Am I seeing this correctly? The temperature gauge next to exhaust from an outdoor AC unit? It would have hot air blowing all over it.
The USHCN is the United States Historical Climate Network, a group of data collection sites which was designated, circa 1990, and is described by the NOAA as a "High Quality" network. Part of the criteria for being designated as part of the USHCN network was that the site had to be at least 80 years old, and to have had few environment changes, and few substantial site moves over its history. The sites were originally set up not for climatology, but rather for weather forecasting, which is just fine collecting data in cities to predict what the weather will be like where people actually live. One of the recurring problems with the network is that since the early 80s the sties have been transitioning over to MMTS sensors, which appear to have been set up with a very short standard cable length...which resulted in sensor stations being moved adjacent to buildings. If you look at the picture of Detroit Lakes, you will see a little louvred box, which is a Stevenson hut...the old style station. If you look at the pole with the little cylindrical louvred object on top, that is the MMTS temperature guage which replaced it.
Here is the NOAA guideline for the temperature guage siting (Somewhere I have a link to how they classify the station quality, and the margins of error for each class, but can't locate it right now): Temperature sensor siting: The sensor should be mounted 5 feet +/- 1 foot above the ground. The ground over which the shelter [radiation] is located should be typical of the surrounding area. A level, open clearing is desirable so the thermometers are freely ventilated by air flow. Do not install the sensor on a steep slope or in a sheltered hollow unless it is typical of the area or unless data from that type of site are desired. When possible, the shelter should be no closer than four times the height of any obstruction (tree, fence, building, etc.). The sensor should be at least 100 feet from any paved or concrete surface.
The USHCN Network, per the NOAA:
Strengths of the U.S. Historical Climate Network: The USHCN is a high-quality network of COOP stations with maximum, minimum, and mean temperature and precipitation, specially selected for analyzing long-term variability and change in the 48 contiguous United States. The stations in the network were chosen based on length of record, spatial distribution, and to minimize the number of station changes that can affect the homogeneity of the record. A methodology has been developed and is applied to test known station changes for their impact on the homogeneity, and data are adjusted if the change causes a statistically significant change in the time series. An urban warming correction based on population is also applied. The data set is a consistent network through time, which minimizes any biasing due to network changes through time.
Weaknesses of the U.S. Historical Climate Network: The start date for stations in the USHCN vary so that the stations used to compute the national value may change from year to year, especially for the earliest years. At present, ancillary variables are not available in the USHCN. Information for geographical sub-regions such as climate regions, river basins, and agricultural regions are currently not available, although they could be computed eventually. Data for the USHCN are not available in near real-time status.
Our Recommendations: We recommend using USHCN whenever possible for long-term climate analyses. The careful selection of each station and the series of adjustments applied to the data make the USHCN database the best long-term monthly temperature and precipitation data set available for the contiguous United States. It provides an accurate, serially complete, modern historical climate record that is suitable for detecting and monitoring long-term climatic changes. Other data sets, such as the Climate Division Dataset, may produce misleading trends due to artificial station changes. When performing analyses on scales smaller than regional, we recommend a review of the metadata in order to identify the stations most suitable for specific research needs.