Natural Forming of Ice Info
As a naturally occurring crystalline solid, ice is considered a mineral.
An unusual fact of ice frozen at a pressure of one atmosphere is that the solid is some 8% less dense than liquid water. Water is the only non-metallic substance to expand when it freezes. Ice has a density of 0.917 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is most dense, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the temperature drops to 0 °C. (In fact, the word "crystal" derives from Greek word for frost.) This is due to hydrogen bonds forming between the water molecules, which line up molecules less efficiently (in terms of volume) when water is frozen. The result of this is that ice floats on liquid water, an important factor in Earth's climate. Density of ice increases slightly with decreasing temperature (density of ice at −180 °C (93 K) is 0.9340 g/cm³).
It is also theoretically possible to superheat ice beyond its equilibrium melting point. Simulations of ultra-fast laser pulses acting on ice shows it can be heated up to room temperature for an extremely short period (250 ps) without melting it. It is possible that the interior of an ice crystal has a melting point above 0 °C and that the normal melting at 0 °C is just a surface effect.
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 semi-liquid state, providing lubrication regardless of any object exerting pressure against the ice.
This phenomenon does not seem to hold true at all temperatures. The extreme conditions found on the South Pole have been observed to make ice and snow not slippery.
Everyday ice and snow are hexagonal ice (ice Ih). Subjected to higher pressures and varying temperatures, ice can form in roughly a dozen different phases. Only a little less stable (metastable) than Ih is the cubic structure.
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. Ices XI, XIII, and XIV are hydrogen-ordered forms of ices Ih, V, and XII respectively.
As well as crystalline forms, solid water can exist in amorphous states as amorphous solid water (ASW), low density amorphous ice (LDA), high density amorphous ice (HDA), very high density amorphous ice (VHDA) and hyperquenched glassy water (HGW).
Rime is a type of ice formed on cold objects when drops of water crystalize on them. This can be observed in foggy weather, when the temperature drops during night. Soft rime contains a high proportion of trapped air, making it appear white rather than transparent, and giving it a density about one quarter of that of pure ice. Hard rime is comparatively denser.
Aufeis is layered ice that forms in arctic and sub-arctic stream valleys. Ice frozen in the stream bed blocks normal groundwater discharge and causes the local water table to rise, resulting in water discharge on top of the frozen layer. This water then freezes, causing the water table to rise further and repeat the cycle. The result is a stratified ice deposit, often several meters thick.
Ice can also form icicles, similar to stalactites in appearance, as water drips and re-freezes.
Clathrate hydrates are forms of ice that contain gas molecules trapped within its crystal lattice. Pancake ice is a formation of ice generally created in areas with less calm conditions.
In outer space hexagonal crystalline ice, the predominant form on Earth, is extremely rare. Amorphous ice is more common; however, hexagonal crystalline ice can be formed via volcanic action.
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
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 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.
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 (
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 adversly 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 carburetor. As air is sucked in through this for burning in the engine the local air pressure is lowered, which causes adiabatic cooling. So, in humid close-to-freezing conditions, the carburetor 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 carburetor air intake heaters for this reason. Jet engines do not experience the problem.
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.
Amorphous ice is an ice lacking crystal structure. Amorphous ice exists in three forms: low-density (LDA) formed at atmospheric pressure, or below, high density (HDA) and very high density amorphous ice (VHDA), forming at higher pressures. LDA forms by extremely quick cooling of liquid water ("hyperquenched glassy water", HGW), by depositing water vapour on very cold substrates ("amorphous solid water", ASW) or by heating high density forms of ice at ambient pressure ("LDA").
Normal hexagonal crystalline ice. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic.
Metastable cubic crystalline variant of ice. The oxygen atoms are arranged in a diamond structure. It is produced at temperatures between 130-150 K, and is stable for up to 200 K, when it transforms into ice Ih. It is occasionally present in the upper atmosphere.
A rhombohedral crystalline form with highly ordered structure. Formed from ice Ih by compressing it at temperature of 190-210 K. When heated it undergoes transformation to ice III.
A tetragonal crystalline ice, formed by cooling water down to 250 K at 300 MPa. Least dense of the high-pressure phases. Denser than water.
Metastable rhombohedral phase. Does not easily form without a nucleating agent.
A monoclinic crystalline phase. Formed by cooling water to 253 K at 500 MPa. Most complicated structure of all the phases.
A tetragonal crystalline phase. Formed by cooling water to 270 K at 1.1 GPa. Exhibits Debye relaxation.
A cubic phase. The hydrogen atoms' position is disordered, the material shows Debye relaxation. The hydrogen bonds form two interpenetrating lattices.
A more ordered version of ice VII, where the hydrogen atoms assume fixed positions. Formed from ice VII by cooling it beyond 5 °C.
A tetragonal metastable phase. Formed gradually from ice III by cooling it from 208 K to 165 K, stable below 140 K and pressures between 200 and 400 MPa. It has density of 1.16 g/cm³, slightly higher than ordinary ice.
Proton-ordered symmetric ice. Forms at about 70 GPa.
A tetragonal metastable dense crystalline phase. It is observed in the phase space of ice V and ice VI. It can be prepared by heating high-density amorphous ice from 77 K to about 183 K at 810 MPa.
Black ice, also known as "glare ice" or "clear ice," typically refers to a thin coating of glazed ice on a surface, often a roadway. While not truly black, it is transparent, allowing the usually-black asphalt/macadam roadway to be seen through it, hence the term. It also is unusually slick compared to other forms of ice on roadways.
It is usually deposited by extremely cold rain droplets, mist, or fog. The process of freezing is slowed down due to latent heat given off in sublimation, allowing the rain droplets to flow and merge together on the surface forming a film before freezing into clear ice. Nevertheless, because it contains relatively little entrapped air in the form of bubbles, black ice is transparent and thus very difficult to see (as compared to snow, frozen slush, rime ice, or other typical forms of ice on roadways). In addition, it often has a matte appearance rather than the expected gloss; and often is interleaved with wet pavement, which is identical in appearance. For this reason it is especially hazardous when driving or walking because it is both hard to see and extremely slick.
Black ice may form even when the ambient temperature is several degrees above the NTP freezing point of water (0°C). This occurs typically (and treacherously) when terrain contours and/or prevailing winds cause a local steep differential of atmospheric pressure and/or temperature, or when the atmosphere has warmed up after a prolonged cold spell that leaves the temperature of the ground and roadway well below the freezing point.
Bridges and overpasses can be especially dangerous. Black ice forms first on bridges and overpasses because air can circulate both above and below the surface of the elevated roadway, causing the temperature to drop more rapidly than on regular pavement. This is often indicated with "Bridge Ices" warning signs.
De-icing is the process of removing ice from a surface. Deicing can be accomplished by mechanical methods (scraping), through the application of heat, by use of chemicals designed to lower the freezing point of water (various salts or alcohols), or a combination of these different techniques.
When there are freezing conditions and precipitation, it is critical that an aircraft be de-iced. Failure to do so means the surface of the aircraft's wings will be too rough to provide for the smooth flow of air and thereby greatly degrading the ability of the wing to generate lift, possibly resulting in a crash. If large pieces of ice separate once the aircraft is in motion, they can be ingested into turbine engines or impact moving propellers and cause catastrophic failure. Thick ice can also lock up the control surfaces and prevent them from moving properly. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to dip below the freezing point.
Deicing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of contamination caused by ice or snow.
De-icing on the ground is usually done by spraying aircraft with a deicing fluid such as monopropylene glycol, similar to the toxic ethylene glycol antifreeze used in some automobile engine coolants. Ethylene glycol is still in use for aircraft deicing in some parts of the world, but Monopropylene glycol is more common due to the fact that it is classified as non-toxic, unlike ethylene glycol. Nevertheless, it still must be used with a containment system to capture all of the used liquid, so that it cannot seep into the ground and streams. Even if it is classified as non-toxic, it still has negative effects in nature, as it uses oxygen as it breaks down, causing other life to suffocate. (In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.) Many airports successfully recycle used deicing fluid, separating out water and solid contaminants in order to be able to reuse the fluid.
Though there are several different formulations of deicing fluid, they fall into two basic categories: Heated glycol diluted with water for deicing and snow/frost removal, also referred to as "Newtonian fluids", and unheated, undiluted glycol that has been thickened (imagine half-set gelatin), also referred to as "Non-Newtonian fluids", that is applied as an agent to retard the future development of ice or to prevent falling snow or sleet from accumulating. In some cases both types of fluid are applied, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is referred to as "a two-step procedure".
Inflight ice buildups are most frequent on the leading edges of the wings, tail and engines (including the propellors or fan blades). Lower speed aircraft frequently use pneumatic boots on the leading edges of wings and tail to affect de-icing in flight. The rubber coverings are periodically inflated, causing ice to crack and flake off in the slipstream. Once the system is activated by the pilot, the inflation/deflation cycle is automatically controlled. In the past, it was thought such systems can be defeated if they are inflated too soon; that the pilot must allow a fairly thick layer of ice to form before inflating the boots. More recent research shows “bridging” does not occur with any modern boots (ref: http://www.aopa.org/asf/publications/sa11.pdf).
Some aircraft may also use electrically heated resistive elements embedded in a rubber sheet cemented to the leading edges of wings and tail surfaces, propeller leading edges, and helicopter rotor blade leading edges. Such systems usually operate continuously. When ice is detected, they first function as de-icing systems, then as anti-icing systems for the duration of flight in icing conditions. Some aircraft use chemical de-icing systems which pump antifreeze such as alcohol or propylene glycol through small holes in the wing surfaces and at the roots of propeller blades, causing the ice to melt and making the surface inhospitable to further ice formation. A fourth system, developed by the National Aeronautics and Space Administration detects ice on the surface by sensing a change in resonant frequency. Once an electronic control module has determined that ice has formed, a large current spike is pumped into the transducers to generate a sharp mechanical shock, cracking the ice layer and causing it to be peeled off by the slipstream.
Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled off the powerplants and is ducted into a cavity just under the surface to be anti-iced. The warm air heats the surface up to a few degrees above zero, preventing ice from forming on that surface. The system may operate completely autonomously, switching itself on and off as the aircraft enters and leaves icing conditions.
De-icing of roads has traditionally been done with salt, spread by snowplows or other dump trucks designed to spread it, along with sand and gravel, on slick roads. Sodium chloride (rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, since salt water still freezes at -18°C or 0°F (the basis for Fahrenheit's thermometer scale), it is of no help when the temperature falls below this point. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar used in concrete bridges. More recent snowmelters use other salts, such as calcium chloride and magnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce an exothermic reaction. They are somewhat safer for concrete sidewalks, but excess should still be removed.
More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are generated as byproducts of agricultural operations such as sugar beet refining or the distillation process that produces ethanol.
Since the 1990s, use of liquid chemical melters has been increasing, being sprayed on roads by nozzles instead of a spinning spreader. Liquid melters are more effective at preventing the ice from bonding to the surface than melting through existing ice.
In Nagano, Japan, relatively inexpensive hot water bubbles up through holes in the pavement to melt snow, though this solution is only practical within a city or town. Some individual buildings may melt snow and ice with electric heating elements buried in the pavement, or even on a roof to prevent ice dams under the shingles, or to keep massive chunks of snow and dangerous icicles from collapsing on anyone below. Small areas of pavement can be kept ice-free by circulating heated liquids in embedded piping systems.
Atmospheric icing occurs when water droplets in the air freeze on objects they contact. This is very dangerous on aircraft, as the built up ice changes the aerodynamics of the flight surfaces, and can cause loss of lift, with an increased risk of a subsequent crash.
Not all water freezes at 0° C or 32° F. Liquid water below this temperature is called super-cooled, and such super-cooled droplets cause the icing problems on aircraft. Below -20°C, icing is rare because clouds at these temperatures usually consist of ice particles rather than super-cooled water droplets. Below -40oC it is generally accepted that icing on aircraft is negligible.