3 COOK INLET MARINE ICE
of freely floating sea ice forming directly from the
3.1 Ice types
The ice types that form in Cook Inlet are described
freezing of seawater. Its formation sequence is as fol-
lows. Water contracts as it cools, until it reaches 4.0C,
in Brower et al. (1988) as
the temperature of maximum density for fresh water.
With further cooling, water expands continuously un-
til it freezes at 0C. Adding salt not only lowers the
freezing point of water but also lowers its tempera-
Estuarine and river ice.
ture of maximum density. Figure 7 shows the relation-
ship between salinity and the temperatures of maxi-
Nelson (1995) categorized the ice types as
mum density and freezing. At 24.7 parts per thousand
(ppt) salt, the two lines converge, indicating that wa-
Floating ice forming under non-turbulent water
ter of even greater salinity will freeze at its point of
and air conditions,
maximum density. Typical seawater has a salinity of
about 35 ppt and freezes at 1.9C.
Floating ice forming under turbulent conditions,
As the temperature of seawater near the surface de-
creases, a vertical density gradient forms in the water
column. The colder and heavier seawater sinks, while
each dependent on the ambient air temperature, wind
warmer, less dense seawater from below rises to take
conditions, water salinity, and tide and turbulence
its place. Ideally the density gradient disappears when
levels at the time of formation. Here, we distinguish
the entire water column becomes uniformly cooled to
between the types of Cook Inlet ice using the terms
its freezing point. However, the seas do not have uni-
form top-to-bottom salinity. Instead, there exists a den-
sity boundary between the surface and the deeper wa-
ters, known as the "halocline," across which vertical
mixing does not occur, in the absence of other forces.
Estuarine and river ice.
The deeper water is warmer, but its higher salinity pre-
vents it from rising to mix with the chilled surface
In reality, sea ice often forms as some combina-
water. The effect is that the ocean surface can freeze
tion of these types but usually can be classified as
when the surface layer alone has attained its freezing
more of one type than others. The World Meteoro-
point, even though there may be considerable heat
logical Organization's nomenclature of sea ice,
trapped below the halocline.
which is useful for further clarification of terminol-
In the absence of turbulence, ice crystals can form
ogy, is included as Appendix A.
anywhere within this upper layer of water. The crys-
tals float to the surface and form a skim, which con-
3.1.1 Pack ice
solidates and thickens progressively downward as
The term "pack ice" can be used to denote any form
more crystals form at the icewater interface. Under