To obtain remotely sensed synoptic measurements of surface water temperatures, use has been made of a Heimann Radiometer (Model KT79). This instrument, sensitive to thermal infrared radiation in the 8–14 µm wavelength range of the electromagnetic spectrum is fitted with a telephoto lens and gives a restricted field of view at the water surface, enabling temperature estimation to ±0.2°C. For example, over a period of 1 h it was possible, from suitably selected vantage points, to record 475 point measurements of temperature over the entire surface of a reservoir some 1 km2 in area (Embalse de Rioseco, northern Spain). Repeated sets of observations permitted the recognition of upwelling and sinking water masses. The technique enabled the detection of influx of cold water from springs along a geological fault zone in the Embalse de Tanes. Additionally, the inception and the temporal and spatial evolution of an influent plunging plume was monitored using this equipment. Although thermal radiometry is restricted to the measurement of surface water temperatures, its speed of data acquisition over a wide area should enhance appreciation of the thermal behaviour of lakes and provide an important complement to existing techniques.