An Evaluation of the Variability of Air Mass Character between Urban and Rural Areas

While numerous studies have examined the magnitude of the urban heat island, no large-scale study has yet analyzed its variability using an air-mass based methodology. The air-mass approach can provide unique insight by discerning how large-scale weather patterns may impact the magnitude of the heat island effect. This study utilizes the Spatial Synoptic Classification (SSC) scheme, a recently-revised air mass classification system which is available for over 300 North American stations for 50 years. For several urban areas across the eastern United States, daily maximum and minimum temperatures were compared between urban and rural sites. Data are segregated by air mass and by season. Overall, the three "dry" air masses show considerably larger urban heat island disparities than the three "moist" air masses. These effects are much more pronounced with minimum temperature than maximum temperature. Summertime minima are slightly more affected than wintertime minima, with the exception of one air mass which shows the opposite trend. The air masses traditionally associated with elevated mortality in urban areas have some of the most intense urban/rural differentiations, with overnight temperatures typically 3° C or more above outlying areas. Temporal trends (over the second half of the century) are also examined. For most urban locations, the summer urban heat island appears to be strengthening, whereas in winter little to no change is observed. This may be related to increased air conditioning use during the summer, although further research is needed to ascertain the effects of air conditioning upon the urban heat island.