Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 26

normalized anomaly of the difference in the heights of the 500 mbar atmospheric pressure
surfaces between the Azores (Ponta Delgada) and Iceland (Stykkisholmur). The NAO-
index is positive, when the westerly winds are stronger than normal and these winters in
Europe are typically much warmer than on average; when the NAO-index is negative, the
westerlies are weaker and the winters are cold. The index value 1 ( 1) means that the

westerly winds at 500 mbar level between Iceland and the Azores are one standard
deviation stronger (weaker) than on average. Much of the variability of lake ice seasons in
Europe is correlated with NAO. This is trivial but it is noteworthy that the correlation is
quite high and therefore general regional climate overcomes the individual characteristics
of lakes (e.g., Blenckner et al. 2004). In land regions around the Paci

−

c Ocean, El

Ni Southern Oscillation (ENSO) is connected to anomalously warm and cold winters

(e.g., Mishra et al. 2011).
The in

ñ o

-

uence of weather and climate on frozen lakes is by the exchange of mass, heat
and momentum between lakes and atmosphere and solar radiation. These

fl

uxes between
the atmosphere and lakes depend on the precipitation, air pressure, cloudiness, air tem-
perature and humidity, and wind. The standard weather station data include air pressure,
temperature and humidity at 2 m altitude, and wind speed and direction at 10 m altitude;
some stations also provide cloudiness. Since direct measurements of solar and terrestrial
radiation components are rarely available, cloudiness becomes a key variable for their
indirect estimation. Table 2.7 presents climatological data from stations Jokioinen (snow
climate, boreal zone) and Utsjoki (polar climate, tundra zone).
The atmospheric mass

fl

flux to a lake is given by the difference between precipitation
and evaporation/sublimation. Precipitation is obtained directly from weather observations,
but evaporation and sublimation need to be estimated using atmospheric boundary layer
and surface temperature data.
Absolute humidity is given by the mass proportion of water vapour (q) or by the partial
atmospheric pressure due to water vapour (e). They are related by

fl

q ¼ 0 : 622 e
p

ð 2 : 9 Þ

The maximum amount of water vapour depends strongly on the temperature (Fig. 2.7 ;
for the exact formulae, see Annex); at temperatures below zero, the saturation level is
given both for liquid water surface and for ice surface. Example the saturation water
vapour pressure at the standard atmospheric pressure is 12.27 mbar at the temperature of
10 ° C, 6.11 mbar at 0 ° C, and 2.86 mbar (water surface) or 2.60 mbar (ice surface) at

− 10 ° C. The relative humidity is the actual humidity in relation to the saturation value:

R ¼ q

q s ¼ e

ð 2 : 10 Þ

e s