Monitoring and understanding the variability of heat within cities is important for urban planning and public health, and there has been a growth in the number of studies measuring intra-urban temperature variability. Recognizing that the physiological effects of heat depend on humidity as well as temperature, some of these measurement campaigns have included measurements of relative humidity alongside temperature. Reported analyses, however, have not reported whether spatial structure in humidity, independent from temperature, contributes significantly to intra-urban heat variability. Here we use summer temperature and humidity from networks of stationary sensors in multiple cities in the USA to examine this issue. It is shown that although there are spatial variations in relative humidity there are only very weak spatial variations in the absolute humidity within these cities.  This variability in absolute humidity plays an insignificant role in the spatial variability of the heat index and humidex, and the spatial variability of the heat metrics is dominated by temperature variability. A practical consequence of this is that a network of sensors that only measure 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 monitoring networks.

Data was collected on Thermocron iButtons placed at 2 m height within trees in each of the cities.