♦  First  ice

Average  first  ice  ♦  Last  ice

Average  last  ice

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Ice  winter  (1991=1990/91)

Figure  9.9.  Beginning  of  freezing  and  end  of  ice  season  in  strong  ice  winters  in  Szczecin  Lagoon

♦  Karnin  ▲  Kamminke  ■  Ueckermunde  Average  number  of  the  days  with  ice

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Ice  winter  (1991=1990/91)

Figure  9.10.  Number  of  days  with  ice  in  strong  ice  winters  in  Szczecin  Lagoon
Figure  9.11  shows  the  monthly  deviations  of  the  ice  parameter  „number  of  days  with  ice“  from  the
mean  value  of  the  1961-1990  period  at  the  Ueckermunde  station.  In  November,  ice  formation  is  rare  in
Szczecin  Lagoon,  but  there  has  been  a  slight  upward  trend  in  the  past  20  years.  In  December,  the
same  number  of  days  with  ice  is  counted  on  average  as  in  the  1961-1990  period.  In  the  winter  months
of  January  and  February  to  mid-March,  there  is  a  very  high  probability  of  ice  occurrence  in  Szczecin
Lagoon  but,  in  comparison  with  the  reference  period,  a  clearly  negative  trend  has  been  observed  for
the  past  20  years.  It  is  only  in  strong  or  very  strong  ice  winters  that  the  last  ice  in  Szczecin  Lagoon
melts  as  late  as  April.  An  analysis  of  the  deviations  of  days  with  ice  from  the  normal  value  does  not
show  any  development  trend  for  April.
Analogous  to  the  parameters  described  above,  the  monthly  deviations  of  air  temperatures  from  their
1961-1990  mean  value  were  determined  for  the  Ueckermunde  station,  see  Figure  9.12.  November
temperatures  in  the  past  20  years  have  decreased  slightly,  which  explains  the  small  increase  in  the
number  of  days  with  ice  in  that  month.  In  December,  the  air  temperature  hardly  deviated  from  the
mean  value.  A  clearly  positive  trend  is  observed  for  January  and  particularly  February.  Also  the  mean
temperatures  in  March  and  April  have  risen  slightly  in  the  past  few  years.
The  parallels  in  the  trends  of  air  temperatures  and  numbers  of  days  with  ice  again  show  clearly  that
ice  formation  and  ice  development  in  calm  and  smaller  waters  depend  primarily  on  the  air  temperature. ­
  Other  factors  are  of  secondary  importance.  But  despite  the  upward  trend  in  air  temperatures
there  has  not  been  a  single  winter  in  the  past  twenty  years  in  which  Szczecin  Lagoon  was  completely
free  of  ice.