Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 25

system into five main classes depending on the temperature T and precipitation

P (Fig. 2.6 ):

A Equatorial climates T Tmin >18 ° C

B Arid climates Threshold depends on T mean and P

3 C \ T Tmin \ 18 C

C Warm temperate climates

D Snow climates T Tmin < - 3 ° C

T max <10 ° C E Polar climates

There are further subclasses based on temperature and precipitation criteria. It is clear
that freezing lakes are found in snow climates and polar climates. Also, there are cold
regions in the arid climate zone, and at the cold boundary of the warm climate zone lakes
may freeze in some winters. Thus climatic conditions have a very large variability across
the zones where lakes freeze. Apart from very large lakes, the size of lakes is small
compared to weather systems, and therefore the atmospheric conditions over a given lake
are fairly homogeneous. However, more variability may be seen in the drainage basins,
which are often much larger than the lake where the water is collected.
Large-scale weather systems are described by various indexes based on the atmo-
spheric pressure distribution. In Europe, widely used is the NAO (North Atlantic Oscil-
lation) index, which tells of the intensity of the westerlies. In exact terms, NAO equals the

Fig. 2.6 K ö ppen-Geiger classification of climates (Kottek et al. 2006)