Water leak. Thermogram of warm water leaking onto a city pavement. The source of the leak is an underground pipe carrying warm waste water to homes in the city which use it as a energy source for central heating. A thermogram shows variation in temperature on the surface of an object, measured by the long-wave infrared radiation it emits. The temperature scale is colour-coded and runs from dark blue (coldest, -9.6 degrees Celsius), through green, yellow, orange and red to cream (warmest, 12.1 degrees Celsius). Photographed in the Netherlands.