Geology Reference
In-Depth Information
metamorphoze into a depth hoar with a corresponding
reduction in
K
s
. With more time, crystal size in the hoar
layer increases, and this metamorphism causes
K
s
to
decrease to values approaching that of new snow. In the
spring, melting and refreezing of snow produces ice
features with much higher values of
K
s
.
than it does over land. The high specific heat capacity of
water is caused by the hydrogen bond between molecules of
water. This is the same reason for the decrease of water den-
sity when it cools below 4 °C as explained in section 3.3. To
overcome or establish this bond, high energy has to be
added or released. That is why it is more difficult to heat or
cool water than to cool or heat metals.
When freshwater freezes, ice exhibits less specific heat.
Freshwater has specific heat around 4.187 kJ/kg · K, but this
drops to values between 2.10 and 1.924 kJ/kg · K for ice with
temperature between −2 and −22 °C, respectively. Seawater
shows different behavior when it freezes. Immediately after
freezing the specific heat increases significantly, then
decreases as ice temperature goes down. The decrease
is sharp within the first 4° below the freezing point, then
becomes mild and eventually the values stabilize below
−7 °C. Table 3.7 includes values of specific heat of ice that
freezes from salt water at 10‰. The data are obtained from
a larger data set of calculations presented in
Ono
[1967]. At
this salinity the water has specific heat of 4.142 kJ/kg · K
and freezes at −0.6 °C. At the freezing temperature the
specific heat is infinity (all the heat is consumed in the
freezing). It then decreases sharply with temperature, as
shown in the first few points in the table. At the same
temperature the specific heat of saline ice increases with
salinity. The increase is tangible at subfreezing tempera-
tures and becomes small as the temperature decreases.
While the specific heat of saline water is less than that of
freshwater at the same temperature, the reverse is true after
freezing. Saline ice has higher specific heat than freshwater
ice at the same temperature. That is because more heat is
required to melt frozen ice at the boundaries of the brine
pockets. Moreover, heat loss is manifested in precipitation
of salts inside the pockets. For example, at −8 °C freshwater
ice has specific heat of 2.0 kJ/kg · K, but the value for saline
ice with 8‰ is 4.4 kJ/kg · K. In general, the heat capacity of
sea ice is very sensitive to salinity when the ice is at subsur-
face temperatures. Tables of specific heat of sea ice for dif-
ferent salinities and temperatures are presented in
Malmgren
[1927],
Schwerdtfeger
[1963],
Pounder
[1965], and
Ono
[1967].
One of the widely used equations to calculate the
specific heat of sea ice
C
si
was developed by
Untersteiner
[1961] based on the analysis of in situ measurements from
previous studies. It applies to saline FY ice. It does not
take into consideration the precipitation or the dissolu-
tion of different salts at specific temperatures. Hence its
application may be limited to the range 0 to −8.7 °C when
3.5.3. Specific Heat of Sea Ice
The specific heat is defined as the amount of heat required
to raise the temperature of a unit mass of a substance by
one degree. This is derived from the definition of the heat
capacity, which is the heat required to raise the temperature
of a substance by one degree, regardless of its mass. The
higher the specific heat the more difficult it is to change the
temperature of the material. Freshwater has high specific
heat capacity, around 4.187 kJ/kg · K compared to metals
that have values mostly between 0.3 and 0.9 kJ/kg · K and
air that has a value around 1.0 kJ/kg · K. Saline water has
less specific heat than freshwater, and it decrease slightly
with the water salinity. For example, the specific heat of sea-
water with 35‰ salinity is 3.898 kJ/kg · K. The specific heat
of water is a string function of salinity (decreases with
salinity) and weak function of temperature (almost con-
stant up to 80 °C). The high specific heat of water tempers
the rate at which air temperature changes. That is the rea-
son air temperature over ocean changes more gradually
Table 3.6
Snow type codes used in Figure 3.24.
Code
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Snow Type
New
Recent
Fine‐grained snow
Medium‐grained snow
Soft wind slab
Moderately hard slab
Hard wind slab
Very hard wind slab
Wind slab turning to depth hoar
Depth hoar
Vertical chains of depth hoar
Chains of depth hoar, indurated
chains of depth hoar with large
Voids
Icy depth hoar
Snow ice (refrozen snow)
Note
: See
Colbeck et al.,
[1990] for full description of each type.
Table 3.7
Specific heat of saline ice with salinity 10‰ at different temperatures.
Temperature
−1.0
−1.5
−2.0
−2.5
−3
−4
−5
−6
−7
−8
Specific Heat
182.6
82.10
47.18
30.94
22.11
13.35
9.29
7.07
5.73
4.86
Note
: Temperature is in °C and specific heat in kJ/kg · K.
