Influence of the Temperature on Ice Conditions in the Bay of Bothnia (Baltic Sea) 
25 
Data series of temperature and ice thickness were fixed for the period 1988-2003. The main 
annual air temperature of each year of this period are from the meteorological station of Oulu 
airport. The observation spots were almost at the same place until 1995, and there was a 
relocation of them after that. Some homogeneity tests of the climatic series were realised by 
the FMI (Finnish Meteorological Institute). The monthly variability of the mean air temperature is 
important for this study. The most important variations of the temperature take place during the 
heart of the winter. It especially concerns the months of December, January and February (Fig. 
1)- 
A succession of cold and warm winters can be noticed. Could it be possible to conclude to a 
significant trend? The mean air temperature of January 2003 was the coldest one recorded for 
the period 1988-2003, but January 2003 does not belong to the coldest winters of the end of 
the XXe century. 
Temperature data can be in relation with ice data. The ice thickness data series presents 
measurements made for the period 1988-2003 by researchers from the station of Virpiniemi. 
The analysis of a statistic relation between two quantitative variables - temperature and ice 
thickness - is for validating or not the hypothesis mentioned before. So the intensity of the 
relation of two variables can be measured by a correlation coefficient. 
Results 
The intensity of the relation temperature / ice thickness 
In the Oulu region, the maximum annual thickness of ice is 40 to 90 centimetres, and the winter 
mean air temperatures is -3 to -7.50 from 1988 to 2003. The relationship between the winter 
air temperature and the Virpiniemi annual maximum ice thickness is shown for the period 1988- 
2003 in Figure 2. The Bravais-Pearson coefficient (r) is 0,607 and shows a good correlation 
between the winter mean temperature and the maximum annual ice thickness. Nevertheless 
some winters are some exceptions. For instance, the winter 1988/89 was warm, the mean air 
temperature (December 1998 - April 1989) about -30, but the maximum ice thickness was 
large, 77 cm.