Urban Influences on Temperatures of Baltimore and Phoenix Urban LTER Sites (presentation)
Gordon M. Heisler
A goal of research in the urban LTER sites, Baltimore, MD and Phoenix, AZ is to examine similarities and differences in their ecosystems, including impacts on climate. As a first step, we analyzed long-term urban minus rural temperature trends using the GHCN (Global Historical Climate Network) database for several weather stations in and near Baltimore and Phoenix. This study will aid in planning and analysis of climate measurements made in LTER research. For the Baltimore area, the stations included the downtown Baltimore station at the Customs House; the Baltimore-Washington International Airport (BWI); the major Washington, DC airports, Dulles and National; and a rural station near Woodstock, MD, about 15 miles west of Baltimore. For Phoenix, the climate data came from the Sky Harbor airport, downtown Phoenix, Mesa, Tempe, and a rural location near Sacaton. The useable climate records began as early as 1908 and extended to 1997 for some stations. For the Baltimore region, the analysis used average daily maximum and minimum temperatures for July. For Phoenix, it used data from May.
Time series of the urban minus rural maximum temperatures (ΔTmaxU-R) show a difference between the humid, forested East and the arid desert regions. Urban maximum temperatures tend to be cooler than rural maximums in Phoenix (that is, values of ΔTmaxU-R tend to be negative in Phoenix) and warmer than rural in Baltimore (Figure 1, top). This results largely from extensive watering of plants in urban areas in arid climates. There are only slight long-term trends for changing ΔTmaxU-R. In downtown Baltimore City, ΔTmaxU-R averaged about 1.5 °C toward the end of the period, up about 1 °C since 1950 (Figure 1, top). Generally, Baltimore downtown was warmer than BWI. Maximum temperatures at BWI were close to maximums at Woodstock. National Weather Service studies suggest that the temperatures measured at the Customs House in downtown Baltimore may be especially high because the station was located on a building roof (Personal communication, Robert Leffler of NWS). The station was moved to a ground-level location over grass at the Baltimore Science Center in May 1999.
Urban minus rural minimum temperature differences (ΔTminu-r) are greater than differences in maximums and tend to reflect population trends. The long-term average ΔTminu-r for Baltimore City peaked at 4.5 °C about 1970, and decreased slightly since then (Figure 1, bottom), apparently because of development encroaching on Woodstock, rather than because population decreased in Baltimore. A similar trend appears for the BWI-Woodstock ΔTminu-r since the BWI record began in 1951. Long-term average ΔTminu-r for Phoenix increased substantially from about 2.5 °C in 1908 to 6.5 °C in 1995. The rural comparison site for Phoenix, Sacaton, has remained with little development since the beginning of the century.
Thus, as has been found in many other cities, the so-called "urban heat island" in both Baltimore and Phoenix is primarily manifested in increased nighttime temperatures rather than in greatly increased temperatures during the warmest part of the day. Planned LTER studies will examine the differences in temperature within cities that result from variations in urban structure. The complete results of the analysis outlined here will appear in a forthcoming article in Climate Research by Anthony Brazel and the other authors of this abstract.
Figure 1. Difference in average July maximum (top) and minimum (bottom) daily temperatures between downtown Baltimore and Woodstock (filled circles) and between Baltimore-Washington International Airport (BWI) and Woodstock (open circles). The curves, fitted by the LOWESS smoothing method in SYSTAT, show long term trends by averaging over year to year variation.