Abstract: An apparatus for determining heat flow at an interface between a water surface and the atmosphere comprising a floatable transparent panel member having a lower surface in contact with the water and an upper surface covered with a thin layer of water. A plurality of thermocouples are carried on both surfaces of the panel member for sensing the temperature of the water and the water layer in contact with the atmosphere. Electronic circuit means operatively associated with the thermocouple means records the temperature readings whereby the difference between the temperatures provides a measure of the heat flow at the interface.