I came across blog posts mentioning two papers about UHI.

Roger Pielke Sr et al  –  Unresolved issues with the assessment of multidecadal global
land surface temperature trends

Essentially the ground temperature is record is so contaminated by UHI that Ocean Heat Content should be the only metric … but if you want to use thermometers you should never use nighttime (Tmin) temps and therefore Tavg should not be used.

Only Tmax should be used.

A major conclusion is that, as a climate metric to
diagnose climate system heat changes (i.e., ‘‘global warming’’),
the surface temperature trend, especially if it includes
the trend in nighttime temperature, is not the most suitable
climate metric. As reported by Pielke [2003], the assessment
of climate heat system changes should be performed using
the more robust metric of ocean heat content changes
rather than surface temperature trends. If temperature
trends are to be retained in order to estimate large-scale
climate system heat changes (including a global average),
the maximum temperature is a more appropriate metric
than using the mean daily average temperature. This paper
presents reasons why the surface temperature is inadequate
to determine changes in the heat content of the Earth’s
climate system.

 

Leeper et al – Impacts of Small-Scale Urban Encroachment on Air Temperature Observations

UHI contaminates the temperature record.

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.