A comparison of natural wet bulb and wet globe thermometers as used in industry showed that a priori insignificant design parameters may influence the readings considerably. Detailed specifications are given for the natural wet bulb thermometer. Models are presented, describing the way the two thermometers integrate the primary environmental parameters: dry and wet bulb temperature, air speed and mean radiant temperature. These models appear to describe the physical phenomena more closely than similar formulae found in the literature.