The ability of airborne instruments to measure temperature in cloud is studied using theoretical analyses and experimental data. Theoretical predictions of the effects of sensor wetting are reviewed and modified, and are then compared to measurements. Two airborne immersion thermometers, the NCAR “reverse-flow” thermometer and the Rosemount 102 thermometer, are compared to each other and to a new radiometric thermometer. The comparisons show that out of cloud all three thermometers agree well with each other. However, there is clear evidence that the immersion thermometers become wet in some clouds and measure erroneously low temperatures as a result. The evidence, particularly from measurements in unmixed parcels, supports the validity of the measurements from the radiometric thermometer both inside and outside clouds. Supporting evidence that the immersion sensors are susceptible to wetting is provided from tests in a wind tunnel and from measurements using a conductivity sensor placed at the location of the immersion sensors. The scientific consequences of these measurement errors, particularly in studies of entrainment and of cloud buoyancy, are discussed.