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).[citation needed]
When ice melts, it absorbs as much heat energy (the heat
of fusion) as it would take to heat an equivalent mass of water by
80 °C, while its temperature remains a constant 0 °C.
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.[3] 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.
Some other substances (particularly solid
forms of those usually found as fluids) are also called "ice": dry ice, for
instance, is a popular term for solid carbon
dioxide.
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.
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Phase |
Characteristics |
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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"). |
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Normal hexagonal crystalline ice. Virtually all ice in the biosphere is ice Ih, with the exception only of a small amount of ice Ic. |
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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. |
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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. |
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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. |
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Metastable rhombohedral phase. Does not easily form without a nucleating agent. |
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A monoclinic crystalline phase. Formed by cooling water to 253 K at 500 MPa. Most complicated structure of all the phases. |
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A tetragonal crystalline phase. Formed by cooling water to 270 K at 1.1 GPa. Exhibits Debye relaxation. |
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A cubic phase. The hydrogen atoms' position is disordered, the material shows Debye relaxation. The hydrogen bonds form two interpenetrating lattices. |
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A more ordered version of ice VII, where the hydrogen atoms assume fixed positions. Formed from ice VII by cooling it beyond 5 °C. |
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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. |
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Proton-ordered symmetric ice. Forms at about 70 GPa. |
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An orthorhombic low-temperature equilibrium form of hexagonal ice. It is ferroelectric. |
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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.[citation needed]
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.[1]
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.
Icing also occurs on towers, wind
turbines, boats,
oil
rigs, trees and other objects exposed to low temperatures and water
droplets.
