Ice-Covered Lakes Environment - Freezing of Lakes and the Evolution of their Ice Cover - page 253

the horizontal resolution of operational satellite systems. Therefore utilization of satellite
remote sensing for lake ice monitoring was for long performed for large lakes only. In the
last 10 20 years the situation has improved, in particular with Synthetic Aperture Radar

(SAR). This method has problems with wet-surface ice and with thin, clear ice, which the
radar signal penetrates through the ice leaving a weak backscatter. The strength of radar
backscatter in general is related to the surface roughness and gas bubbles.
Physically the best channel for ice thickness is the thermal infrared, since the surface
temperature does feel the thickness of ice (e.g., Lepp

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ranta and Lewis 2007, 2015) but for
the other windows the connections between surface layer and ice thickness are weak. SAR
images have shown promises in distinguishing different lake ice types (Nghiem and
Leshkevich 2007; Sobiech and Dierking 2013) but their connection to ice thickness is not
very good. In the melting period, the contrasts weaken due to natural reasons. This is most
unfortunate, since then the ice cover deteriorates fast and ice conditions change signifi-

ä

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cantly on a daily basis.
In large freezing lakes, satellite remote sensing is a feasible method for ice monitoring
(Nghiem and Leshkevich 2007; Filatov and Kondratyev 1999; Semovski et al. 2000).
Then, in practice, ground-based mapping does not provide a full view of the ice cover.
The Canadian Ice Service reports regularly of the ice conditions in the Great Lakes of
North America, based on satellite data (Fig. 8.5 ).

8.2.2 Use of Lake Ice

Ice-covered lakes have been a part of the environment in northern settlements. Life has
adjusted to the presence of the ice, and the ice itself has been utilized for its cold content.
For long in the history, in Finland until the 1960s, ice blocks were taken in winter
(Fig. 8.6 ) and stored under sawdust to survive through summer. It was especially the
custom in dairy farms and food stores. In fact, machine-based cooling techniques have
been introduced only at the end of 1800s.

Example 8.2. c applied

science story from northern Finland is claimed to be true. Children knew about a treasure
at the bottom of a shallow lake. They could not get it up in summer but found a winter
solution. As the ice grew, top layer was cut away and the growth rate became faster at the
bottom. This way ice

There are tales and stories about lake ice cover but one speci

finally reached the bottom, and the treasure could be digged out.
Holes have been cut into the ice cover to maintain a free surface water spot. This acted
as a source of liquid water for the household and farm production, and to wash clothes and
linen (see Fig. 1.1 ). A particular activity has been bathing in an ice hole (Fig. 8.7 ),
believed by many to be healthy, a way of strengthening oneself against cold. Fishing
techniques were developed for ice conditions with nets and drags.