The Urban Heat Island and Local Temperature Variations in Orlando, Florida
Yow, Donald M., Carbone, Gregory J., Southeastern Geographer
Cities artificially alter local climates affecting economic and biological processes. This study examined air temperatures in Orlando's urban canopy layer using a network of twenty-nine fixed-point stations from September 1999 to December 2001. Urban Heat Island (UHI) statistics were calculated using two stations that were representative of an urban and rural setting. Orlando's UHI develops best on calm, clear nights during dry months: its maximum magnitude exceeds 8[degrees]C Orlando's UHI, however, is predominantly a nocturnal phenomenon with intense heat islands sometimes occurring during warm afternoons. These events are most likely attributable to isolated thundershowers. Local temperature variations between urban and rural extremes were examined by calculating environmental indices for all stations. The range in monthly cooling degree-day totals exceeded 100 degree days in six months of the thirty-six month study period. Heating degree totals and number of freezing hours were also highly variable.
KEY WORDS: urban climate, urban heat island, temperature variability, Orlando
Urban climates differ significantly from conditions in the surrounding region. The higher evening and early morning temperatures of cities, commonly called the urban heat island (UHI) effect, results from differences in thermal and physical properties of construction materials, building geometry, surface roughness, factors contributing to decreased evapotranspiration, and anthropogenic heat sources (Tyson, Garstang, and Emmitt 1973; Oke 1987). Urban heat islands have been studied in numerous cities around the world because the myriad factors leading to the existence of an UHI differ from city to city. It is important to understand heat islands in many cities to detect the impact artificially elevated temperatures have on human comfort (Palecki, Changnon, and Kunkel 2001), city water and energy demand (Bailing and Brazel 1988; Jauregui 1998; Palecki, Changnon, and Kunkel 2001), air pollution (Viras 2002), and the introduction of bias into long-term temperature records (Lowry 1977; Kukla, Gavin, and Karl 1986; Karl, Diaz, and Kukla 1988; Jones, Kelly, and Goodess 1989; Karl and Jones 1989; Jones et el. 1990; Hughes and Bailing 1996; Bohm 1998; Brazdil and Budikova 1999).
Urban heat islands are important from an economic standpoint because air temperature adversely affects electricity consumption (Svensson and Eliasson 2002; Matsuura 1995; Quayle and Diaz 1980). Urban warmth may actually reduce annual energy consumption in cold climates, but the reverse is true in warm cities where summer air conditioning loads far outweigh potential savings in energy use for heating during winter. Elevated urban temperatures seriously impact energy consumption for air conditioning (Sanatmouris et al. 2001), which has considerable economic implications for southern cities like Orlando. Sailor (2001) found that Florida's per capita energy consumption is extremely sensitive to higher temperatures, citing increases in residential energy consumption of 5.3%, 11.6%, and 18.8% when temperature increased by 1[degrees], 2[degrees], and 3[degrees]C, respectively. The corresponding increases in the commercial sector were 2.4%, 5%, and 8%.
In addition to affecting local energy consumption, urban heat islands also affect humans and the biosphere. Extreme nocturnal humidity and temperatures, particularly within the heat islands of Chicago and St. Louis, were major factors contributing to more than 1,000 deaths in 1995 and over 300 deaths in 1999 during heat waves in the central United States (Palecki, Changnon, and Kunkel 2001). Populations exposed to prolonged periods of high temperatures experience increased mortality not only due to heatstroke, but also due to various diseases associated with the heart and lungs (Kilbourne 1998). While the elderly are often cited as having increased risk during warm conditions, other cohorts also experience adverse heat-related effects. For example, Marzuk et al. (1998) found that elevated temperatures contributed to significant increases in death from cocaine overdose. Plants and animals are also adversely affected by urban heating. Impacts associated with recent large-scale warming at various locations around the world illustrate the interconnectedness of climate and the biosphere. Among these are changes in bio-cycles, flowering dates, migration patterns, and habitat range (Montaigne 2004). Collectively, these impacts can affect individual species and/or overall ecosystem health. The magnitude of local urban warming in many cities far exceeds that of larger-scale regional or global warming and effects on the biosphere in those cities increase accordingly. Examining the relationship between urban climate and plant phenology, Matsumoto and Fukuoka (2003) found that the heat island of Kumagaya City in Japan has caused variation in the flowering dates of Prunus yedosnsis both spatially and historically since 1956. It is likely that urban warmth also leads to increased incidence of infectious disease because heat allows large numbers of pests to breed, including mosquitoes, which spread a number of dangerous diseases such as encephalitis and the West Nile virus (Baron-Faust 2000). It is also possible that various allergen producers such as molds, mildews, and weeds may find favorable habitats in cities with artificially elevated temperatures. For instance, ragweed grows faster, flowers earlier, and produces stronger pollen when under the influence of warmer temperatures and higher C[O.sub.2] concentrations (Newman 2006).
This study examined air temperatures in Orlando's urban canopy layer below roof level from July 1999 to June 2002. The influence that Orlando's urbanization has on local temperatures was quantified by calculating the magnitude of Orlando's UHI using data from two stations that were selected to be representative of an urban and rural setting. Observations from the rural station provided a baseline temperature of the natural environment and the urban station approximated the maximum amount of influence urban development in Orlando has on local temperatures. Therefore, UHI statistics estimate the magnitude of temperature variability that occurs in Orlando.
In addition to quantifying Orlando's UHI intensity, statistical analysis was used to describe temporal variation of this phenomenon and how weather conditions influence the difference between urban and rural temperatures. Heat island magnitude typically increases rapidly after sunset, reaches its maximum sometime near the middle of the nocturnal period, and diminishes shortly after sunrise (Landsburg 1981; Oke 1982). This diurnal character of UHI development has been well documented in numerous cities around the globe, but the heating/cooling rates of urban and rural areas differ from one city to the next and should be quantified in heat island studies. Some researchers have found seasonal patterns in UHI behavior as well (e.g., Jauregui 1997; Figuerola and Mazzeo 1998; Magee, Curtis, and Wendler 1999). This study investigated both diurnal and seasonal aspects of Orlando's UHI. It also analyzed the role synoptic conditions play in Orlando's UHI development. Synoptic conditions can dramatically affect UHI magnitude, especially wind and clouds (Landsburg 1981; Oke 1987). Nocturnal urban heat islands are generally more intense when wind speeds are light because of less mixing between urban and rural air, and when skies are clear because rural cooling rates are greater (Landsburg 1981).
This study also explored the spatial
variability of temperatures between urban and rural extremes. While UHI intensity provides a handy measure of maximum temperature differences that occur within a city, it does not address the issue of spatial variability. Previous studies have attempted to map air temperature patterns in cities using traverse studies and fixed point networks. Traverse studies are much better at describing the intricate nature of heterogeneous urban environments because fixed stations are limited in the area their observations represent. One can, however, obtain insight into local scale variability with a fixed point network if instrument locations are carefully chosen (Oke 2004). In this study, three temperature-based environmental indices were calculated for each station in a network of 29 weather stations: cooling degree-days, heating degree-days, and number of freezing hours. Environmental indices have numerous practical applications relating to human comfort and health, the growth and behavior of plants and animals, and energy demand. Thus, environmental indices are quite useful in describing a given area's thermal characteristics. The findings from this portion of the study demonstrate how much local temperature varies among some of the landscapes that are commonly found in the Orlando area. It is hoped that this information will help urban planners, businesses, and homeowners in the Orlando area reduce energy consumption and mitigate future losses caused by thermal …
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Publication information: Article title: The Urban Heat Island and Local Temperature Variations in Orlando, Florida. Contributors: Yow, Donald M. - Author, Carbone, Gregory J. - Author. Journal title: Southeastern Geographer. Volume: 46. Issue: 2 Publication date: November 2006. Page number: 297+. © 2009 University of North Carolina Press. COPYRIGHT 2006 Gale Group.
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