What was done
The authors examined the influence of built-up areas on the near-surface air temperature field of Debrecen, Hungary -- which sits on nearly flat terrain in the Great Hungarian Plain with a population of 220,000 -- via mobile measurements made under different types of weather conditions between March 2002 and March 2003.

What was learned
Bottyan et al. found that "the area of the mean maximum UHI [urban heat island] intensity of higher than 2°C is 76 times larger in the non-heating season than in the heating season (0.5% and 38% respectively)," while "the strongest developments of UHI occurring in the warmer and colder periods were 5.8°C and 4.9°C respectively."  They also say they "proved a strong linear relationship between the mean UHI intensity and the urban parameters studied, such as built-up ratio and its areal extensions, in both seasons."

What it means
Once again we are reminded of the huge magnitude of the urban heat island effect compared to the global warming of the past decade or even century, as well as its dependency upon the specific nature of the urban landscape, which facts suggest to us that it is essentially impossible to adequately adjust temperature measurements made within an urban area to the degree required to correctly quantify background or rural climate change, which may be an order of magnitude smaller.