The ultimate performance of high resolution electron microscopes depends on a variety of factors including the stability of the temperature surrounding the instrument. Variations in temperature can cause drift of the specimen, the microscope electronics, and the mechanical tolerances in components such as microscope lenses and scan coils. For high resolution work or for image reconstruction that requires acquisition of multiple exposures over extended periods of time, it is critical that the temperature variations be strictly controlled. A typical manufacturer's specification for temperature stability of a room housing a transmission electron microscope (TEM) is a tolerance of <0.5°C/hour with fluctuations of <0.05°C/minute. While modern instrumentation rooms are carefully designed to maintain stable environmental conditions (O'Keefe etal, 2004), it seems unlikely that these stringent specifications are routinely met in practice when the microscope is in operation and personnel are present in the room.