Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 23

Table 2.4 Classification of lakes according to the quality of the ice season
Ice cover Explanation Surface temperature T 0 Limnological type
Ephemeral Ice formation possible Winter T 0 ≤ max(T m , T f ) DM, MM
Seasonal Ice forms and melts Winter T 0 = T f DM, CM, PM, MM
Perennial Multi-year ice Summer T 0 ≈ T f CM, AM, PM, MM
DM Dimictic; CM Cold monomictic; PM Polymictic; AM Amictic; MM Meromictic

Physical properties of fresh water can be taken constants for many applications
(Table 2.5 ). Temperature, salinity and pressure modify the physical properties of water,
but apart from electromagnetic properties the in uence is small. The density is, however,

an important exception, since beneath the ice cover very small differences have a major
role in the circulation of lake water masses. Very high pressure, or equivalently great
depth, needs to be considered in subglacial lakes and cooling of very deep lakes. In the
other extreme, at the surface of high mountain lakes the pressure is low that has in

fl

fl uence

primarily on the saturation levels of dissolved gases.
Optical classi

cation of lake waters goes for the colour and transparency (Arst 2003;
Arst et al. 2008). Coloured dissolved organic matter (CDOM), suspended matter, and
chlorophyll a are the basis of this classi cation. The strength of brown colour is related to

the concentration of CDOM, while turbidity is related to the concentration of suspended
matter. Because humic substances give a large contribution to CDOM in boreal lakes, in
classical limnology, the colour of water is given by the degree of brown colour in

filtered
water samples, obtained by comparing the water sample colour with the colour of platinum

Table 2.5 Physical properties of fresh water (S < 0.5 ‰ ) at the temperature of 0 ° C and comments
on the variability of these properties in cold water. Standard atmospheric pressure is assumed
Property

Value (T=0 ° C, S = 0) Variability

Molecular weight

18 : 02 gmol 1 -

1,000 kg m − 3 Density Equation of state

- 6.7 × 10 − 5

Thermal expansion

→ 0 at 3.98 ° C, >0 at higher T

1.79 × 10 − 3 Nsm − 1 Viscosity

1 : 57 10 3 Nsm 1 at 4 C

75.6 N m -1 <1 % (75.1 N m − 1 at 4 ° C) Surface tension

0.51 × 10 − 10 Pa − 1 Compressibility Very small

1,402 m s − 1 1,421 m s − 1 at 4 ° C Speed of sound

4.22 kJ kg − 1 ° C − 1 Specific heat <1 ‰

0.561 W m − 1 C − 1 Thermal conductivity ° ≈ 1%

333.5 kJ kg − 1 Latent heat of freezing -

2.49 MJ kg − 1 Latent heat of evaporation <1 %

Highly sensitive to salinity a
Relative permittivity 87.9 Sensitive to salinity
a In pure water the electric conductivity is extremely small but then rises fast with salinity

<1 ʼ Scm − 1 Electric conductivity