Ice is the name given to any one of the 14 known solid phases of water. However in non-scientific contexts, it usually describes ice I_h, which is the most abundant of these phases. It is a crystalline solid, which can appear transparent or an opaque bluish-white color depending on the presence of impurities such as air. The addition of other materials such as soil may further alter appearance. The most common phase transition to ice I_h occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. However, it can also deposit from a vapor with no intervening liquid phase such as in the formation of frost. Ice appears in varied forms such as hail, ice cubes, and glaciers. It plays an important role with many meteorological phenomena. The ice caps of the polar regions are of significance for the global climate and particularly the water cycle.

Characteristics
Another consequence of ice's lower density than water is that pressure decreases its melting point, potentially forcing ice back into a liquid state. Until recently it was widely believed that ice was slippery because the pressure of an object in contact with it caused a thin layer to melt. For example, the blade of an ice skate, exerting pressure on the ice, melted a thin layer, providing lubrication between the ice and the blade.
This explanation is no longer widely accepted. There is still debate about why ice is slippery. The explanation gaining acceptance is that ice molecules in contact with air cannot properly bond with the molecules of the mass of ice beneath (and thus are free to move like molecules of liquid water). These molecules remain in a semiliquid state, providing lubrication regardless of any object exerting pressure against the ice.
This may also be observed when, at times, a block of ice, such as is commonly found in freezers can stick to skin or other surfaces. This only happens when the block of ice is again, cold enough to allow the outer layer to fully harden into ice, and is thus not a common phenomenon.

Slipperiness
Everyday ice and snow are hexagonal ice (ice I_h). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than I_h is the cubic structure (I_c).
With both cooling and pressure more types exist, each being created depending on the phase diagram of ice. These are II, III, V, VI, VII, VIII, IX, and X. With care all these types can be recovered at ambient pressure. The types are differentiated by their crystalline structure, ordering and density. There are also two metastable phases of ice under pressure, both fully hydrogen disordered, these are IV and XII. Ice XII was discovered in 1996. In 2006, XIII and XIV were discovered.

Types of ice

Uses of ice
Ice has long been valued as a means of cooling. Until recently, the Hungarian Parliament building used ice harvested in the winter from Lake Balaton for air conditioning. Icehouses were used to store ice formed in the winter to make ice available year-round, and early refrigerators were known as iceboxes because they had a block of ice in them. In many cities it was not unusual to have a regular ice delivery service during the summer. For the first half of the 19th century, ice harvesting had become big business in America. New Englander Frederic Tudor, who became known as the "Ice King," worked on developing better insulation products for the long distance shipment of ice, especially to the tropics. The advent of artificial refrigeration technology has since made delivery of ice obsolete.
In 400 BC Iran, Persian engineers had already mastered the technique of storing ice in the middle of summer in the desert. The ice was brought in during the winters from nearby mountains in bulk amounts, and stored in specially designed, naturally cooled refrigerators, called yakhchal (meaning ice storage). This was a large underground space (up to 5000 m³) that had thick walls (at least two meters at the base) made out of a special mortar called sārooj, composed of sand, clay, egg whites, lime, goat hair, and ash in specific proportions, and which was resistant to heat transfer. This mixture was thought to be completely water impenetrable. The space often had access to a Qanat, and often contained a system of windcatchers that could easily bring temperatures inside the space down to frigid levels in summer days. The ice was then used to chill treats for royalty during hot summer days.

Ice harvesting
Ice also plays a role in winter recreation, in many sports such as ice skating, tour skating, ice hockey, ice fishing, ice climbing, curling and sled racing on bobsled, luge and skeleton. A sort of sailboat on blades gives rise to iceboating.
The human quest for excitement has even led to ice racing, where drivers must speed on lake ice while also controlling the skid of their vehicle (similar in some ways to dirt track racing). The sport has even been modified for ice rinks.

Sports on ice
Ice can also be an obstacle; for harbors near the poles, being ice-free is an important advantage, ideally all-year round. Examples are Murmansk (Russia), Petsamo (Russia, formerly Finland) and Vardø (Norway). Harbors that are not ice-free are opened up using icebreakers.
Ice forming on roads is a dangerous winter hazard. Black ice is very difficult to see because it lacks the expected glossy surface. Whenever there is freezing rain or snow that occurs at a temperature near the melting point, it is common for ice to build up on the windows of vehicles. Driving safely requires the removal of the ice build-up. Ice scrapers are tools designed to break the ice free and clear the windows, though removing the ice can be a long and labor-intensive process.
Far enough below the freezing point, a thin layer of ice crystals can form on the inside surface of windows. This usually happens when a vehicle has been left alone after being driven for a while, but can happen while driving if the outside temperature is low enough. Moisture from the driver's breath is the source of water for the crystals. It is troublesome to remove this form of ice, so people often open their windows slightly when the vehicle is parked in order to let the moisture dissipate, and it is now common for cars to have rear-window defrosters to combat the problem. A similar problem can happen in homes, which is one reason why many colder regions require double-pane windows for insulation.
When the outdoor temperature stays below freezing for extended periods, very thick layers of ice can form on lakes and other bodies of water (although places with flowing water require much colder temperatures). The ice can become thick enough to drive onto with automobiles and trucks. Doing this safely requires a thickness of at least 30 centimeters (one foot).
For ships, ice presents two distinct hazards. Spray and freezing rain can produce an ice build-up on the superstructure of a vessel sufficient to make it unstable and to require it to be hacked off or melted with steam hoses. And large masses of ice floating in water (typically created when glaciers reach the sea) can be dangerous if struck by a ship when under way. These masses are called icebergs and have been responsible for the sinking of many ships - a notable example being the Titanic.
For aircraft, ice can cause a number of dangers. As an aircraft climbs, it passes through air layers of different temperature and humidity, some of which may be conducive to ice formation. If ice forms on the wings or control surfaces, this may adversely affect the flying qualities of the aircraft. During the first non-stop flight of the Atlantic, the British aviators Captain John Alcock and Lieutenant Arthur Whitten Brown encountered such icing conditions - heroically, Brown left the cockpit and climbed onto the wing several times to remove ice which was covering the engine air intakes of the Vickers Vimy aircraft they were flying.
A particular icing vulnerability associated with reciprocating internal combustion engines is the carburettor. As air is sucked through the carburettor into the engine the local air pressure is lowered, which causes adiabatic cooling. So, in humid close-to-freezing conditions, the carburettor will be colder and tend to ice up. This will block the supply of air to the engine, and cause it to fail. Modern aircraft reciprocating engines are provided with carburettor air intake heaters for this reason. Jet engines do not experience the problem.

Ice travel

Engineers leveraged pack ice's formidable strength when they constructed Antarctica's first floating ice pier in 1973. Such ice piers are used during cargo operations to load and offload ships. Fleet operations personnel make the floating pier during the winter. They build upon naturally occurring frozen seawater in McMurdo Sound until the dock reaches a depth of about 22 feet. Ice piers have a lifespan of three to five years.
Ice is used in cooling instead of the air cooler.
The manufacture and use of ice cubes or crushed ice is common for drinks.
Pagophagia, a type of pica eating disorder, is the compulsive consumption of ice.
Structures and ice sculptures are built out of large chunks of ice. The structures are mostly ornamental (as in the case with ice castles) and not practical for long-term habitation. Ice hotels exist on a seasonal basis in a few cold areas. Igloos are another example of a temporary structure, made primarily from snow.
During World War II, Project Habbakuk was a British program which investigated the use of pykrete (wood fibres mixed with ice) as a possible material for warships, especially aircraft carriers due to the ease with which a large deck could be constructed, but the idea was given up when there were not enough funds for construction of a prototype.
It has been shown on Mythbusters that ice can be used to start a fire by carving it into a lens that will focus sunlight onto kindling. When one waits long enough, a fire will start.
In global warming, ice plays an important part because it reflects 90% of the sun's rays. Other uses of ice
Most liquids freeze at a higher temperature under pressure because the pressure helps to hold the molecules together. However, the strong hydrogen bonds in water make it different: water freezes at a temperature below 0 °C under a pressure higher than 1 atm. Consequently water also remains frozen at a temperature above 0 °C under a pressure lower than 1 atm. The melting of ice under high pressures is thought to contribute to why glaciers move. Ice formed at high pressure has a different crystal structure and density than ordinary ice. Ice, water, and water vapor can coexist at the triple point, which is 273.16 K at a pressure of 611.73 Pa.

Ice at different pressures

In fiction