Now, an innovative study is about to be published - see the manuscript here: Measurement and statistical modeling of the urban heat island of the city of Utrecht (the Netherlands) by Theo Brandsm and Dirk Wolters from KNMI. The trick was, to do the year long observations by bike, as described in this abstract:
Mobile temperature and humidity measurements have been performed along a 14 km transect through the city of Utrecht (311,000 inhabitants) in the period March 2006 - January 2009. The measurements took place on a bicycle during commuter tra c and resulted in 106 nighttime profiles (before sunrise) and 77 daytime (afternoon) profiles. It is shown how the intensity of the urban heat island depends on wind direction, cloudiness and wind speed. Statistical models are constructed that relate the mean and maximum nighttime urban heat island intensity profiles to area-averaged sky-view factors and land use combined at both the micro and local scale. Sky-view factors are estimated from a 0:5 x 0:5 m surface elevation database and land use is obtained from a 25 x 25 m land use database. The models are calibrated using the mobile measurements and provide estimates of the spatial distribution of the mean and maximum nighttime urban heat island intensity in Utrecht. Both models explain more than 75% of the variance. A separate non-linear model is introduced that relates the temperature differences between the warmest and coolest part of the transects to wind speed and cloudiness.For unknown reasons, the overall strength of the UHI is not discussed in this abstract - the maximum difference between more urban areas and the location of the weather service KNMI in de Bilt is of the order of 1 degree, stronger at nighttime than on daytime (see Figs. 1 and 4.)
One would hope that more such innovative (and low budget!) projects would be done to help sorting out the important question, to what extend we are seeing in urban areas the effect of urbanization and of global warming, and to what extent planers could in principle seek to reduce the urban effect for mitigating the consequences of global warming.