Limited Role of Absolute Humidity in Intraurban Heat Variability

Monitoring and understanding the variability of heat within cities is important for urban planning and pub-lic health, and the number of studies measuring intraurban temperature variability is growing. Recognizing that the physio-logical effects of heat depend on humidity as well as temperature, measurement campaigns have included measurements of relative humidity alongside temperature. However, the role the spatial structure in humidity, independent from tempera -ture, plays in intraurban heat variability is unknown. Here we use summer temperature and humidity from networks of sta-tionary sensors in multiple cities in the United States to show spatial variations in the absolute humidity within these cities are weak. This variability in absolute humidity plays an insignificant role in the spatial variability of the heat index and hu-midity index (humidex), and the spatial variability of the heat metrics is dominated by temperature variability. Thus, results from previous studies that considered only intraurban variability in temperature will carry over to intraurban heat variabil-ity. Also, this suggests increases in humidity from green infrastructure interventions designed to reduce temperature will be minimal. In addition, a network of sensors that only measures temperature is sufficient to quantify the spatial variability of heat across these cities when combined with humidity measured at a single location, allowing for lower-cost heat moni-toring networks.SIGNIFICANCE STATEMENT: Monitoring the variability of heat within cities is important for urban planning and public health. While the physiological effects of heat depend on temperature and humidity, it is shown that there are only weak spatial variations in the absolute humidity within nine U.S. cities, and the spatial variability of heat metrics is dominated by temperature variability. This suggests increases in humidity will be minimal resulting from green infra-structure interventions designed to reduce temperature. It also means a network of sensors that only measure tempera -ture is sufficient to quantify the spatial variability of heat across these cities when combined with humidity measured at a single location.