Novel bicycle traverse meteorological measurements were made in Rotterdam to assess the spatial variation of temperature during a tropical day. Nocturnal spatial urban temperature differences of 7¿K were found to be related to city morphology. The coolest residential areas were green low-density urban areas. During midday measurements the downtown was up to 1.2¿K warmer than the surrounding rural area while a city park was 4.0¿K cooler than downtown. A regression analysis showed that the nocturnal measured urban heat island (UHI) can be linked to land use, namely plan area fraction of vegetation, built up area water and is most significant for vegetation. The vegetated area was derived from visible and near infrared aerial images. Neighbourhoods with vegetation (within an upwind radius of 700¿m) had a significantly reduced UHI during the night. From the traverse observation data a multiple linear regression model was constructed and independently validated with 3-year summertime UHI statistics derived from 4 urban fixed meteorological stations. In addition, two fixed rural stations were used; a WMO station at Rotterdam airport and a rural station further away from the city. Wind rose analysis shows that UHI is strongest from easterly directions and that the temperature signal of the WMO station is influenced by an UHI signal from both the airport runways and urban directions. A regression model reproduced the nighttime spatial variability of the UHI within a fractional bias of 4.3% and was used to derive an UHI map of Rotterdam and surroundings. This map shows that high density urban configurations lacking greenery or close to large water bodies are vulnerable to high nocturnal temperatures during heat waves. This warming effect of water bodies is also evident for an urban weather station located in the harbor area, which had a similar nocturnal UHI frequency distribution as the downtown urban weather station. The UHI map can be used as a valuable planning tool for mitigating nocturnal urban heat stress or identifying neighborhoods at risk during heat waves.
- climatic change
- urban areas