USHCN UHI adjustments are less than 0.1F – about 0.05C.
This is a complete joke, and the result of confirmation bias. The National Weather Service reports eight degrees UHI in Phoenix and four degrees in New York. On my bicycle I typically see 5-10 degrees UHI between open space and a nearby neighborhood.
A study in tropical Hong Kong found between 1.5C and 3.8C UHI:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3613573/
3C UHI effect in Atlanta and Nashville:
http://www.enn.com/climate/article/47575
Dutch cities had a mean UHI of 2.3K, with some extremes reaching up to 7K:
http://onlinelibrary.wiley.com/doi/10.1029/2011JD015988/pdf
Is there a recognised figure for what percentage of the US counts as urbanised? Is it possible for a 5-10deg uhi at any particular city to be compatible with a nationwide figure of 0.1deg? The issue of contamination of rural sites with adjustments from urban sites is of course a separate thing entirely.
Any location which has nearby asphalt is suffering from UHI.
Those are rare “rural” areas in the US which don’t have paved roads & air-conditioners. I’m not bothered by the fact that it’s not pristine wilderness anymore, but an asphalted parking lot creates a 100yd UHI zone around it.
And if that parking lot is surrounded by thousands of square miles of nature then that few degrees of extra heat will be diluted down to the 0.1f that was claimed. If though, as is likely, the weather station for the area is next to the parking lot, then that raised temp. will be allocated (minus a 0.1f adjustment) to the whole region.
To make this into a professional joke, leave it to George Carlin:
“The temperature at the airport is 75 degrees which is STUPID because I don’t know anybody who lives at the airport. Downtown is on FIRE, man.”
From his AM/FM album from the early 1970’s.
Ahhh, the good ol’ days when nobody lived at the airport. The pre-Snowden era…
Off subject- but came home Tuesday night to Minneapolis-St. Paul Airport- Slightly after midnight I would say over 50 were spread out with mattresses and blankets.
I see nothing on the chart labeled as “UHI”
Indeed you don’t.