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15. Definitions 
Accumulated areal ice volume, or V AI 
The accumulated areal ice volume was introduced in 1989 as a new way of classifying 
ice winters. Besides the duration of ice cover, also the ice concentration and thickness is 
included in the calculation, thus taking into account interruptions of freezing periods 
during each winter season. The daily areal ice volume data at the 13 ice climatological 
stations are summed, and the accumulated areal ice volume is obtained by computing 
the average of the 13 stations as 
n 3 k 
with n = number of stations 
N = ice concentration (in tenths) 
H = ice thickness 
j = index of the 13 stations 
k = index of days with ice. 
Close drift ice 
Drift ice with concentration of 7/10 to 8/10, composed of floes mostly in contact 
Difference of dewpoint 
Difference between the air temperature and the dewpoint 
Drift ice 
Term used in a wide sense to include any area of sea ice, other than fast ice, no mat 
ter what form it takes or how it is disposed 
Fast ice 
Sea ice which forms and remains fast along the coast. Fast ice may be formed from 
sea water or by freezing of drift ice 
Freezing period 
Period in which ice formation occurs in an observations area or ice increase continues 
after a stagnation of ice development (ice decrease) 
Ice concentration 
The ratio expressed in tenths describing the amount of the sea surface covered by ice 
as a fraction of the whole area being considered 
Ice season 
Period between the first and the last ice occurrence in the area being considered; Ice 
season may contain several freezing periods 
Level ice 
Sea ice which has not been affected by deformation 
Maximum ice thickness 
Thickness of the level ice which is achieved in the maximum ice development of an ice 
season 
Modified ‘Winterzahl’, or W* z 
Winterzahl (W z ) was introduced by Müller-Annen (1964, 1965) as W z ( = (n m / N),KS, 
with n WT = number of days with negative means of the air temperature 
N = period from 1 st of November to 31 st of March 
KS = sum of coldness 
W* z (T m < -2,0 °C) takes into consideration the daily mean of air temperature starting 
with temperatures below -2.0 °C and the period from the date with water temperatures 
below 4 °C until 15 th March