Ice cover as an indicator of winter air temperature changes: case study of the Polish Lowland lakes

Observations of ice cover and winter air temperature measurements were carried out on six lakes in northern Poland during the period 1961-2000. Detailed analyses of the dates of formation and termination of the ice cover, the duration of maximum thickness and ice-free period during winter were carried out. Various tendencies were found in the time series of the earliest freeze-up dates, whereas the latest ice break-up dates were recorded to occur much earlier than in the past on all the lakes, with time advance being on average from 0.6 to 0.8 day year(-1). The period with ice cover has been getting shorter at the rate of 0.8 to 0.9 day year(-1), with the exception of Lake Hancza, the deepest lake in the European Lowland, where the rate of 0.4 day year(-1) was recorded. Similarly, there was a decreasing tendency in the maximum thickness of the ice cover, at the rate of 0.26 to 0.60 cm year(-1). Despite similar tendencies, all those changes showed diverse dynamics in particular lakes. The proposed indicator of the ice cover stability confirms the above statements, and thus, the undergoing climatic changes.