Influence of Meteorological Conditions on Urban/Rural Temperature and Humidity Differences for a Small City

Southeastern Geographer Vol. 27, No. 2, November 1987, pp. 90-100 INFLUENCE OF METEOROLOGICAL CONDITIONS ON URBAN/RURAL TEMPERATURE AND HUMIDITY DIFFERENCES FOR A SMALL CITY* DavidJ. Travis, Vernon Meentemeyer and Philip W. Suckling INTRODUCTION. Urban environments have a considerable impact upon meteorological conditions such as temperature and humidity. (1) The concept of the "urban heat island", whereby cities are often warmer than their rural surroundings, is probably the most documented aspect of urban climatology. Even small cities can develop significant heat islands . (2) Warmer urban conditions are best developed under calm clear nocturnal conditions indicating that meteorological controls such as windspeed and cloud cover have a profound effect upon heat island development for a particular city. (3) Urban/rural humidity differences are not as clearly distinguished. (4) It is the purpose of this study to examine urban/rural temperature and humidity differences and the relationship between the day-to-day changes in the state of the atmosphere and the maximum intensity of the urban heat island produced by a small city. A simple model of urban heat island magnitude versus regional meteorological conditions will be presented. METHODOLOGY. This study was conducted during the autumn of 1986 for the urban environment of Athens, Georgia (34°N; 83°W). It has been shown in previous research that urban heat islands are especially welldeveloped in summer and autumn for temperate latitude cities. (5) Athens is a small city with a population of approximately 50,000 within the city limits and a metropolitan population of approximately 100,000. To monitor urban/rural meteorological differences, an "urban" site was selected near the city center close to the area of warmest temperatures detected in a previous temperature transect study. (6) A "rural" site was chosen in an area of small fields and pine forest (typical of the region) located 6 km north of the city center beyond the urban boundary. The * The technical assistance of Doug Oetter is gratefully acknowledged. Mr. Travis is a geography undergraduate student, Dr. Meentemeyer is Professor of Geography and Dr. Suckling is Associate Professor of Geography at the University of Georgia in Athens, GA 30602. Vol. XXVII, No. 2 91 use of single urban and rural sites is a potential limitation to the representativeness of the data obtained. The study was conducted during the autumn of 1986 from 1 October through 10 November. It was the purpose of this study to detect well-established urban/ rural temperature and humidity differences and examine their relationship to regional meteorological conditions. Previous studies have shown that the urban heat island primarily develops due to a lack of urban cooling around and shortly after sunset, i.e., a lesser urban cooling rate. (7) Thus, the maximum heat island magnitude is established within a few hours of sunset. Based on these facts, data were collected for the present study beginning 90 minutes after official sunset. The rural site was monitored first, then the urban site, followed by a return to the rural site. All measurements were completed within two hours of sunset. The two rural measurements were averaged to yield the final rural data in order to compensate for temporal change. The two rural temperature measurements consistently differed by less than 1.50C indicating that cooling rates, typically rapid around sunset, had moderated considerably by the measurement times for this study. Wet and dry bulb temperatures were monitored at both sites using a Cole-Parmer type K thermocouple probe with wet and dry bulb sensors. A standard sling Psychrometer was used to confirm the response of the thermocouple probe. Due to instrument failure, temperature and humidity data for November 9 were missed while humidity data for October 16 were lost. The number of available observations of urban/rural temperature conditions was therefore 40, while humidity observations were available for 39 days. To represent the general regional meteorological conditions, hourly values of temperature (T), relative humidity (RH), vapor pressure (e), cloud cover amount (cc), and windspeed (u) were obtained from the National Weather Service (NWS) station located at Athens airport (approximately 6 km east of the city center) for appropriate times corresponding to the urban/rural field measurements. The NWS site, located on the suburban fringe...