The vertical temperature distribution in the open ocean can be represented as two layers separated by an interface. The upper layer is warmed by the sun and mixed to depths of about 100 m by wave motion. The bottom layer consists of colder water formed at high latitudes. The interface or thermocline is sometimes marked by an abrupt change in temperature but more often the change is gradual. This implies that there are two reservoirs providing the heat source and the heat sink required for a heat engine. A practical application is found in a system designed to transform the thermal energy into electricity. This is referred to as OTEC for Ocean Thermal Energy Conversion.