Inside the ASOS HO83 Temp/dewpoint sensor

HO83 ASOS Hygrothermometer (temperature/dewpoint sensor)

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 including the all time high of 117 degrees F, became part of the official climate record and still stand today. Here is a New York Times article that highlights the problem and a research paper from Kessler et al outlining similar problems in Albany New York as well as Tucson.

But we haven’t really known much about the detail and inner workings of the HO83. Fortunately, I’ve located a NOAA training course on the original model HO83 and its improved replacement, the model 1088  The NOAA online tutorial provides some detail on its inner workings, with pictorials and schematics.

See the NOAA HO83/1088 online training course. 

In an internal NOAA Document from 2002 that outlines a software upgrade that was designed to improve performance and reliability of the ASOS temperature and dewpoint system, they have a description of its operation:

1.1.2 New Dew Point Temperature Replacement Sensor Currently ASOS uses a hygrothermometer (H083 or 1088) sensor for measuring both ambient and dew point temperatures. This sensor uses a platinum wire Resistive Temperature Device (RTD) to measure ambient temperature and a chilled mirror to determine dew point temperature. The mirror is cooled by a thermoelectric or Peltier cooler until dew or frost begins to condense on the mirror surface. The body of the mirror contains a platinum wire RTD and the mirror’s temperature is measured and reported as the dew point temperature. The ambient temperature sensor for both hygrothermometers meets ASOS performance requirements.

The dew point temperature sensor performance is below expectations.

In order to improve the performance of the dew point temperature sensor, the NWS looked for a more reliable technology. The new sensor measures relative humidity via capacitance and then the dew point temperature is calculated and processed through the ASOS algorithms. The ASOS data processing algorithms have not changed; only the dew point temperature sensor has been replaced. The new ASOS dew point temperature replacement sensor is the Vaisala DTS1.

They also issued the ASOS Product Improvement Implementation Plan in 2002 which outlines all of the stations in the USA that were scheduled to get the upgraded temperature/dewpoint sensor.

One of the biggest problems was that the early design of the HO83 allowed exhaust air (warmed by the warm side of Peltier chip) to recirculate from the mushroom shaped cap down the sides of the chamber, and back into the air inlet at the bottom. The problem was solved a few years later by the addition of a metal skirt which deflects the exhaust air.

Unfortunately, even though NOAA has modernization plans in place for the ASOS network, there are still some of the original designs that remain in operation today, such as this USHCN station which is the official climate station of record for New Orleans:

Photo from sufracestations.org volunteer Fred Perkins 8/25/07 click for larger photo.

Thus, the HO83 induced bias first noted in the mid 1980’s continues in the surface temperature record even 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 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.