Heat Exposure in Cities: Connecting the Dynamics of Temperature and Population

Metropolitan areas are usually warmer than their rural surroundings, a common phenomenon known as the urban heat island (UHI). This effect is compounded by heat waves, which are a leading cause of weather related human mortality in many countries worldwide. Previous studies have shown heat waves also change the urban heat island patterns, by disproportionally enhancing temperature in urban or rural areas. Studies of population vulnerability to heat traditionally analyze population as a static phenomenon.  However, urban population patterns change in a diurnal cycle due to commute. In order to better describe the population exposure to excess heat during heat waves, we combined spatiotemporal information of urban population and urban temperature distribution.  In this study, we use recently developed dataset of near-surface air and dew temperature from MODIS atmospheric profiles and the new method for the UHI quantification -- urban heat island curve -- to quantify changes in the urban heat distribution during heat waves.  We generate hourly population distribution at traffic analysis zone scale based on the Census Transportation Planning Products. Combining these datasets allows us to identify different levels of heat exposure among urban population spatially, four times a day.  Taking Chicago and Phoenix as examples, our results show that different cities exhibit different patterns of UHI response to heat waves and different dynamics of population exposure to heat.  The methodology presented in this paper has a wide application to other cities in the United States. This study has a significant implication for public health management during heat waves.