Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania

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Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania Danielle Doyle Department of Geography and Earth Science Shippensburg University Shippensburg, PA 17257 Timothy W.

1 Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania Danielle Doyle Department of Geography and Earth Science Shippensburg University Shippensburg, PA Timothy W. Hawkins* Department of Geography and Earth Science Shippensburg University Shippensburg, PA twhawk@ship.edu *Corresponding Author ABSTRACT The development of cities potentially has a significant impact on climate. Buildings and infrastructure that replace natural vegetation often create new microclimates through changes in the energy balance associated with the built environment. The urban heat island effect, the phenomenon where air temperatures within an urban area are warmer than the surrounding rural areas, has been thoroughly examined for large urban areas. The purpose of this study is to determine the magnitude and extent of the urban heat island for a small urban area surrounded by agricultural land. Temperature data collected from several urban and rural locations over a four-month period indicate that urban areas are, on average, 0.8 C warmer than rural areas. The average difference between the most urban and most rural stations was 1.9 C. The maximum difference between these same stations was 10 C. Initial results, requiring further inquiry, suggest that the magnitude of the urban heat island is smaller during the summer season compared to the spring and fall. Key Words: urban heat island, agriculture, Shippensburg, Pennsylvania INTRODUCTION The development of cities has a significant impact on climate. Infrastructure and buildings that replace natural vegetation often create new microclimates in and around urban areas (Golden and Kaloush 2006). The urban heat island (UHI) effect describes the phenomenon where air temperatures within an urban area are higher than those in surrounding rural areas. Roads, buildings, and other infrastructure absorb solar radiation and store it more effectively than do rural surfaces. In addition, human activities such as industrial processes and air conditioning create heat (Chapman 2005; Voogt 2004). Consequently, temperatures in downtown The Geographical Bulletin 50:??-?? 2008 by Gamma Theta Upsilon 1

2 Danielle Doyle and Timothy W. Hawkins areas can be much warmer than the surrounding suburban and rural areas. Air temperatures can be as much as 5.5 C hotter in urban areas than in the surrounding rural areas (EPA 2007). Most UHI studies, however, have focused on larger urban areas. This particular study tested to see if a UHI is associated with the small urban area of Shippensburg, Pennsylvania (population 5,590). Urban areas are expanding across the globe. Over 70% of the population of developed countries lives in urban areas. In developing countries only about 40% of the population lives in urban areas. However, more and more rural people are flocking to the cities. By 2020, about 60% of the world s population will live in urban areas (PRB 2008). This migration can lead to the development of more and more heat islands, which can have a substantial impact on human health as well as air quality (Coutts et al. 2007; Chapman 2005). The urban heat island phenomenon is based on heat energy gains and losses (Chapman 2005; Souch and Grimmond 2006). Building structures and roads are built with rocklike materials such as brick, asphalt, and concrete, which have thermal characteristics that allow them to store heat more effectively (Chapman 2005; Voogt 2004; Golden and Kaloush 2006). These structures replace natural porous vegetative surfaces with resistant surfaces that convert precipitation to runoff rather than soil moisture, thereby eliminating the cooling process of evapotranspiration that would occur from a vegetated surface (Coutts et al. 2007). The UHI is also enhanced by taller buildings that cause multiple reflections of solar radiation, but ultimately greater overall radiation absorption (Chapman 2005; Voogt 2004). Tall buildings also decrease windaided cooling (Voogt 2004). In addition, industry, motor vehicles, and human activities such as heating and air conditioning can also contribute to the UHI effect through the addition of waste heat. These same activities often release pollutants that form a blanket over larger cities that can trap outgoing radiation, and therefore contribute to the UHI effect (Chapman 2005; Golden and Kaloush 2006; Voogt 2004). Considerable attention has been paid to both the seasonal and diurnal variations of the UHI phenomenon. UHIs typically are strongest during the nighttime hours (Oke 1982; Runnals and Oke 2000). Urban surfaces generally absorb more solar and heat energy during the day and release that energy more slowly at night compared to rural surfaces, which results in the most pronounced difference between urban and rural temperatures at nighttime. From a seasonal standpoint, the largest UHI for a location typically occurs during the driest and least windy season. For example, the UHI of Phoenix is typically largest in April and May after the winter rainy season and prior to the summer monsoon season (Hawkins et al. 2004). Most studies have focused on the UHI magnitude in mid-size or larger cities such as New York City (Holt and Pullen 2007); Phoenix, Arizona (Hawkins et al. 2004); Orlando, Florida (Yow and Carbone 2006); Toledo, Ohio (Schmidlin 1989); London, England (Meyer 1991); Melbourne, Australia (Coutts et al. 2007); Lisbon, Portugal (Alcoforado and Andrade 2006); Eilat, Israel (Potcher and Sofer 2006); Buenos Aires, Argentina (Figuerola and Mazzeo 1998); and a variety of Mediterranean cities (Mihalakakou et al. 2004). Few have studied the effect in smaller urban areas (e.g. Kopec 1970), but urban planners need to know whether this phenomenon is happening in smaller areas to enable better urban design and planning methods (e.g. allocation of green space or use of less heat absorbing building materials) (Coutts et al. 2007, Souch and Grimmond 2006, Kopec 1970). The purpose of this study is to determine if there is an UHI for the small urban area of Shippensburg, Pennsylvania. Shippensburg is a unique location to study the UHI effect due to its small size and its primarily agricultural setting. We hypothesize that even a small ur- 2

Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania ban area will have significantly warmer temperatures than surrounding rural agricultural lands during the summer months.

3 Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania ban area will have significantly warmer temperatures than surrounding rural agricultural lands during the summer months. The borough of Shippensburg is located in south-central Pennsylvania and is surrounded by Southampton Township (Fig. 1). Shippensburg lies in the central part of the Cumberland Valley and has an approximate elevation of m above sea level, with a population of about 5,590 people living in an area of 5.2 km 2 (U.S. Census Bureau 2000). Southampton Township has a population of 4,790 people living in an area of km 2 (U.S. Census Bureau 2000). Shippensburg s land use mainly consists of areas of high and low urban density with much smaller amounts of agriculture, forests, and wetlands (Fig. 1). The surrounding Southampton Township is mostly agriculture in the areas adjacent to the downtown Shippensburg area (Fig. 1). The primary crops grown in this area are corn and soybeans. The southern part of Southampton Township is mostly forest land (Fig. 1). The transition zone between forest and agriculture also represents the edge of the valley floor where elevations begin to increase. The average annual daily temperature for Shippensburg, as recorded by the National Weather Service/National Climatic Data Center s Cooperative Observers Network station at Shippensburg Universitym, is 12 C, with an average annual high of 17 C, and an average low of 6 C (Shippensburg University 2007) Table Figure 1. Land use and temperature sensors for Shippensburg and the surrounding townships. The land use map extent represents the Southampton Township boundaries. 3

4 Danielle Doyle and Timothy W. Hawkins Table 1. Average monthly summer temperatures for Shippensburg. Data are from the Coop. station # located at Shippensburg University. May Jun Jul Aug Sep Average Daily Temperature ( C) Average Daily Maximum Temperature ( C) Average Daily Minimum Temperature ( C) presents the temperatures for May through September when this study took place. DATA AND METHODS Temperature data were collected using HOBO Pro T/RH sensors 1. Figure 1 shows the locations of the seven temperature sensors and Table 2 shows the station characteristics. The stations are numbered one (most urban) to seven (least urban). Urban is defined based on proximity to the center of the urban area. Table 2 lists the site characteristics for each of the seven stations. We classified the first four stations as urban, and the final three as rural. We chose sites based on geographic distribution, land use, and access to the site. Hourly temperature measurements were taken from May 27, 2007 to September 19, This time frame insured that the study would encompass the summer season when it was believed that the UHI would be largest. Due to time and budget constraints, a longer study was not feasible. The magnitude of a UHI often varies with the seasons due to the varying dominant meteorological conditions (He et al. 2007). Relatively dry and calm conditions often result in the largest UHI (Alcoforado and Andrade 2006, Hawkins et al. 2004). Examination of the summer months limits the scope of the findings for this study. Important information regarding the seasonality and potential magnitude of the UHI is not possible to discern. Plans are underway to examine the UHI for this area on a longer time scale. To assess whether temperature differences between the sites were partly a function of elevation differences, the mean elevation of the urban sites was calculated as meters Table 2. Characteristics of study sites. 4 Site Coordinates Elevation NLCD Land Use General Land Use N W N W N W N W N W N W N W m High Urban Density Urban m High/Low Urban Density Urban m Low Urban Density Urban m Low Urban Density/ Mixed Forest Urban m Mixed Forest Rural m Hay Pasture/Row Crop Rural m Row Crop Rural

Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania and the mean elevation of the rural sites was 207.0 meters, a difference of only 1.1 meters.

5 Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania and the mean elevation of the rural sites was meters, a difference of only 1.1 meters. In addition, the biggest elevation difference occurs between an urban site having an elevation of meters (site 2) and a rural site having an elevation of meters (site 5), a difference of 34.7 meters. Based on a dry adiabatic lapse rate of 9.8 C/1000 m, elevation differences can account for a maximum temperature difference of a third of a degree. Any temperature differences greater than 0.3 C can be attributed to something other than elevation. RESULTS AND DISCUSSION Figure 2 shows the average hourly temperatures for the urban and rural stations for the study period. A clear UHI develops around 1900 and is at a maximum from 0000 to The largest average UHI value is 0.8 C during this time. A transition to a daytime urban oasis (i.e. rural temperatures are warmer than urban temperature) occurs from 0800 to 1200 where urban temperatures are slightly cooler than rural temperatures. The maximum urban oasis is 0.5 C. Urban oases often develop due to enhanced shading in urban areas during the daytime hours. Figure 3 shows the average daytime and nighttime temperatures for each of the seven sites over the study period. Daytime was defined as 0800 to 2000 and nighttime was Temperature ( C) Urban Rural Hour Figure 2. Hourly average urban and rural temperatures. Stations 1-4 are urban and 5-7 are rural. Temperature ( C) Night 1 Day Station Figure 3. Average night and day temperatures for each of the study sites. See Table 2 for a description of the sites and Figure 1 for the location of the sites. Day is defined as 0800 to Night is defined as 2100 to defined as 2100 to Station 1 represents the most urban and station 7 represents the most rural site. Little difference can be seen between the urban (1-4) and rural (5-7) sites during the daytime. There is a pattern of decreasing temperatures from station one (most urban) to station 7 (most rural). The average nighttime temperature difference between station 1 and station 7 is 1.9 C. Site 5, while classified as rural is actually located in a housing development outside of Shippensburg and consequently displays similar temperature characteristics to sites 2, 3, and 4, the low-density urban sites. Sites 6 and 7 are both located on single-family home properties larger than one acre that are adjacent to large tracts of agricultural land. Consequently, these stations are the two coolest. The greatest UHI of 10.0 C was observed between station 1 (25.2 C) and station 7 (15.2 F) on August 27, 2007 at There were 62 times (2% of the observations) when the UHI measured between station one and seven was larger than 5.0 C. To assess conditions that favor the development of a large UHI, Figure 4 shows a comparison of hourly temperature differences between stations 1 and 7 and hourly meteorological conditions recorded at Shippensburg University. The urban location is generally 5

1-7) ( C) 10 0 R 2 =0.14-10 5 10 15 20 25 30 35 40 10 0 Temperature ( C) R 2 =0.

6 Danielle Doyle and Timothy W. Hawkins 6 Temperature Difference (site 1-7) ( C) Temperature Difference (site 1-7) ( C) Temperature Difference (site 1-7) ( C) 10 0 R 2 = Temperature ( C) R 2 = Dew Point ( C) Wind Speed (m/s) Figure 4. Hourly temperature difference between station 1 (most urban) and 7 (least urban) versus temperature (a), dew point (b), and wind speed (c). a. b. c.

Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania warmer than rural location when the overall meteorological conditions are cooler (Figure 4a), less humid (Figure 4b),

7 Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania warmer than rural location when the overall meteorological conditions are cooler (Figure 4a), less humid (Figure 4b), and less windy (Figure 4c). Increased humidity serves to moderate temperatures and does not allow for larger temperature differences to occur. Lower humidity conditions are often associated with cooler temperatures as the result of advection of a continental polar airmass. Likewise, windy conditions serve to better mix the atmosphere resulting in smaller urban-rural temperature differences. This phenomenon can be clearly seen in Figure 4c as the temperature differences approach zero with increasing wind speed. The urban oasis effect (where rural areas are warmer than urban areas during the morning hours) is larger during warmer and more humid conditions. Increased radiation during these times is likely to heat up rural areas while urban areas remain shaded for the morning hours. Note that this meteorological analysis is cursory and is the subject of ongoing research. Seasonality and time of day were not examined in Figure 4. Of particular interest for future study is the impact of agricultural growth throughout the summer on rural microclimates and thus on the regional UHI. Figures 5 and 6 show the spatial pattern of average nighttime and daytime temperatures. Note that the temperature ranges represented by each symbol are different for the two figures. Nighttime temperatures (Fig. 5) are warmest at station 1 (most urban) and Figure 5. Nighttime average temperatures. Night is defined as 2100 to Land use is also shown for reference. 7

Danielle Doyle and Timothy W. Hawkins are progressively cooler moving out of the urban area. Station 5, the rural site to the south of Shippensburg, is relatively warm.

8 Danielle Doyle and Timothy W. Hawkins are progressively cooler moving out of the urban area. Station 5, the rural site to the south of Shippensburg, is relatively warm. The housing development helps to explain this temperature feature. Daytime differences are not as evident (Fig. 6). While an urban oasis is evident from 0800 to 1200 (Fig. 2), warmer afternoon temperatures quickly dissipate the overall oasis effect in Figure 6. The rural station 6 is warmest and the urban station 2 is next warmest. The other three urban stations as well as station 5, which is in the rural housing development, are the coolest during the day. Note that the daytime temperature differences between all stations are much smaller compared to the nighttime temperature differences. Figure 7 shows the monthly average urbanrural temperature differences for both nighttime and daytime values. There is no clear seasonal pattern in the daytime values. However, at nighttime, the UHI is smaller during June, July, and August than it is in May and September. The summer months are typically more humid and therefore do not allow for as large an UHI to form, since increased water vapor moderates temperatures, thereby reducing urban-rural temperature differences. Humidity may also be increased in the summer due to agricultural production and the associated transpiration thereby warming the rural areas. Because data were not collected for the entire month of May or September, caution must be taken in making generalizations of this nature. Figure 6. Daytime average temperatures. Day is defined as 0800 to Land use is also shown for reference. Note that the temperature ranges represented by each symbol are different between figures. 8

9 Assessing a Small Summer Urban Heat Island in Rural South Central Pennsylvania Urban-Rural Temperature Difference ( C) Day May Night Jun Jul Aug Sep Month Figure 7. Monthly average urban rural temperature difference for night and day. Day is defined as 0800 to Night is defined as 2100 to Note that the temperature ranges represented by each symbol are different between figures. CONCLUSION Analysis of temperature data collected around Shippensburg, Pennsylvania revealed that a small yet consistent UHI exists for this small urban area located within a surrounding agricultural landscape. Specific results drawn from this study can be summarized as follows: A 0.8 C UHI exists on average from 1900 to A smaller urban oasis effect exists from 0800 to The average UHI as measured between the most urban and most rural station was 1.9 C. The maximum observed UHI was 10.0 C. A UHI greater than 5.0 C was observed 2% of the time. The UHI is largest under cooler, less humid, and less windy conditions. The average UHI is greater in May and September compared with June, July, and August. The UHI for this small urban area was smaller than those typically found in larger cities (Souch and Grimmond 2006). The diurnal timing of the UHI and urban oasis effect were similar to other UHI studies (e.g. Holt and Pullen, 2007; Hawkins et al. 2004; Yow and Carbone 2006). Another key difference between this study and other UHI studies is the size of the rural area relative to the urban area. In this case, agricultural land uses dominate the study area, compared to most UHI studies where urban land use dominates the study area. As urban areas increase in geographic size and population, it is important to observe the characteristics of urban environments and further study the effects these characteristics have on a city s climate. The findings of this study indicate that even small urban areas can develop UHI s. Such knowledge is important when considering the future development of small urban areas. A critical next step will be to evaluate what effect the seasonality associated with agriculture has on the magnitude and extent of a small rural UHI. NOTES 1. HOBO temperature sensors have an operating range of -30 C to 50 C. The sensors have a standard deviation of ± 0.2 C between 0 C and 40 C, and a standard deviation of ± 0.3 C to 0.6 C when the temperature is greater than 40 C or less than 0 C. ACKNOWLEDGEMENTS Thank you to editor Steven Schnell and three anonymous reviewers whose comments greatly improved this article. REFERENCES Alcolforado, M. J. and H. Andrade Nocturnal Urban Heat Island in Lisbon (Portugal): Main Features and Modeling Attempts. Theoretical and Applied Climatology, 84(10): Chapman, D It s HOT in the City! Geodate 18(2): 1-4. Coutts, A. M., J. Beringer and N.J. Tapper Impact of Increasing Urban Density on Local Climate: Spatial and Temporal Variations in the Surface Energy Balance 9

10 Danielle Doyle and Timothy W. Hawkins in Melbourne, Australia. Journal of Applied Meteorology and Climatology, 46: EPA [Environmental Protection Agency]. Heat Island Effect. Washington D.C. [ Accessed September Figuerola, P.I., and N.A. Mazzeo Urban-Rural Temperature Differences in Buenos Aires. International Journal of Climatology, 18: Golden, J.S., and K.E. Kaloush Mesoscale and Microscale Evaluation of Surface Pavement Impacts on the Urban Heat Island Effects. The International Journal of Pavement Engineering, 7(1): Hawkins, T.W., A.J. Brazel, W.L. Stefanov, W. Bigler, and E.M. Saffell The Role of Rural Variability in Urban Heat Island Determination for Phoenix, Arizona. Journal of Applied Meteorology, 43: He, J.F., J.Y. Liu, D.F. Zhuang, W. Zhang, and M.L. Liu Assessing the Effect of Land Use/Land Cover Change on the Change of Urban Heat Island Intensity. Theoretical and Applied Climatology, 90: Holt, T. and J. Pullen Urban Canopy Modeling of the New York City Metropolitan Area: A Comparison and Validation of Single and Multilayer Parameterizations. Monthly Weather Review, 135: Kopec, R. J Further Observations of the Urban Heat Island in a Small City. Bulletin American Meteorological Society, 51(7): Meyer, W Urban Heat Island and Urban Health: Early American Perspectives. Professional Geographer, 43(1): Mihalakakou, G., M. Santamouris, N. Papanikolaou, C. Cartalis, and A. Tsangrassoulis Simulation of the Urban Heat Island Phenomenon in Mediterranean Climates. Journal of Pure and Applied Geophysics, 16: Oke, T.R The Energetic Basis of the Urban Heat Island. Quarterly Journal of the Royal Meteorological Society, 108(455):1-24. Potchter, O. and M. Sofer The Urban Heat Island of a City in an Arid Zone: the Case of Eilat, Israel. Theoretical and Applied Climatology, 85: PRB [Population Reference Bureau]. Has the World s Population Distribution Changed over Time. [ PopulationGrowth/QuestionAnswer.aspx]. Accessed July Runnals, K.E., and T.R. Oke Dynamics and Controls of the Near-Surface Heat Island of Vancouver British Columbia. Physical Geography, 21(4): Schmidlin, T. W The Urban Heat Island at Toledo, Ohio. Ohio Journal of Science, 89(3): Shippensburg University Weather Page. Climatology Page. Shippensburg, Pennsylvania. < Accessed September Souch, C. and S. Grimmond Applied Climatology: Urban Climate. Progress in Physical Geography, 30(2): U.S. Bureau of the Census Census Washington D.C. [ census.gov/main/www/cen2000.html]. Accessed September Voogt, J., Urban Heat Islands: Hotter Cities. American Institute of Biological Sciences, Washington D.C. Yow, D.M. and G.J. Carbone The Urban Heat Island and Local Temperature Variations in Orlando, Florida. Southeastern Geographer, 46(2):

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