It is well-known that urban areas possess higher temperatures than nearby rural locations, but it is uncertain whether certain air masses contribute more significantly to the urban heat island effect. For example, are all air masses similarly warmer in the urban areas? Or are some air masses more significantly affected than others? If the latter is the case, this can have important urban health implications, especially if hot, tropical air masses are the ones that constitute the majority of the urban heat island warming. Although there is a proliferation of urban heat island studies, we have identified none which attempts to determine the differential effect of air masses on the urban heat island thermal regime. In addition, if there is a difference in urban warming among air masses, there is no research which has tried to ascertain why this would be the case.

The goals of this proposed research are as follows:

• to determine which air masses (if any) possess the greatest urban/rural disparity,
• if certain air masses carry the majority of the heat island burden, to determine if the urban/rural disparity within these air masses has been increasing over the past 50 years.
• if there is evidence of either of the above, to determine why this might be the case.

• DP, dry polar air mass; very cold and very dry
• DM, dry moderate air mass; mild and dry
• DT, dry tropical air mass; very hot and very dry
• MP, moist polar air mass; cold, cloudy, rainy
• MM, moist moderate air mass; Aoverrunning@, cool, drizzle, foggy
• MT, moist tropical air mass; hot, very humid
• TR, transition situation; a change from one air mass to the next, e.g. frontal passage.

The proposed work would encompass one or more major urban areas in the United States, and would compare urban/rural maximum and minimum temperature differences for each air mass for each season of the year. It is very possible that the urban/rural thermal differential varies from one season to the next within each air mass. For the city of Philadelphia, which is the first area we would like to study, we have identified at least 30 weather stations within a 35-mile radius of Center City which could provide us with the meteorological data that we need. We have access to the most up-to-date data free of charge through an agreement we have with NOAA=s National Climatic Data Center.

The implications of this work are enormous in terms of both public health and urban energy usage. For example, a pilot study has determined that the DT air mass in summer, traditionally associated with greatly elevated human mortality, shows the greatest urban/rural disparity in the Philadelphia area. A cursory analysis indicates that dry air masses seem to show a greater temperature difference between urban and rural stations than moist air masses; summertime differentials are greater than wintertime. Of the moist air masses, MT shows the greatest urban/rural thermal disparity; this is also important because this air mass is associated with the greatest number of heat-related deaths in the summer. Our initial results should be viewed with caution because of the cursory nature of our investigation. Only one urban and one rural station was used in our evaluation, and this was done for one eastern city only. Obviously, to determine if these results represent systematic differentials, we must use numerous locales, which would help us determine the Adistance decay@ effect of thermal differentials as we move away from the central city.

One other interesting feature of the urban/rural thermal difference among air masses involves whether the disparity has been increasing over the past 50 years. Initial analysis suggests that only the two hottest air masses (DT and MT) are showing an increased disparity. If this result is borne out in the wider study, it might suggest that increased air conditioning usage through the period may be a major contributor in increasing the urban heat island effect within these hot air masses. Of course, these are the air masses associated with elevated air conditioning use in the summer.

The air conditioning implication is important, because over 25 percent of most center city air sheds up to 300 feet in elevation represent air conditioned buildings (the number is considerably higher in Manhattan). Thus, while these buildings are cooled during hot weather, the heat which has been removed from the buildings is extracted into the outside air by the air conditioning systems. Using data from many weather stations within a 35-mile radius of the central city, we can determine if this increasing thermal disparity extends beyond the downtown buildings and into urban and suburban residential areas as well. Also, if the DT and MT urban/rural temperature disparity increased largely in the 1960s and 1970s, when downtown buildings were initially air conditioned, this may provide strong evidence that air conditioning is causing a local heating phenomenon within the hottest air masses in our downtown urban areas.

This project will quantify: (1) the urban/rural temperature differences for each air mass for each season and for a number of large urban areas in different climate regimes; (2) the actual increase in thermal disparity (if any exists) through time (1948 to the present) within each air mass; and (3) whether or not air conditioning appears to be the reason for the increasing disparity. By plotting temperature differentials for each air mass at all the stations surrounding an urban area, we can actually map the extent of the urban heat island effect for each air mass, and determine the distance decay from the center of the city.

REFERENCE

Kalkstein, LS, SC Sheridan and DY Graybeal, 1998. A determination of character and frequency changes in air masses using a spatial synoptic classification. International Journal of Climatology 18:1223-1236.