Refreshing the role of open water surfaces on mitigating the maximum urban heat island effect

• Urban water bodies influence the urban microclimate.
• We explore whether urban water bodies improve the urban heat island effect.
• Observations show that urban lakes increase rather than decrease the urban heat island effect.

During warm summer episodes citizens in urban areas are subject to reduced human thermal comfort and negative health effects. To mitigate these adverse effects, land use planners and urban designers have used the evaporative power of water bodies as a tool to limit the urban heat island effect (UHI) and undesirable human thermal comfort. Based on weather observations by Dutch hobby meteorologists and a station network in Rotterdam (Netherlands), we show that water bodies increase rather than decrease the 95 percentile of the daily maximum UHI. The high heat capacity of water suppresses the diurnal and annual cycle over water, and water temperatures remain relatively high after evening and season transitions. This is reflected to the 2 m temperature above and in the surround of the water body, and in a relatively high UHI. Our result has consequences for the daily practice in urban design concerning microclimate effects.