Based on the data collected for the Surface Station Project and analysis papers describing the results, my friend Anthony Watts has been saying for years that “surface temperature measurements (and long term trends) have been affected by encroachment of urbanization on the placement of weather stations used to measure surface air temperature, and track long term climate.”
When Ellen and I traveled across the country in the RV we visited weather stations in the historical weather network and took photos of the temperature measurement stations and the surrounding environments.
Now, NOAA has validated Anthony’s findings — weather station siting can influence the surface station long temperature record. Here some samples that were taken by other volunteers :
Impacts of Small-Scale Urban Encroachment on Air Temperature Observations
Ronald D. Leeper, John Kochendorfer, Timothy Henderson, and Michael A. Palecki
A field experiment was performed in Oak Ridge, TN, with four instrumented towers placed over grass at increasing distances (4, 30, 50, 124, and 300 m) from a built-up area. Stations were aligned in such a way to simulate the impact of small-scale encroachment on temperature observations. As expected, temperature observations were warmest for the site closest to the built environment with an average temperature difference of 0.31 and 0.24 °C for aspirated and unaspirated sensors respectively. Mean aspirated temperature differences were greater during the evening (0.47 °C) than day (0.16 °C). This was particularly true for evenings following greater daytime solar insolation (20+ MJDay−1) with surface winds from the direction of the built environment where mean differences exceeded 0.80 °C. The impact of the built environment on air temperature diminished with distance with a warm bias only detectable out to tower-B’ located 50 meters away.
The experimental findings were comparable to a known case of urban encroachment at a U. S. Climate Reference Network station in Kingston, RI. The experimental and operational results both lead to reductions in the diurnal temperature range of ~0.39 °C for fan aspirated sensors. Interestingly, the unaspirated sensor had a larger reduction in DTR of 0.48 °C. These results suggest that small-scale urban encroachment within 50 meters of a station can have important impacts on daily temperature extrema (maximum and minimum) with the magnitude of these differences dependent upon prevailing environmental conditions and sensing technology.
My blue emphasis added
Anthony has a more complete write up of the Surface Station Project HERE.