The rise in temperature follows the growth of the city and at least some of the temperature rise is due to the Urban Heat Island effect and the placement of the thermometer. Below is a photo of the station in 1923; the thermometer is housed in the white box. We see that the site is fairly open and is not near building or asphalt which can absorb and radiate heat.
In contrast, here is where the thermometer is today:
The thermometer is over asphalt and between buildings. The radiation from these structures gives an artificial warming bias. NOAA, which runs USHCN applies “correction factors” to the raw data, supposedly to account for the site-induced warming bias.
A private volunteer group has been surveying the USHCN stations. You can see their results at www.surfacestagions.org. So far they have surveyed 854 of the 1,221 stations. Only 11% comply with the Class one or Class two standards specified by the National Climate Data Center:
NCDC Climate Reference Network Handbook, 2002, specifications for siting of NOAA’s new Climate Reference Network:
Class 1- Flat and horizontal ground surrounded by a clear surface with a slope below 1/3 (<19deg). Grass/low vegetation ground cover <10 centimeters high. Sensors located at least 100 meters from artificial heating or reflecting surfaces, such as buildings, concrete surfaces, and parking lots. Far from large bodies of water, except if it is representative of the area, and then located at least 100 meters away. No shading when the sun elevation >3 degrees.
Class 2- Same as Class 1 with the following differences. Surrounding Vegetation <25 centimeters. No artificial heating sources within 30m. No shading for a sun elevation >5deg.
There are five classes of site specifications. The survey has found that 69% of sites have warming biases of greater than 2 degrees Centigrade and 11% of sites have errors of more than 5 degrees Centigrade.