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During the past five years, the National Weather Service (NWS) has replaced over half of its liquid-in-glass maximum and minimum thermometers in wooden Cotton Region Shelters (CRSs) with thermistor-based Maximum–Minimum Temperature Systems (MMTSs) housed in smaller plastic shelters. Analyses of data from 424 (of the 3300) MMTS stations and 675 CRS stations show that a mean daily minimum temperature change of roughly +0.3°C, a mean daily maximum temperature change of−0.4°C, and a change in average temperature of −0.1 °C were introduced as a result of the new instrumentation. The change of −0.7°C in daily temperature range is particularly significant for climate change studies that use this element as an independent variable. Although troublesome for climatologists, there is reason to believe that this change (relative to older records) represents an improvement in absolute accuracy. The bias appears to be rather sharp and well defined. Since the National Climatic Data Center (NCDC) station history database contains records of instrumentation, adjustments for this bias can be readily applied, and we are reasonably confident that the corrections we have developed can be used to produce homogeneous time series of area-average temperature.
Quayle et al. (Fri,) studied this question.