Urban Heat Island, a US-versus-Them Update

March 11th, 2010 by Roy W. Spencer, Ph. D.

My post from yesterday showed a rather unexpected difference between the United States versus the rest of the world for the average urban heat island (UHI) temperature-population relationship. Updated results shown below have now reduced that discrepancy…but not removed it.

I have now included more station temperature and population data by removing my requirement that two neighboring temperature measurement stations must have similar fractions of water coverage (lakes, coastlines, etc.). The results (shown below, second panel) reveal less of a discrepancy between the U.S. and the rest of the world than in my previous post. The US now shows weak warming at the lowest population densities, rather than cooling as was presented yesterday.

Also, I adjusted the population bin boundaries used for averaging to provide more uniform numbers of station pairs per bin. This has reduced the differences between individual years (top panel), suggesting more robust results. It has also increased the overall UHI warming effect, with about 1.0 deg. C average warming at a population density of 100 persons per sq. km.

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