Winter ice cover can be related to the temperature of air, but morphological and meteorological differences cause large variations. Physically based modelling help their study, as results can be controlled and verified. A lot of data is needed, and in the process liability and interpretation of data are important. In this study long series of data are used to study winter ice coverage of morphologically different lakes in Southern Finland. Physical factors in the vertically resolving lake model, the Probe model, were considered. The ice model is based on the use of the temperature of air and it is successful in describing regular ice formation and break-up. Suitability for individual lakes could be improved by changing the factors for melting, the wind limit for breaking the ice and the temperature limit for determining ice coverage. Depth delays and mixing increases cooling of water. For freezing date, the differences between lakes are about one month. Ice break-up is strongly determined by air temperature, but the model can help to relate differences to morphology. During warmer winters horizontal variations have more importance; there can also be periods of partial ice-cover. For further study of those the model should be developed; modelling over years in succession is then a necessity.

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