Inside the HO83 Hygrothermometer

Here.

Excerpts:

Much has been written about problems with artificially high temperature readings due to the HO83 aspirated air temperature/dewpoint temperature sensor used on NOAA Automated Surface Observing Stations (ASOS). The most famous problem occurred in Tucson, AZ in the mid 1980’s where a malfunctioning HO83 unit created dozens of new high temperature records for the city, even though surrounding areas had no such measured extremes. Unfortunately those new high temperature records includign the all time high of 117 degrees F, became part of the official climate record and still stand today.
...
While only 5% of the USHCN network is ASOS, the biases produced by the HO83 are quite large, and there appears to be no site specific adjustments to remove the bias. Since determining the individidual maintenance records and biases of each ASOS station would be a significant task, the simplest solution would be to remove all ASOS stations from the USHCN record set.