in the mountain ranges outside the polar regions. This loss of grounded ice—that which rests on land, as opposed to floating sea ice —is the primary, and frankly, unavoidable, contributor to global sea level rise . Humanity, in its wisdom, continues to observe.

For millennia, humans, with their characteristic blend of ingenuity and stubbornness, have exploited ice for a multitude of purposes. Some historical structures, designed specifically to store ice for cooling, date back over 2,000 years, a testament to enduring needs. Before the advent of modern refrigeration technology, the only reliable way to preserve food without resorting to chemical preservatives or complex modification was to simply use ice. Beyond preservation, sufficiently solid surface ice has historically rendered waterways navigable for land transport during winter, and in some hardy regions, dedicated ice roads are still meticulously maintained. Naturally, ice also forms the unyielding stage for a significant portion of winter sports , because what else would you do with it?

Physical properties

Further information: Water (properties) § Density of water and ice

The three-dimensional crystal structure of H₂O ice I h, as depicted in (c), is built upon fundamental H₂O molecules (b) precisely positioned on the lattice points within a two-dimensional hexagonal space lattice (a). [^4][^5]

Ice, in its structured form, exhibits a regular crystalline arrangement, fundamentally based on the ubiquitous molecule of water. This molecule, as you might recall from basic chemistry, consists of a single oxygen atom covalently bonded to two individual hydrogen atoms , elegantly represented as H–O–H. However, a significant number of the peculiar physical properties that both water and ice possess are not merely a function of these internal covalent bonds, but are instead profoundly influenced by the formation of transient hydrogen bonds between adjacent oxygen and hydrogen atoms. While these hydrogen bonds are individually weak, their collective influence is absolutely critical in orchestrating the intricate structure of both liquid water and solid ice. [^6]

One of the most counterintuitive, yet fundamentally important, properties of water is its peculiar behavior upon freezing: its solid form—ice, specifically when frozen at atmospheric pressure —is approximately 8.3% less dense than its liquid counterpart. This translates to a volumetric expansion of roughly 9%. To put it plainly, ice insists on taking up more space. The precise density of ice is measured at 0.9167 [^1] –0.9168 [^2] g/cm³ at 0 °C and standard atmospheric pressure (101,325 Pa). In stark contrast, water at the same temperature and pressure has a density of 0.9998 [^1] –0.999863 [^2] g/cm³. Liquid water achieves its maximum density, essentially 1.00 g/cm³, at a temperature of 4 °C. Below this point, as the water cools further, its molecules begin the intricate process of arranging themselves into the open, hexagonal crystals of ice , leading to a gradual decrease in density. This counterintuitive phenomenon is primarily due to the dominance of hydrogen bonding over other intermolecular forces, which dictates a less compact arrangement of molecules in the solid state compared to the liquid. As temperatures plummet further, the density of ice does increase slightly, reaching a value of 0.9340 g/cm³ at a chilling −180 °C (93 K). [^7]

When water transitions into ice, it undergoes a significant increase in volume—around 9% for fresh water. [^8] This expansive force during freezing is not just a scientific curiosity; its effects can be quite dramatic and destructive. It stands as a fundamental cause of freeze-thaw weathering, relentlessly breaking down rock formations in nature, and is a persistent menace to human infrastructure, causing damage to building foundations and roadways through the insidious process of frost heaving . Less grand, but equally disruptive, it’s also a distressingly common reason for household flooding when water pipes, unable to withstand the pressure of expanding freezing water, inevitably burst. [^9]

The fact that ice is less dense than liquid water, and thus floats, is not merely a trivial observation; it is a profound ecological safeguard. This buoyancy prevents bodies of water from freezing solid from the bottom up, a scenario that would be catastrophic for aquatic life. Instead, a protective, sheltered environment is maintained beneath the floating ice layer, shielding the underlying water and its inhabitants from the immediate, brutal extremes of surface weather, such as corrosive wind chill . Furthermore, if this floating ice is sufficiently thin, it allows enough light to penetrate, sustaining the vital photosynthesis carried out by bacterial and algal colonies. [^10] In the more complex realm of seawater, when it freezes, the resulting ice is not solid but permeated with intricate brine-filled channels. These channels are far from empty; they host thriving communities of sympagic organisms , including various bacteria, algae, tiny copepods , and segmented annelids . This microscopic ecosystem, in turn, forms the base of a critical food web, providing sustenance for larger animals such as krill and specialized fish like the bald notothen , which are then preyed upon by even larger creatures, including majestic emperor penguins and colossal minke whales . [^11] It’s a delicate balance, easily disrupted.

Frozen waterfall in southeast New York

When ice melts, it doesn’t just transition; it demands energy. Specifically, it absorbs as much energy as would be required to heat an equivalent mass of liquid water by a substantial 80 °C (176 °F). [^12] Throughout this entire melting process, the temperature of the ice-water mixture remains stubbornly constant at 0 °C (32 °F). Any energy introduced during this phase is not used to increase the kinetic energy of the molecules (and thus the temperature), but rather to break the strong hydrogen bonds that rigidly hold the ice (water) molecules in their solid lattice. Only once a sufficient number of these hydrogen bonds are broken, allowing the substance to be considered fully liquid water, does additional energy become available to increase the thermal energy and, consequently, the temperature. The precise quantity of energy consumed in this bond-breaking transition from ice to water is scientifically termed the heat of fusion . [^12][^8]

Much like liquid water, ice exhibits a selective absorption of light, preferentially absorbing wavelengths at the red end of the visible spectrum. This phenomenon is a direct result of an overtone of an oxygen–hydrogen (O–H) bond stretch within the water molecule. Compared to liquid water, this absorption in ice is subtly shifted towards slightly lower energies. Consequently, large masses of ice tend to appear blue, often with a slightly greener tint than one might observe in deep liquid water. Since this light absorption is a cumulative effect, the intensity of the blue coloration naturally increases with greater thickness of the ice, or if internal reflections within the ice cause light to traverse a longer path. [^13] Of course, other colors can emerge, but only in the unfortunate presence of light-absorbing impurities. In such cases, the impurity itself, rather than the intrinsic properties of the ice, dictates the observed hue. For example, icebergs laden with various impurities—such as sediments, algal blooms, or even trapped air bubbles—can present as earthy brown, muted grey, or even distinct green. [^13]

Given that ice in its natural environments is typically found at temperatures uncomfortably close to its melting point, its inherent hardness displays rather pronounced variations with temperature. At its melting point, ice registers a Mohs hardness of 2 or even less, making it surprisingly yielding. However, as the temperature plummets, its hardness steadily increases, reaching approximately 4 at a bone-chilling −44 °C (−47 °F), and a remarkable 6 at a frigid −78.5 °C (−109.3 °F)—a temperature equivalent to the vaporization point of solid carbon dioxide , commonly known as dry ice. [^14]

Phases

Main article: Phases of ice

Log-lin pressure-temperature phase diagram of water. The Roman numerals correspond to some ice phases listed below. An alternative formulation of the phase diagram for certain ices and other phases of water [^15]

Most liquids, when subjected to increased pressure, predictably freeze at higher temperatures. This is because the external pressure effectively assists in forcing the molecules closer together, facilitating their transition into a more ordered solid state. Water, however, ever the contrarian, behaves differently due to its strong hydrogen bonds. For pressures exceeding 1 atm (0.10 MPa), water actually freezes at a temperature below 0 °C (32 °F). This is not an error; it’s just water being itself. The unique condition where ice, liquid water, and water vapour can all coexist in a delicate equilibrium is known as the triple point . This precise point occurs at exactly 273.16 K (0.01 °C) at a pressure of 611.657 Pa . [^16][^17] For a time, the kelvin unit of temperature was defined as precisely ¹⁄₂₇₃.₁₆ of the difference between this triple point and absolute zero , [^18] although, as with many things, this definition shifted in May 2019. [^19] Unlike the majority of other solids, ice is remarkably difficult to superheat . In one fleeting experiment, ice at −3 °C was momentarily superheated to approximately 17 °C for a mere 250 picoseconds . [^20] Such are the fleeting triumphs of science.

When subjected to increasingly higher pressures and varying temperatures, ice reveals an astonishing complexity, capable of forming in nineteen distinct, currently known crystalline phases, each with its own unique density. Furthermore, there are hypothetical proposed phases that scientists have yet to definitively observe. [^21] With meticulous care, at least fifteen of these phases (with ice X being one of the notable exceptions) can be retrieved and stabilized at ambient pressure and low temperature in a metastable form. [^22][^23] These diverse types of ice are meticulously differentiated not only by their specific crystalline structure but also by the ordering of their protons [^24] and their individual densities. Beyond these ordered structures, two metastable phases of ice exist under pressure, both characterized by a complete disordering of their hydrogen atoms: these are Ice IV and Ice XII. Ice XII, a relatively recent discovery, was first identified in 1996. A decade later, in 2006, Ice XIII and Ice XIV were added to the roster. [^25] It is worth noting that Ices XI, XIII, and XIV are simply the hydrogen-ordered counterparts of ices I h, V, and XII, respectively. In 2009, yet another phase, ice XV, was unearthed at profoundly high pressures and a chilling −143 °C. [^26] At even more extreme pressures, scientific prediction suggests that ice is destined to transform into a metal —a truly alien state for water. This metallic transition has been variously estimated to occur at pressures of 1.55 TPa [^27] or, more conservatively, 5.62 TPa. [^28]

Beyond its crystalline manifestations, solid water can also exist in amorphous states, collectively known as amorphous solid water (ASW), which themselves come in various densities. In the frigid expanse of outer space, while hexagonal crystalline ice does manifest in the spectacular form of ice volcanoes , [^29] it is otherwise exceedingly rare. Even seemingly icy moons, such as Ganymede , are largely expected to be composed of other, more exotic crystalline forms of ice. [^30][^31] The water found within the vast interstellar medium is overwhelmingly dominated by amorphous ice, solidifying its status as likely the most common form of water in the entire universe. [^32] Low-density ASW (LDA), sometimes known by the rather poetic name of hyperquenched glassy water, may be a key component of noctilucent clouds on Earth and typically forms through the deposition of water vapor in intensely cold or vacuum conditions. [^33] Conversely, high-density ASW (HDA) is generated by compressing ordinary ice I h or LDA under immense GPa pressures. An even denser variant, very-high-density ASW (VHDA), is produced by slightly warming HDA to 160 K while maintaining pressures of 1–2 GPa. [^34]

Theorized superionic water ice may possess a dual crystalline structure. At pressures exceeding 500,000 bars (7,300,000 psi), this superionic ice is predicted to adopt a body-centered cubic arrangement. However, if pressures continue to escalate beyond 1,000,000 bars (15,000,000 psi), the structure may undergo a further shift to a more stable face-centered cubic lattice. It is speculated, with a certain cosmic indifference, that such superionic ice could constitute the dense, enigmatic interiors of ice giants like Uranus and Neptune. [^35]

Friction properties

Takahiko Kozuka figure skating – an act which is only possible due to ice’s low frictional properties

Ice is famously, almost comically, “slippery ” due to its remarkably low coefficient of friction . This seemingly simple observation became the subject of serious scientific inquiry as far back as the 19th century. The prevailing explanation at the time, “pressure melting ,” posited that the sheer pressure exerted by, say, the blade of an ice skate, would melt a minuscule, thin layer of ice, thereby providing the necessary lubrication for effortless gliding. [^36] However, research conducted in 1939 by the venerable Frank P. Bowden and T. P. Hughes introduced a rather inconvenient truth: if pressure melting were the sole explanation, skaters would experience significantly more friction than they actually do. Furthermore, the optimal temperature for the rather elegant art of figure skating is a brisk −5.5 °C (22 °F; 268 K), while for the more aggressive sport of ice hockey, it’s an even colder −9 °C (16 °F; 264 K). According to the pressure melting theory, skating below −4 °C (25 °F; 269 K) would be utterly impossible. [^37] Bowden and Hughes, ever the pragmatists, argued that the heating and subsequent melting of the ice layer was primarily caused by friction itself, rather than pressure. Yet, even this theory doesn’t quite explain why ice retains its slipperiness even when one is simply standing still, even at sub-zero temperatures. [^36] The universe, it seems, enjoys its mysteries.

Subsequent research, ever attempting to fill the gaps, suggested that ice molecules at the very interface—the boundary layer—are unable to properly bond with the bulk mass of ice beneath them. This leaves these surface molecules in a perpetual semi-liquid state, providing a constant, if subtle, lubrication, irrespective of any pressure exerted by an object resting on the ice. However, even this seemingly elegant hypothesis has its detractors, with experiments utilizing atomic force microscopy demonstrating a surprisingly high coefficient of friction for ice under certain conditions. [^37] Thus, the precise mechanism governing the frictional properties of ice remains an active, and somewhat exasperating, area of scientific study. [^38] A truly comprehensive theory of ice friction, should one ever fully materialize, would need to meticulously account for all these aforementioned mechanisms to accurately estimate the friction coefficient of ice against various materials as a function of both temperature and sliding speed. Recent research from 2014, with a hint of resignation, suggests that frictional heating is, under most typical conditions, the most significant process at play. [^39] Sometimes, the simplest explanation, however unsatisfying, is the correct one.

Natural formation

Frozen landscape in the Northwest Territories of Canada . A large ice circle can be clearly seen floating on water. [^40][^41]

The collective term that encompasses all parts of the Earth’s surface where water exists in its frozen form is the cryosphere . Ice, in its many guises, is an indispensable component of the global climate system, particularly in its profound influence on the water cycle . Glaciers and expansive snowpacks serve as crucial, albeit temporary, reservoirs for fresh water; over time, these vast stores either sublimate directly into the atmosphere or melt, releasing their precious contents. The seasonal phenomenon of snowmelt is a particularly vital source of fresh water for countless ecosystems and human populations. [^42][^43] The World Meteorological Organization , in its meticulous fashion, categorizes numerous types of ice based on their origin, size, shape, environmental influence, and other distinguishing characteristics. [^44] Among the more intriguing forms are clathrate hydrates , specialized types of ice that possess a crystal lattice capable of trapping gas molecules within their structure. [^45][^46]

In the oceans

Main article: Sea ice

Ice encountered at sea is not a monolithic entity; it manifests in several distinct forms. It can appear as drift ice , untethered and floating freely in the water, or as fast ice , rigidly anchored to a shoreline. A more peculiar variant is anchor ice , which, defying expectations, attaches itself directly to the seafloor. [^47] Majestic masses of ice that calve —that is, break off—from an ice shelf or a coastal glacier venture forth as formidable icebergs . [^48] The turbulent aftermath of such calving events often leaves behind a chaotic, loose mixture of snow and ice, aptly named ice mélange . [^49]

The formation of sea ice is a progression through several discernible stages, each with its own subtle characteristics. Initially, minuscule, millimeter-scale ice crystals begin to accumulate on the water surface, a delicate stage known as frazil ice . As these nascent crystals coalesce and grow somewhat larger and more consistent in their shape and coverage, the water surface takes on an almost “oily” sheen when viewed from above—a phase aptly termed grease ice . [^50] Following this, the ice continues its aggregation, solidifying into flatter, more cohesive pieces, the familiar ice floes . These ice floes are the fundamental, modular building blocks of the expansive sea ice cover, and their horizontal dimensions, measured as half of their diameter , can vary dramatically, from mere centimeters for the smallest fragments to hundreds of kilometers for the most immense formations. [^51] An area where over 70% of the surface is covered by ice is officially classified as being blanketed by pack ice. [^52]

Fully formed sea ice is not static; it can be subjected to immense forces from ocean currents and powerful winds, which can relentlessly push and pile these ice formations together, creating impressive pressure ridges that can tower up to 12 meters (39 ft) in height. [^53] Conversely, vigorous wave activity can relentlessly break down and “polish” sea ice into small, remarkably regular, circular pieces, a phenomenon known as pancake ice . [^54] In even rarer instances, the combined action of wind and waves can refine sea ice into perfectly spherical pieces, whimsically referred to as ice eggs . [^55][^56]

• Grease ice in the Bering Sea
• Loose drift ice on the east coast of Greenland
• Ice eggs (diameter 5–10 cm) on Stroomi Beach, Tallinn, Estonia
• Ice floes in Antarctica, 1919
• A first-year sea ice ridge in the Central Arctic, photographed by the MOSAiC expedition on 4 July 2020
• Ice mélange on Greenland’s western coast, 2012
• Anchor ice on the seafloor at McMurdo Sound , Antarctica.

On land

NASA image of the Antarctic ice sheet

The most monumental ice formations on Earth are undoubtedly the two colossal ice sheets that almost entirely engulf the world’s largest island, Greenland , and the vast continent of Antarctica . These monolithic ice sheets boast an average thickness exceeding 1 kilometer (0.6 miles) and have persisted for millions of years, silent witnesses to geological time. [^57][^58]

Beyond these two giants, other significant terrestrial ice formations include smaller ice caps , sprawling ice fields , dynamic ice streams , and the more familiar glaciers . The Hindu Kush region, for instance, has earned the evocative moniker of the Earth’s “Third Pole” due to the staggering number of glaciers it harbors. These glaciers collectively cover an expansive area of approximately 80,000 km² (31,000 sq mi), holding a combined volume estimated between 3,000 and 4,700 km³. [^42] These vital glaciers are affectionately, and accurately, nicknamed “Asian water towers,” as their life-giving meltwater run-off feeds into a network of rivers that ultimately provide water for an estimated two billion people. [^43] A precarious dependency, to say the least.

Permafrost is defined as soil or underwater sediment that remains continuously at or below 0 °C (32 °F) for a minimum of two consecutive years. [^59] The ice embedded within permafrost is typically categorized into four main types: pore ice, vein ice (also known as ice wedges), buried surface ice, and intrasedimental ice (resulting from the freezing of subterranean waters). [^60] A fascinating example of ice formation in permafrost regions is aufeis —a layered ice deposit that characteristically forms in Arctic and subarctic stream valleys. Here, ice frozen within the stream bed acts as a blockage, impeding the normal discharge of groundwater. This obstruction causes the local water table to rise, forcing water to discharge on top of the already frozen layer. This newly exposed water then freezes, which in turn causes the water table to rise even further, perpetuating the cycle. The cumulative result is a stratified ice deposit, often several meters thick, a slow-motion geological sculpture. [^61] The concepts of the snow line and snow fields are intrinsically linked, with snow fields accumulating atop an ice deposit and ablating away, ultimately reaching an equilibrium point defined by the snow line. [^62]

On rivers and streams

A small frozen rivulet

Ice that forms on moving water, as one might expect, tends to be less uniform and inherently less stable than its counterparts on calm bodies of water. Ice jams (sometimes ominously referred to as “ice dams”), which occur when broken chunks of ice pile up in a chaotic manner, represent the most significant ice-related hazard on rivers. These jams can trigger destructive flooding, inflict severe damage upon structures located in or near the river, and pose a threat to vessels navigating the waterway. In industrial contexts, particularly for hydropower facilities, ice jams can necessitate a complete shutdown of operations. A related phenomenon, an ice dam, refers to a blockage caused by the movement of a glacier, which can lead to the formation of a proglacial lake . Furthermore, heavy flows of ice in rivers can damage vessels and frequently require the deployment of specialized icebreaker ships to maintain navigation. [^63][^64]

Ice discs are a particularly captivating natural phenomenon: perfectly circular formations of ice that rotate gracefully on river water. Their formation is attributed to localized eddy currents , and their continuous, slow rotation is a result of asymmetric melting caused by their position within these currents. [^65][^66] A subtle dance between ice and fluid dynamics.

On lakes

Candle ice in Lake Otelnuk, Quebec, Canada

On calm bodies of water like lakes, ice typically begins its formation from the shores, gradually spreading inward as a thin layer across the surface, and then extending downward into the water column. Lake ice is generally categorized into four distinct types: primary, secondary, superimposed, and agglomerate. [^67][^68] Primary ice is the initial layer that forms. Secondary ice then develops beneath this primary layer, growing in a direction parallel to the flow of heat. Superimposed ice forms on top of the existing ice surface, usually from rain or water that seeps up through cracks in the ice, often settling when burdened by a layer of snow. A more dramatic event, an ice shove , occurs when the expansive movement of ice, driven by thermal expansion and/or wind action, becomes so pronounced that it violently pushes onto the shores of lakes, frequently displacing the very sediment that constitutes the shoreline. [^69]

Shelf ice is a perilous formation, created when floating pieces of ice are driven by the wind and pile up against the windward shore. This type of ice often conceals large, treacherous air pockets beneath a deceptively thin surface layer, rendering it particularly hazardous for anyone attempting to traverse it on foot. [^70] Another notoriously dangerous form of rotten ice for pedestrians is candle ice, which develops in vertical columns, perpendicular to the surface of a lake. Its lack of a firm horizontal structure means that a person unfortunate enough to fall through it would find virtually nothing solid to grasp onto, making self-extraction incredibly difficult. [^71]

As precipitation

Main article: Precipitation

Snow and freezing rain

Main articles: Snow and Snowflake

Snowflakes by Wilson Bentley , 1902

Snow crystals, those delicate marvels, begin their existence when minuscule supercooled cloud droplets—typically around 10 μm in diameter—succumb to the cold and freeze . These tenacious droplets possess the remarkable ability to remain in a liquid state at temperatures as low as −18 °C (255 K; 0 °F). This is because, for them to freeze, a few molecules within the droplet must, by pure chance, spontaneously align themselves into an arrangement similar to that found in an ice lattice; the droplet then freezes around this nascent “nucleus.” Experimental evidence suggests that this “homogeneous” nucleation of cloud droplets only occurs at temperatures plummeting below −35 °C (238 K; −31 °F). [^72] In warmer clouds, where such spontaneous alignment is less probable, an aerosol particle, or “ice nucleus,” must be present within (or in direct contact with) the droplet to initiate the freezing process. Our current understanding of precisely which particles serve as efficient ice nuclei remains, regrettably, rather poor. What we do know is that they are exceedingly rare compared to the cloud condensation nuclei upon which liquid droplets initially form. Clays, desert dust, and certain biological particles may act as effective nuclei, [^73] though the extent of their influence is still debated. Artificial nuclei, of course, are deliberately employed in the practice of cloud seeding . [^74] Once nucleated, the droplet then grows steadily through the ongoing condensation of water vapor onto its expanding ice surfaces. [^75]

An ice storm is a specific type of winter storm, characterized by the insidious phenomenon of freezing rain . This precipitation creates a slick, transparent glaze of ice on virtually all exposed surfaces, including treacherous roads and vulnerable power lines . In the United States, approximately a quarter of all winter weather events produce this glaze ice, necessitating that utility companies maintain a state of preparedness to mitigate the inevitable damages. [^76]

Hard forms

Further information: Ice crystal

A large hailstone, about 6 cm (2.4 in) in diameter

Hail forms within the turbulent confines of storm clouds when supercooled water droplets instantly freeze upon contact with available condensation nuclei , such as particles of dust or dirt . The powerful updraft within the storm then carries these embryonic hailstones to the upper reaches of the cloud. As the updraft eventually dissipates, the hailstones fall, only to be caught by another updraft, lifted skyward once more, and so the cycle repeats. To be classified as hail, these ice formations must possess a diameter of 5 millimeters (0.20 inches) or more. [^77] Within the cryptic lexicon of METAR code, “GR” is reserved for larger hail, specifically those with a diameter of at least 6.4 millimeters (0.25 inches), while “GS” denotes smaller variants. [^78] In North America, the most frequently reported hail sizes are 19 millimeters (0.75 inches), 25 millimeters (1.0 inch), and 44 millimeters (1.75 inches). [^79] However, hailstones are capable of growing to truly formidable dimensions, reaching diameters of up to 15 centimeters (6 inches) and weighing in excess of 0.5 kilograms (1.1 pounds). [^80] In particularly large hailstones, the latent heat released by further freezing can paradoxically cause the outer shell of the hailstone to melt. The hailstone then enters a phase of ‘wet growth,’ where this liquid outer shell efficiently collects other, smaller hailstones. [^81] Through this process, the hailstone progressively gains additional layers of ice, growing ever larger with each ascent into the cloud. Once a hailstone achieves a weight too substantial to be supported by the storm’s updraft, it finally succumbs to gravity and falls from the cloud. [^82] A rather dramatic life cycle, if you ask me.

Soft hail, or graupel, in Nevada

Hail preferentially forms in vigorous thunderstorm clouds, particularly those characterized by intense updrafts, a high liquid water content, significant vertical extent, large water droplets, and where a substantial portion of the cloud layer remains below the freezing point of 0 °C (32 °F). [^77] Hail-producing clouds are often, somewhat ominously, identifiable by their distinct green coloration. [^83][^84] The growth rate of hailstones is maximized at approximately −13 °C (9 °F), becoming negligibly small much below −30 °C (−22 °F) as the availability of supercooled water droplets diminishes. For this reason, hail is most common within the continental interiors of the mid-latitudes, as its formation is considerably more probable when the freezing level is situated below an altitude of 11,000 feet (3,400 meters). [^85] The entrainment of dry air into strong continental thunderstorms can, counterintuitively, increase the frequency of hail by promoting evaporative cooling, which effectively lowers the freezing level of the thunderstorm clouds, thereby providing a larger volume for hail to grow within. Accordingly, despite a much higher frequency of thunderstorms, hail is actually less common in the tropics because the atmosphere over tropical regions tends to be warmer over a significantly greater vertical depth. In the tropics, hail primarily occurs at higher elevations, where the necessary cold temperatures can be found. [^86]

An accumulation of ice pellets

Ice pellets (METAR code PL [^78]) constitute a form of precipitation comprising small, translucent balls of ice, which are generally smaller than true hailstones. [^87] This particular form of precipitation is also colloquially referred to as “sleet” by the United States National Weather Service . [^88] (It’s worth noting, for clarity, that in British English , “sleet” typically refers to a mixture of rain and snow .) Ice pellets commonly form in conjunction with freezing rain, a scenario that arises when a moist warm front becomes sandwiched between colder and drier atmospheric layers. Within this cold layer, falling raindrops will both freeze solid and simultaneously shrink in size due to the effects of evaporative cooling. [^89] So-called snow pellets, or graupel , form when multiple individual water droplets freeze onto existing snowflakes, eventually creating a soft, ball-like shape. [^90] Finally, the ethereal phenomenon known as “diamond dust ” (METAR code IC [^78]), also referred to as ice needles or ice crystals, forms at intensely cold temperatures approaching −40 °C (−40 °F). This occurs when air with slightly higher moisture content from aloft mixes with colder, surface-based air, resulting in the direct crystallization of water vapor into tiny ice prisms. [^91]

On surfaces

As water drips and then repeatedly re-freezes, it can create elegant, hanging icicles , or, on the ground, build up into stalagmite -like structures. [^92] On sloped roofs, the accumulation of ice can lead to the formation of an ice dam , a problematic barrier that prevents meltwater from properly draining. This can, and often does, result in damaging leaks into the structure below. [^93] More broadly, the deposition of water vapor onto cold surfaces, driven by high relative humidity and subsequent freezing, gives rise to various forms of atmospheric icing , commonly known as frost . Within buildings, this can be observed as a delicate layer of ice forming on the interior surface of un-insulated windows. [^94] Hoar frost, with its feathery crystals, is a common sight in natural environments, particularly prevalent in low-lying areas such as valleys . [^95] In the extreme cold of Antarctica, temperatures can plunge so low that electrostatic attraction between ice crystals intensifies significantly. This allows hoarfrost on the snow to stick together when propelled by the wind, forming peculiar, tumbleweed -like balls known as yukimarimo . [^96] The universe has a strange sense of humor.

Occasionally, water droplets crystallize onto cold objects as rime rather than the more common glaze. Soft rime, characterized by a high proportion of trapped air, has a density typically between a quarter and two-thirds that of pure ice, [^97] which also gives it its characteristic white, opaque appearance. Hard rime, by contrast, is denser, more transparent, and is more frequently observed accreting on ships and aircraft, posing a significant hazard. [^98][^99] A specific type of frost, known as advection frost, is caused by cold wind directly colliding with objects. When this phenomenon occurs on plants, it frequently leads to damage and crop loss. [^100] Various methods have been devised to protect agricultural crops from frost—ranging from the simple act of covering them to the deployment of large wind machines. [^101][^102] In recent decades, irrigation sprinklers have been ingeniously calibrated to spray precisely enough water to preemptively create a protective layer of ice. This layer forms slowly, preventing a sudden temperature shock to the plant, and is carefully controlled to avoid becoming so thick as to cause damage through its sheer weight. [^101] A testament to human persistence, if nothing else.

• Grass partially covered in hoarfrost, 2008
• Frost on a thistle in Hausdülmen , North Rhine-Westphalia , Germany
• A spiderweb covered in frost
• Ice on deciduous tree after freezing rain
• Icicles on a stairway in Seattle , 1968
• Fern frost on a window
• Hoar frost atop snow
• Yukimarimo at South Pole Station , Antarctica, in 2008

Ablation

Main article: Ablation

Different stages of ice melt in a pond The melting of floating ice

The term “ablation” of ice refers comprehensively to both its melting and its dissolution . [^103] It’s the universe’s way of saying, “nothing lasts forever.”

The melting of ice fundamentally involves the breaking of the intricate hydrogen bonds that rigidly hold water molecules in their solid, crystalline structure. As these bonds yield, the highly ordered arrangement of molecules in the solid state breaks down into a more chaotic, less ordered liquid state. This transformation is achieved by augmenting the internal energy of the ice beyond its critical melting point . When ice melts, it demands a significant amount of energy, absorbing as much as would be required to heat an equivalent mass of water by a substantial 80 °C. During this entire melting process, the temperature of the ice surface remains stubbornly constant at 0 °C. The rate at which this melting occurs is directly dependent on the efficiency of the energy exchange process. An ice surface immersed in fresh water melts predominantly through free convection , with a rate that exhibits a linear dependence on the water temperature, T∞, when T∞ is less than 3.98 °C. However, the relationship becomes superlinear when T∞ is equal to or greater than 3.98 °C, with the rate being proportional to (T∞ − 3.98 °C)α, where α = ⁵⁄₃ for T∞ significantly greater than 8 °C, and α = ⁴⁄₃ for intermediate temperatures. [^104]

In environments characterized by salty conditions, dissolution rather than pure melting often becomes the primary mechanism for ice ablation. For example, the ambient temperature of the Arctic Ocean is typically below the conventional melting point of the sea ice found there. In such cases, the phase transition from solid to liquid is achieved through the mixing of salt and water molecules, a process analogous to the dissolution of sugar in water, even if the surrounding water temperature is considerably below the sugar’s melting point. However, this dissolution rate is inherently limited by the concentration of salt, and is consequently a slower process than direct melting. [^105]

Role in human activities

Cooling

A schematic showing how the ancient yakhchals used ice to provide radiative cooling

Ice has, for millennia, been highly valued as a simple yet effective means of cooling. As far back as 400 BC in Iran, resourceful Persian engineers had already perfected sophisticated techniques for storing ice within the arid desert climate throughout the scorching summer months. During the winter, ice was meticulously transported from harvesting pools and nearby mountains in substantial quantities, destined for storage in specially designed, naturally cooled structures known as yakhchal (a term that literally translates to “ice storage”). These yakhchals were massive underground spaces, some reaching volumes of up to 5000 m³, characterized by incredibly thick walls—at least two meters at the base—constructed from a unique type of mortar called sarooj . This sarooj was an ingenious composite material, formulated from sand, clay, egg whites, lime, goat hair, and ash. The mortar was specifically engineered to be highly resistant to heat transfer, effectively insulating the ice and maintaining temperatures low enough to prevent melting; it was also remarkably impenetrable by water. Yakhchals frequently incorporated a qanat (an ancient underground aqueduct system) and an intricate network of windcatchers that could dramatically lower internal temperatures to frigid levels, even amidst the peak heat of summer. One luxurious application for this painstakingly preserved ice was the creation of chilled treats for royalty. [^106][^107] Because nothing says “power” like ice cream in the desert.

Harvesting

Main articles: Ice cutting and Ice trade

In 16th–17th century England, a thriving industry emerged where low-lying areas along the Thames Estuary were deliberately flooded during the winter months. The resulting ice was then harvested, transported in carts, and stored inter-seasonally in insulated wooden houses, serving as a provision for the icehouse typically found on large country estates. This ice was widely utilized to keep fish fresh, particularly those caught in distant waters. This practice was purportedly adopted by an Englishman who had observed similar activities in China. By as early as 1823, ice was being imported into England from Norway on a considerable scale, highlighting its growing commercial importance. [^108]

Across the Atlantic, in the United States, the inaugural cargo of ice was dispatched from New York City to Charleston, South Carolina , in 1799. [^108] By the first half of the 19th century, ice harvesting had burgeoned into a substantial and profitable business. Frederic Tudor , a visionary entrepreneur who earned the moniker “Ice King,” dedicated his efforts to developing superior insulation products for the long-distance shipment of ice, especially to tropical climates. This ambitious endeavor became known as the ice trade . [^109]

Harvesting ice on Lake St. Clair in Michigan , c. 1905

The international flow of ice was surprisingly extensive: Trieste exported ice to distant locales such as Egypt , Corfu , and Zante ; Switzerland supplied France; and Germany, at times, relied on imports from Bavarian lakes. [^108] A more recent, and rather charming, example comes from Hungary: from the 1930s until as late as 1994, the majestic Hungarian Parliament building utilized ice harvested during the winter from Lake Balaton for its air conditioning system. [^110]

Ice houses were purpose-built structures designed to store ice formed during the winter, ensuring its availability throughout the year. An early form of refrigerator , known simply as an icebox , was cooled by placing a large block of ice inside. Many cities maintained a regular ice delivery service during the summer months, a logistical feat now rendered obsolete by the widespread advent of artificial refrigeration technology. [^111]

Today, ice is still purposefully harvested, primarily for extravagant ice and snow sculpture events . For instance, a specialized swing saw is employed each year to extract massive blocks of ice from the frozen surface of the Songhua River for the renowned Harbin International Ice and Snow Sculpture Festival in China. [^112] Because some traditions, however cumbersome, must endure.

Artificial production

Main article: Icemaker

Layout of a late 19th-century ice factory

The earliest known written account detailing a process for artificially creating ice can be traced back to the 13th-century writings of the Arab historian Ibn Abu Usaybia . In his comprehensive book on medicine, Kitab Uyun al-anba fi tabaqat-al-atibba, Ibn Abu Usaybia attributes this intriguing process to an even older, largely unknown author named Ibn Bakhtawayhi. [^113]

Today, ice is produced on a formidable industrial scale, serving a diverse array of applications. These include the crucial functions of food storage and processing, various chemical manufacturing processes, the precise mixing and curing of concrete, and, of course, the ubiquitous consumer or packaged ice. [^114] Most commercial icemakers are engineered to produce three fundamental types of fragmentary ice: flake, tubular, and plate, each achieved through a variety of specialized techniques. [^114] Large-batch ice makers possess the capacity to churn out up to 75 tons of ice per day, a testament to modern efficiency. [^115] As of 2002, the United States alone boasted 426 commercial ice-making companies, collectively generating a value of shipments totaling a substantial $595,487,000. [^116] In the domestic sphere, modern home refrigerators are commonly equipped with a built-in icemaker , typically producing convenient ice cubes or crushed ice. The first such integrated device for domestic use was unveiled in 1965 by Frigidaire . [^117] Because even the mundane demands precision.

Land travel

Ice formation on exterior of vehicle windshield

Ice forming on roads is a perennial winter hazard, with black ice presenting a particularly insidious danger due to its extreme difficulty to perceive. It is not only highly transparent but also frequently forms specifically in shaded (and thus cooler and darker) areas, such as beneath overpasses , where its presence is almost impossible to detect until it’s too late. [^118]

Whenever freezing rain or snow occurs at temperatures hovering near the melting point, it is a common and frustrating occurrence for ice to accumulate on the windows of vehicles. Often, snow melts slightly, then re-freezes, forming a fragmented yet stubborn layer of ice that effectively “glues” the remaining snow to the window. In such instances, this frozen mass is typically removed with the aid of a trusty ice scraper . [^119] A thin, crystalline layer of ice can also form on the inside surface of car windows during sufficiently cold weather. In the 1970s and 1980s, some luxury vehicles, such as the Ford Thunderbird , offered the option of heated windshields to combat this. However, this technology eventually fell out of favor due to its high cost and susceptibility to damage, while rear-window defrosters , being more economical to maintain, have become far more widespread. [^120]

1943 US propaganda film explaining how the ice of Lake Ladoga became the Road of Life during WWII

In sufficiently frigid regions, the layers of ice accumulating on water surfaces can reach a thickness robust enough to support the construction of dedicated ice roads . Certain regulations (though which specific ones are often left unstated, as if by mystical decree) specify minimum safe thicknesses: typically 4 inches (10 cm) for a person, 7 inches (18 cm) for a snowmobile , and 15 inches (38 cm) for an automobile weighing less than 5 tonnes. For heavier trucks , the required effective thickness varies significantly with the load carried—for instance, a vehicle with a 9-ton total weight necessitates an ice thickness of 20 inches (51 cm). Notably, the speed limit (though the location of this limit is also often left to the imagination) for a vehicle traveling on an ice road that barely meets its minimum safe thickness is a cautious 25 km/h (16 mph), incrementally increasing to 35 km/h (22 mph) if the road’s thickness is two or more times greater than the minimum safe value. [^121] There is even a documented historical instance where an entire railroad was constructed on ice. [^122]

A particularly poignant example of an ice road’s critical importance is the Road of Life across Lake Ladoga . This vital artery operated during the brutal winters of 1941–1942 and 1942–1943, serving as the only land route available to the Soviet Union for relieving the desperate Siege of Leningrad by the German Army Group North . [^123] Over those unforgiving winters, trucks traversed the frozen lake, delivering hundreds of thousands of tonnes of desperately needed supplies into the besieged city, and simultaneously evacuating hundreds of thousands of suffering civilians. [^124] Today, this historic route stands recognized as a World Heritage Site . [^125] A chilling testament to human endurance and ingenuity under duress.

Water-borne travel

Channel through ice for ship traffic on Lake Huron with ice breakers in background

For ships, ice presents two distinct and potentially catastrophic hazards. Firstly, the combination of ocean spray and freezing rain can lead to a dangerous accumulation of ice on a vessel’s superstructure. This ice build-up can become substantial enough to compromise the ship’s stability, potentially leading to the dire consequence of capsizing . [^126] Historically, crewmembers were routinely forced into the perilous task of manually hacking off ice accumulations. However, since the 1980s, the more civilized methods of spraying de-icing chemicals or melting the ice with hot water/steam hoses have become more commonplace. [^127] The second, and perhaps more famously dramatic, hazard comes from icebergs —colossal masses of ice floating in water, typically formed when glaciers reach the sea. These titans of ice pose an existential threat if struck by a ship underway, as tragically demonstrated by the sinking of numerous vessels, most notably the ill-fated RMS Titanic. [^128]

For harbors situated near the poles , maintaining an ice-free status, ideally throughout the entire year, is a paramount strategic advantage. Notable examples include Murmansk (Russia), Petsamo (Russia, formerly Finland), and Vardø (Norway). Harbors that are not naturally ice-free require specialized vessels, known as icebreakers , to forge open routes. These icebreakers are also indispensable for clearing paths through expansive sea ice for other vessels, as the only natural alternatives are the unpredictable openings called “polynyas ” or “leads ”. The widespread production of icebreakers commenced during the 19th century. Earlier designs were relatively simple, featuring reinforced bows with a spoon-like or diagonal shape, intended to effectively crush the ice. Later, more advanced designs incorporated a forward propeller positioned underneath the protruding bow, addressing the limitation of typical rear propellers which were often incapable of effectively steering the ship through dense ice. [^129]

Air travel

Further information: Icing conditions and Carburetor icing

Rime ice on the leading edge of an aircraft wing. When the build-up is too large, the black deicing boot inflates to shake it off. [^130][^131]

For aircraft, the presence of ice introduces a multitude of potential dangers, a constant, nagging threat in the skies. As an aircraft ascends, it inevitably traverses through atmospheric layers of varying temperature and humidity, some of which are unfortunately conducive to ice formation. Should ice accumulate on critical surfaces like the wings or control surfaces, it can severely compromise the aircraft’s aerodynamic performance and handling qualities, making it unstable and difficult to control. A dramatic historical example occurred in 1919, during the first non-stop flight across the Atlantic . The intrepid British aviators, Captain John Alcock and Lieutenant Arthur Whitten Brown , encountered severe icing conditions. Brown, demonstrating extraordinary bravery (or perhaps sheer desperation), repeatedly left the relative safety of the cockpit, climbing onto the wing of their Vickers Vimy aircraft to manually remove ice that was dangerously obstructing the engine air intakes. [^132]

One particular vulnerability caused by icing, especially associated with reciprocating internal combustion engines, is carburetor icing . As air is drawn through the carburetor into the engine, the local air pressure decreases, leading to a phenomenon known as adiabatic cooling. Consequently, in humid, near-freezing conditions, the carburetor’s temperature can drop below freezing, causing ice to form. This ice accumulation can then restrict the crucial supply of air to the engine, potentially leading to engine failure. Between 1969 and 1975 alone, 468 such instances were officially recorded, resulting in 75 aircraft losses, 44 fatalities, and 202 serious injuries. [^133] To combat this persistent threat, carburetor air intake heaters were developed. Furthermore, modern reciprocating engines equipped with fuel injection systems bypass the need for carburetors entirely, thereby eliminating this specific icing vulnerability. [^134]

Jet engines , while immune to carburetor icing, face their own unique ice-related challenges. The moisture inherently present in jet fuel can freeze and form ice crystals, which possess the potential to clog fuel lines and restrict the flow of fuel to the engine. To mitigate this, aircraft employ fuel heaters and/or specialized de-icing additives. [^135] Because in the air, there’s no margin for error.

Recreation and sports

Main article: Ice sports

Skating fun by 17th century Dutch painter Hendrick Avercamp

Ice, in its frozen ubiquity, plays a central and often thrilling role in winter recreation and a myriad of sports. Activities such as the graceful art of ice skating , the endurance challenge of tour skating , the aggressive dynamics of ice hockey , the lesser-known but equally demanding sport of bandy , the patient pursuit of ice fishing , the vertical challenge of ice climbing , the strategic precision of curling , the chaotic fun of broomball , and the high-speed thrills of sled racing in bobsled , luge , and skeleton all depend entirely on this solid form of water. Many of these distinct sports garner international attention every four years during the spectacle of the Winter Olympic Games . [^136] A peculiar way to spend a global gathering, if you ask me.

Beyond the traditional, small, boat-like craft, fitted with blades, can be propelled across vast expanses of ice by powerful sails . This exhilarating sport is known as ice yachting , a practice that has been enjoyed for centuries. [^137][^138] Another vehicular sport, ice racing , pits drivers against each other on frozen lake surfaces, demanding not only speed but also exceptional control over their vehicle’s inevitable skids—a challenge somewhat akin to dirt track racing . The sport has even been adapted for the confined spaces of ice rinks . [^139] Humanity’s creativity knows no bounds when it comes to finding ways to risk life and limb for entertainment.

Other uses

Carving an ice sculpture

As thermal ballast

• Ice, in its most basic form, continues to be a practical and effective medium for cooling and preserving food within portable coolers . [^111] A classic, enduring solution.
• The humble ice cube or crushed ice is routinely deployed to chill beverages. As the ice undergoes its inevitable melting, it absorbs heat, thus maintaining the drink at a refreshingly cool temperature near 0 °C (32 °F). [^140]
• Ice can be ingeniously integrated as a component of an air conditioning system , utilizing battery- or solar-powered fans to circulate warm air over its cold surface. This particular application proves especially valuable during debilitating heat waves when conventional (electrically powered) air conditioners fail due to power outages. [^141]
• Medically, ice can be applied (much like other cold packs ) to the body to reduce swelling—by constricting blood flow—and to alleviate pain by numbing the affected area. [^142] Simple, yet effective.

As structural material

Ice pier during 1983 cargo operations. McMurdo Station , Antarctica.

• Engineers, ever resourceful, have leveraged the substantial inherent strength of pack ice. A notable example is the construction of Antarctica’s first floating ice pier in 1973. [^143] Such ice piers are strategically employed during cargo operations, facilitating the loading and offloading of ships. Fleet operations personnel meticulously construct these floating piers during the winter months, building upon naturally occurring frozen seawater in McMurdo Sound until the dock achieves a depth of approximately 22 feet (6.7 meters). While inherently temporary structures, some ice piers have demonstrated remarkable longevity, lasting as long as 10 years. Once a pier reaches the end of its operational lifespan, it is simply towed out to sea by an icebreaker. [^144]
• Structures and intricate ice sculptures are meticulously crafted from large, pre-cut chunks of ice or by carefully spraying water. [^122] These structures are predominantly ornamental, as exemplified by elaborate ice castles , rather than practical for long-term habitation. Seasonal ice hotels do exist in a select few cold regions. [^145] Igloos offer another, more traditional, example of a temporary shelter, constructed primarily from compacted snow. [^146]
• Engineers can, paradoxically, also employ ice as a destructive force. In mining operations, the practice of drilling holes into rock structures and then introducing water during cold weather is an accepted alternative to using conventional dynamite . The rock conveniently cracks as the water expands upon freezing into ice. [^9] A rather elegant form of demolition.
• During the tumultuous period of World War II, Project Habbakuk was an ambitious Allied program that explored the feasibility of using pykrete —an unusual composite material made from wood fibers mixed with ice—as a potential material for constructing warships, particularly aircraft carriers. The appeal lay in the theoretical ease with which a vessel immune to torpedoes and equipped with an immense deck could be constructed from ice. A small-scale prototype was indeed built, [^147] but the project soon revealed its fundamental flaws: it would have cost significantly more than a conventional aircraft carrier, been many times slower, and, rather inconveniently, remained vulnerable to the inevitable process of melting. [^148] A grand idea, ultimately undone by reality.
• Ice has even found its way into the realm of musical performance, serving as the raw material for a variety of unique musical instruments, famously utilized by percussionist Terje Isungset . [^149] Because why not?

Impacts of climate change

See also: Climate change in the Arctic and Climate change in Antarctica

Historical

Earth lost 28 trillion tonnes of ice between 1994 and 2017, with melting grounded ice (ice sheets and glaciers) raising the global sea level by 34.6 ±3.1 mm. [^150] The rate of ice loss has risen by 57% since the 1990s − from 0.8 to 1.2 trillion tonnes per year. [^150] On average, climate change has lowered the thickness of land ice with every year, and reduced the extent of sea ice cover. [^150]

Greenhouse gas emissions stemming from human activities have, with predictable consequences, fundamentally unbalanced the Earth’s energy budget , leading to an undeniable accumulation of heat . [^151] Approximately 90% of this excess heat is absorbed into the ocean heat content , while a mere 1% is retained within the atmosphere, and a significant 3–4% is dedicated to melting substantial portions of the cryosphere . [^151] Between 1994 and 2017, a staggering 28 trillion tonnes of ice were lost globally as a direct result of this warming. [^150] The Arctic sea ice decline accounts for the single largest portion of this loss (7.6 trillion tonnes), followed by the melting of Antarctica’s ice shelves (6.5 trillion tonnes), the widespread retreat of mountain glaciers (6.1 trillion tonnes), the melting of the immense Greenland ice sheet (3.8 trillion tonnes), and finally, the melting of the Antarctic ice sheet (2.5 trillion tonnes), along with more limited losses of sea ice in the Southern Ocean (0.9 trillion tonnes). [^150]

With the exception of sea ice —which, due to Archimedes’ principle , already displaces water—these immense ice losses are a major, and accelerating, cause of global sea level rise (SLR). This trend is unequivocally expected to intensify in the future. In particular, the melting of the West Antarctic ice sheet may accelerate substantially as its protective floating ice shelves are lost and can no longer buttress the land-based glaciers. This would trigger poorly understood marine ice sheet instability processes, which could then augment the projected SLR for the end of the century (currently estimated between 30 cm (1 ft) and 1 meter (3½ ft), depending on future warming), by an additional tens of centimeters. [^152]

The loss of ice in Greenland and Antarctica also generates vast quantities of fresh meltwater , which, in turn, disrupts the crucial Atlantic meridional overturning circulation (AMOC) and the Southern Ocean overturning circulation , respectively. [^153] These two components of the global thermohaline circulation are profoundly important for regulating the global climate. A continuation of high meltwater flows could potentially cause a severe disruption, or even a complete “collapse,” of either or both of these circulations. Such an event would be classified as a tipping point in the climate system , because reversing it would be extraordinarily difficult, if not impossible. [^153] While AMOC is generally not anticipated to collapse within the 21st century, our understanding of the Southern Ocean circulation remains regrettably limited. [^152]

Another alarming example of an ice-related tipping point is the widespread thaw of permafrost . As the organic content within the permafrost decomposes upon thawing, it releases significant CO₂ and methane emissions. [^153] Beyond atmospheric impacts, the melting of permafrost ice literally liquefies the ground, causing any infrastructure built above the previously frozen soil to collapse. By 2050, the economic damages resulting from such infrastructure loss are projected to cost tens of billions of dollars. [^154] A rather expensive lesson.

Predictions

Potential regional warming caused by the loss of all land ice outside of East Antarctica, and by the disappearance of Arctic sea ice every year starting from June. [^155] While plausible, consistent sea ice loss would likely require relatively high warming, [^156] and the loss of all ice in Greenland would require multiple millennia. [^157][^158]

Looking ahead, the Arctic Ocean is highly likely to experience a near-complete loss of its sea ice during at least some Septembers (the culmination of the ice melting season), although a portion of this ice would predictably refreeze during the subsequent winter. To be precise, an “ice-free September” is estimated to occur once every 40 years if global warming is limited to 1.5 °C (2.7 °F). However, this frequency would escalate dramatically to once every 8 years at 2 °C (3.6 °F) of warming, and an alarming once every 1.5 years at 3 °C (5.4 °F). [^156] This profound transformation would inevitably impact both regional and global climate systems, largely due to the powerful ice-albedo feedback . Because ice is highly reflective of incoming solar energy, persistent sea ice cover naturally contributes to lower local temperatures. Once this reflective ice cover melts, the darker ocean waters beneath begin to absorb significantly more heat, which in turn further accelerates the melting of any remaining ice. [^159] It’s a self-reinforcing cycle, and not in a good way.

Global losses of sea ice between 1992 and 2018, almost exclusively concentrated in the Arctic, have already exerted an impact equivalent to approximately 10% of total greenhouse gas emissions over the same period. [^160] If all Arctic sea ice were to vanish completely each year between June and September—the period of polar day , when the Sun never sets—temperatures in the Arctic would surge by over 1.5 °C (2.7 °F), while global temperatures would experience an increase of around 0.19 °C (0.34 °F). [^155]

Possible equilibrium states of the Greenland ice sheet in response to different equilibrium carbon dioxide concentrations in parts per million . Second and third states would result in 1.8 m (6 ft) and 2.4 m (8 ft) of sea level rise, while the fourth state is equivalent to 6.9 m (23 ft). [^161]

By the year 2100, at least a quarter of the world’s mountain glaciers outside of Greenland and Antarctica are projected to melt, [^162] and virtually all ice caps on non-polar mountains are likely to be lost approximately 200 years after global warming consistently reaches 2 °C (3.6 °F). [^157][^158] The West Antarctic ice sheet is considered highly vulnerable and will likely disappear even if global warming does not progress further, [^163][^164][^165][^166] although its complete demise could take around 2,000 years. [^157][^158] The Greenland ice sheet will most probably be lost with sustained warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F), [^167] though its total disappearance is a process spanning approximately 10,000 years. [^157][^158] Finally, the colossal East Antarctic ice sheet will require at least 10,000 years to melt entirely, a scenario contingent on a warming of between 5 °C (9.0 °F) and 10 °C (18 °F). [^157][^158] In other words, humanity is setting in motion changes that will unfold over geological timescales, long after we’re gone.

Should all the ice on Earth eventually melt—a truly catastrophic scenario—it would result in an approximate sea level rise of 70 meters (229 feet 8 inches). [^168] A significant portion of this, some 53.3 meters (174 feet 10 inches), would originate from the vast East Antarctic ice sheet . [^58] Due to the phenomenon of isostatic rebound , the now ice-free landmasses would eventually rise: Greenland would become, on average, 301 meters (987 feet 6 inches) higher, and Antarctica a staggering 494 meters (1,620 feet 9 inches) higher. Within the centers of these landmasses, the uplift could be even more pronounced, reaching up to 783 meters (2,568 feet 11 inches) in Greenland and 936 meters (3,070 feet 10 inches) in Antarctica, respectively. [^169] The impact on global temperatures from the loss of the West Antarctic ice sheet, mountain glaciers, and the Greenland ice sheet is estimated at 0.05 °C (0.090 °F), 0.08 °C (0.14 °F), and 0.13 °C (0.23 °F), respectively. [^155] The absence of the East Antarctic ice sheet, however, would trigger a more substantial temperature increase of 0.6 °C (1.1 °F). [^157][^158] Humanity is currently writing a very long, very warm chapter in Earth’s history.

Non-water

Main article: Volatile (astrogeology)

The solid phases of numerous other volatile substances are also, somewhat loosely, referred to as “ices.” Generally, a volatile is categorized as an ice if its melting or sublimation point resides above or around 100 K (−173 °C; −280 °F), assuming standard atmospheric pressure. The most widely recognized example of this broader category is dry ice , which is simply the solid form of carbon dioxide . Its sublimation/deposition point occurs at 194.7 K (−78.5 °C; −109.2 °F). [^170]

A rather intriguing “magnetic analogue” of ice has also been realized in certain insulating magnetic materials. In these substances, the magnetic moments mimic the precise positions of protons in water ice and adhere to energetic constraints strikingly similar to the Bernal-Fowler ice rules , which arise from the inherent geometrical frustration of the proton configuration in actual water ice. These fascinating materials are known as spin ice . [^171] Because the universe, apparently, enjoys repeating itself with subtle variations.

See also

• Water portal
• Ice famine – Historical scarcity of commercial ice
• Ice jacking – Structural damage caused by freezing water
• Jumble ice – Irregular jagged ice formed over water
• Pumpable ice technology – Type of technology to produce and use fluids or secondary refrigerants