Anthony Watts has shown that NOAA temperature anomalies are off by an order of magnitude from legitimate data, largely due to UHI.
h/t to John Silver
Last night I was riding my bike over to the gym about 10 PM, and noticed what I always do. Temperatures in open space are much cooler than temperatures near asphalt. As soon as I leave open space and enter a neighborhood, or particularly a parking lot – the temperature goes up 5-10 degrees Fahrenheit. Most UHI effects are due to proximity to asphalt and brick. Even a small town can suffer significant UHI effects.