Air compressor. Thermogram of an air compressor in operation. 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 black (coldest, 22 degrees Celsius) through indigo, red, orange and yellow to white (hottest, 30 degrees Celsius). The air inside the compression chamber increases in temperature as it is compressed, which causes the surface of the chamber to heat up.