Exposure to excessive heat can lead to adverse health outcomes in both healthy and vulnerable individuals. This study examines the spatiotemporal variability of exposure to severe heat at the sub-neighborhood scale using temperature and relative humidity measurements of a wireless distributed sensor network (WDSN). First, we demonstrate a multi-sensor calibration scheme for the temperature and the relative humidity sensors. Next, exposure to heat was calculated using the heat index (HI) scale, which enables linking exposure to HI and heat-related health risks. We noticed repeated exposures to excessive heat above the safe threshold for about 8 h per day throughout July-August, 2015, in Haifa, Israel. Persistent exposure to such conditions is unhealthy. The areas that experienced high HI were scattered across the study area, with the HI showing spatiotemporal variability. In general, in some microenvironments, the HI peaked earlier during the day than in other microenvironments. This was attributed to variability in urban physical drivers, which were found to be good predictors of the morning HI variability buildup but less so of the HI variability in the afternoon. Our results are consistent with summer HI occurrence in the study area in the past 20 years. Since exposure to excessive heat in the east Mediterranean is expected to increase in the future due to climate changes, it may result in a grave health toll.