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Cities are generally warmer than their adjacent rural land, a phenomenon known as the urban heat island (UHI). Often accompanying the UHI effect is another phenomenon called the urban dry island (UDI), whereby the humidity of urban land is lower than that of the surrounding rural land
. The UHI exacerbates heat stress on urban residents
, whereas the UDI may instead provide relief because the human body can cope with hot conditions better at lower humidity through perspiration
. The relative balance between the UHI and the UDI-as measured by changes in the wet-bulb temperature (T
)-is a key yet largely unknown determinant of human heat stress in urban climates. Here we show that T
is reduced in cities in dry and moderately wet climates, where the UDI more than offsets the UHI, but increased in wet climates (summer precipitation of more than 570 millimetres). Our results arise from analysis of urban and rural weather station data across the world and calculations with an urban climate model. In wet climates, the urban daytime T
is 0.17 ± 0.14 degrees Celsius (mean ± 1 standard deviation) higher than rural T
in the summer, primarily because of a weaker dynamic mixing in urban air. This T
increment is small, but because of the high background T
in wet climates, it is enough to cause two to six extra dangerous heat-stress days per summer for urban residents under current climate conditions. The risk of extreme humid heat is projected to increase in the future, and these urban effects may further amplify the risk.