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 Hañcza, 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.