Heat Waves and the Urban Heat Island: Employing Citywide Observational Networks to Assess Spatiotemporal Variability of Air Temperature in the U.S. Northeast

The urban heat island (UHI) is a well-documented phenomenon that occurs when air temperatures within an urban area are greater than that of the surrounding rural area. This is due to increased sensible heat storage, anthropogenic heating, and many other complex urban factors that change on very short spatial scales. Enhanced temperatures in urban areas have been associated with a negative impact on human health by elevating personal mortality and morbidity risk, exacerbating already harmful heat waves, and not allowing relief from daytime heat with higher overnight temperatures; hence, it is important to study the spatiotemporal variations in the surface UHI and determine areas of greatest risk. There have been many studies completed to understand city-specific UHIs at the mesoscale, yet there has been minimal research conducted using observational data at microscale. Moreover, minimal research has addressed the influence of the synoptic weather type in modifying the intensity of the localized UHI within the urban canopy layer. For cities to manage the growing populations, risks, and vulnerabilities associated with such high heat exposures, as well as to increase their resiliency with climate variability during harmful heat waves, progress is needed for understanding the spatial and temporal variations of UHI development.

This study examines four large northeastern cities during the warm season (May through September): Boston, Baltimore, Philadelphia, and New York. The objectives of this study are: 1) examine inter-city variations of diurnal UHI intensity under different synoptic weather types using meteorological data from over 600 UrbaNet stations (NOAA and Earth Networks); 2) study intra-city temperature patterns, employing GIS software and cross-sectional visualizations; and 3) determine the effect wind direction has on UHI intensity. Analysis is focused on the most oppressively hot weather types (hot and dry, hot and humid) and their UHI intensity in various microclimates.

Results showed that dry weather types yielded the largest UHI intensities at night. Further, the intensities under the dry weather types were found to be significantly different than their moist counterparts. Numerous instances of negative daytime UHIs were also found in each city. Surface air temperatures varied by as much as 4–6oC depending on the city due to different land cover types such as urban parks, asphalt, and white roofs. Such intra-city comparisons helped identify the warmest regions of each city based on the underlying local climate zone, further demonstrating that surface air temperature does not follow the pattern commonly assumed from previous UHI work. New York displayed the largest variations in UHI intensity with the highest nighttime magnitude. In Boston, Baltimore, and New York City, wind direction was found to influence daytime UHI formation with daytime UHI intensity below average during onshore flow, and above average for offshore flow.

The current study provides immense potential to better understand the regions within a city that are most vulnerable to heat during the hottest times of the summer. This reflects areas with the strongest warming potential, and hence areas in need of mitigation and adaptation strategies to help reduce urban risks and hazards to extreme heat.