Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 12

2.1 Lake Types and Characteristics

2.1.1 Classification and Geometry of Lakes

Lakes are found in all climate zones. A first-order zonation of potentially freezing

freshwater lakes can be taken as the continental areas where the January (Northern
Hemisphere) or July (Southern Hemisphere) mean air temperature is less than 0 C

(Fig. 2.1 ). This region is called the cold climate zone in the present topic. There, at least
very shallow freshwater lakes freeze in normal years (Fig. 2.2 ). The cold climate zone
covers most of Eurasia and North America down to around 40

°

° N at sea level (see

Hutchinson and L er 1956; Bates and Bilello 1966). In the Asian mountains the

boundary is more south, while in the Western Europe it is more north. Apart from
Antarctica, in the Southern Hemisphere only some mountain areas are included in the
zone of freezing lakes. Actually, between 45

ö f fl

S and the Antarctic continent the only land
masses are the southern corner of New Zealand and some small islands with eventual
minor freezing inland water basins; e.g., Lake Alta, a small and deep cirque lake in the
New Zealand Southern Alps at 45

°

° 03 ′ S 168 ° 49 ′ E, 1,800 m above sea level. In the high

Andes, the cold zone even reaches the Equator.
However, in the cold climate zone there are lakes, which do not freeze. Very deep lakes
have thermal time scale longer than the winter, geothermal heating in

uences the surface
temperature due to convective mixing in the water body, and salinity depresses the
freezing point. Examples of these lakes are the very deep Lake Shikotsu (mean depth
266 m) in Hokkaido ( http://www.ilec.or.jp/database/asi/asi-16.html ), the geothermal Lake
H

fl

é í °

C in winter although other
lakes in the region are normally frozen ( http://www.lakeheviz.hu ) , and the hypersaline
Don Juan Pond (salinity 440

v z in Hungary, where the water temperature is above 20

) in McMurdo Dry Valley in Antarctica, generally
unfrozen in winter even at temperatures below

‰

− 50 ° C (Meyer et al. 1962).

ne the ice year as the time between the
maxima in the summer heat storage in two consequent summers. In the Northern
Hemisphere the maximum occurs normally in July

For lake ice climatology, it is convenient to de

August, while in the south it is Jan-

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uary February. The ice years are labelled as the year they begin, i.e. ice year 2006 is the

period between the summer maxima of 2006 and 2007. In this way ice-free years and
perennial ice periods can be properly treated in ice phenology analyses. Perennial ice or
multi-year ice is ice that has survived at least one summer, while ice-free ice year is such
where no ice occurs between two summer maxima.
A natural geological classi

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cation of lakes is according to the origins of the depres-
sions, where lakes have formed (Table 2.1 ). The time scale of the depressions is usually
very long. On a short time-scale, new ones can be created by landslides or environmental
engineering. The formation of glacial lakes has taken place at glaciers and ice sheets. In
the Northern Europe and North America there are large lake districts, which originate
from ice sheet land processes after the Last Glacial Maximum. Epiglacial, supraglacial

and subglacial lakes constitute the class of proglacial lakes, which are found at existing

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