sentences:  During winter any additional cooling below 4 °C results in stratification of water column, so dimictic lakes usually have an inverse thermal stratification, with water at 0 °C below ice and then with temperatures increasing to near 4 °C at the lakes base.Once the ice melts, the water column can be mixed by the wind.In large lakes the upper water column is often below 4 °C when the ice melts, so that spring is characterized by continued mixing by solar driven convection, until the water column reaches 4 °C.