Human-biometeorological assessment of the urban heat island in a city with complex topography – The case of Stuttgart, Germany

• Analysis of urban heat island and intra-urban variability using air temperature and thermal indices.
• UHI is much higher using thermal indices compared to air temperature.
• Strong influence of topography and land-use on UHI intensity.
• UHI classification based on weather types.

The spatial and temporal differences of climate, urban heat island and future conditions were assessed for Stuttgart, a city located in complex topography in Southwest Germany. The present and future urban climate conditions were analysed using hourly measured data from 2000 to 2011 of 5 measuring stations and data from regional climate simulations. The urban heat island intensity was quantified applying thermal indices as Physiologically Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI) and compared to weather type classifications. In Stuttgart, wind speed was mostly less than 3 ms−1, the wind roses were very inconsistent and local wind could be clearly observed. The average annual urban heat island of air temperature was between 0.3 K in the suburb areas to 2 K in the city centre and the maximum up to 12 K. The assessed urban heat island with PET was in average 3.3 K and maximum around 20 K. Based on regional climate simulation we found that the amount of days with heat stress (PET ⩾⩾ 35 °C) is estimated to increase by about 17 days until the end of the 21st century. Urban heat island and intra-urban variability were most obvious using thermal indices rather than air temperature.