Congelation ice is

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

that forms on the bottom of an established ice cover.

Seawater

seawater Seawater, or sea 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 approximat ...

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

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

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 (or briny water) is a high-concentration solution of salt (typically sodium chloride or calcium chloride) in water. In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawat ...

, 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. The term excludes seawater and brackish water, but it does include non-salty mi ...

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

(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 Human-powered transport, 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. ...

, skate sailing,

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. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

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

References

Water ice Sea ice {{ocean-stub