The seasonal stratification in lakes is examined by a qualitative evaluation of important physical processes and with the aid of a mathematical model. From the description of the physical processes it is realized that the observed vertical temperature distribution is the synthesis of a number of complicated and inter-dependent phenomena. The mathematical model is employed for a quantitative estimate of the importance of lake morphometry and transparency. It is found that the vertical variation of horizontal area and variations in transparency strongly influence the thermal structure of a lake. Measurements of the temperature distribution in Lake Velen are compared to predictions obtained by the mathematical model. The agreement is found to be satisfactory and it is concluded that the mathematical model provides a realistic way of studying and predicting the summer stratification.