By Dave Hoopman, Wisconsin Energy Cooperative News

A growing pile of record daily highs from the National Weather Service (NWS) office at Tucson International Airport (in the late 1980s) —and the absence of corresponding new records from Davis-Monthan Air Force Base on very similar terrain just three miles away—suggested a problem with NWS monitoring equipment, specifically with a device called the HO-83 hygrothermometer, newly installed at the airport in 1986. In the early 1990s, articles in the scientific and technical literature (starting with Gall 1992) began examining design flaws that might cause any HO-83 to read significantly higher than the real ambient air temperature and to produce higher readings the longer it remained in service.

It is common practice for records of the U.S. Historical Climatology Network (USHCN) to be adjusted to account for biasing factors such as differing times of recording data or changes in the location of monitoring equipment. But the USHCN records are evidently not adjusted to compensate for measurement errors introduced by the HO-83.

One reason for that could be that the HO-83 is installed at just 5 percent of the USHCN’s 1,221 monitoring stations, the vast majority of which are not NWS offices but rather are sited on private property and operated by citizen volunteers. Even so, in 1995 Thomas Karl, current head of the USHCN’s parent organization, the National Climatic Data Center (NCDC), and a strong proponent of the theory of human-induced climate change, authored a paper indicating replacement of older equipment with the HO-83 may have polluted the record of U.S. maximum temperature averages since the 1980s, raising them by an error of as much as 0.5 degree, Celsius.

Read more of this story which goes on to detail problems with this and other new instrumentation and highlights the work and findings of Anthony Watts.
See also the Climate Audit Blog on this issue.