AN INVESTIGATION OF TEMPERATURE DISCONTINUITIES INTRODUCED BY THE INSTALLATION OF THE HO-83 THERMOMETER

The introduction and subsequent redesign of the HO-83 thermometer have brought the homogeneity of temperature time series from many U.S. Wea ther Service stations into question. The nature of these inhomogeneiti es was investigated using data from a 15-month side-by-side comparison of the old and new versions of the HO-83 thermometers. Examination of differences between the two instruments found that the original versi on of the HO-83 read approximately 0.6 degrees C warmer than the redes igned instrument. Significant changes in the differences between the t wo instruments were noted between winter and summer. It is suggested t hat, for stations with climatology similar to the ones used in this st udy, monthly mean temperatures reported by the original version of the HO-83 be adjusted by adding -0.4 degrees C to June, July, August, and September observations and by adding -0.7 degrees C for the remainder of the year. The physical basis of the problem with the old HO-83 see ms to be related to heating of the instrument housing by internal heat sources coupled with inadequate ventilation. The mean differences bet ween the two instruments were negative for all hours, in contrast with a change in sign between day and night as would be expected if the ho using was heated and cooled by radiation. The effects of the wind spee d and ambient temperature on the observed temperature differences are consistent with this finding.