Congelation ice is

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

that forms on the bottom of an established ice cover.

Seawater

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

, congelation ice is ice that forms on the bottom of an established

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...

cover, usually in the form of platelets which coalesce to form solid ice. Only the water freezes to ice, the salt from the seawater is concentrated into

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for br ...

, some of which is contained in pockets in the new ice. Due to the brine pockets, congelation ice on seawater is neither as hard nor as transparent as fresh water ice.

Fresh water

On the surface of lakes, or other bodies of still

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

, ''congelation ice'' is often called ''black Ice''. This ice has frozen without many air bubbles trapped inside, making it

transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...

. Its transparency reveals the colour, usually black, of the water beneath it, hence the name. This is in contrast to ''snow ice'', sometimes called ''slush ice'', which is formed when

slush Slush, also called slush ice, is a slurry mixture of small ice crystals (e.g., snow) and liquid water. In the natural environment, slush forms when ice or snow melts or during mixed precipitation. This often mixes with dirt and other pollutant ...

(water saturated snow) refreezes. Snow ice is white due to the presence of air bubbles. Black ice grows downward from the bottom of the existing ice surface. The growth rate of the ice is proportional to the rate that heat is transferred from the water below the ice surface to the air above the ice surface. The total ice thickness can be approximated from

Stefan's equation In glaciology and civil engineering, Stefan's equation (or Stefan's formula) describes the dependence of ice-cover thickness on the temperature history. It says in particular that the expected ice accretion is proportional to the square root of th ...

. Black ice is very hard, strong and smooth, which makes it ideal for

ice skating Ice skating is the self-propulsion and gliding of a person across an ice surface, using metal-bladed ice skates. People skate for various reasons, including recreation (fun), exercise, competitive sports, and commuting. Ice skating may be per ...

skate sailing Skate sailing is a method of moving over ice standing on ice skates utilizing the force of the wind. A small sail is held in ones hands or leaned against with the whole body. Using a metal blade under foot and the height of the ice skates is of m ...

ice yachting An iceboat (occasionally spelled ice boat or traditionally called an ice yacht) is a recreational or competition sailing craft supported on metal runners for traveling over ice. One of the runners is steerable. Originally, such craft were boats ...

and some other ice sports. Thin, clear ice also has acoustic properties which are useful to tour skaters. Skating on clear ice radiates a tone whose

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

depends on the thickness of the ice. The Acoustics of Thin Ice, by Gunnar Lundmark

References

Water ice Sea ice {{ocean-stub