The Urban Heat Island and Local Temperature Variations in Orlando, Florida

Article excerpt

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

INTRODUCTION

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. …