Measurement and Statistical Modeling of the Urban Heat Island of the City of Utrecht (the Netherlands)

Mobile temperature and humidity measurements have been performed along a 14-km transect through the city of Utrecht, in the Netherlands (311 000 inhabitants), during the period March 2006-January 2009. The measurements took place on a bicycle during commuter traffic 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 scales. Sky-view factors are estimated from a 0.5 m X 0.5 m surface elevation database, and land use is obtained from a 25 m 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 nonlinear model is introduced that relates the temperature differences between the warmest part and coolest part of the transects to wind speed and cloudiness.