Daily Spatial and Secular Variations of Atmospheric Humidity in a Small City

Abstract

A multiscale temporal analysis of the urban heat island (UHI) for a large, rapidly growing, subtropical city (Charlotte, North Carolina) is conducted using hourly surface observations from a regional network of 12 weather and air-quality stations over a 5-yr period and monthly mean surface temperatures from two stations over a 40-yr period. Each station was classified as urban, suburban, or rural after detailed site analysis. During the 5-yr period, from temperature differences between the most central urban site and the rural reference site, over 70% of nights exhibited prominent nocturnal UHIs. The most intense UHIs occurred on winter nights with light winds, clear skies, low humidity, strong low-level stability, and no precipitation or frontal passage. The UHI maxima occurred either just after sunset or near sunrise. Maximum urban and rural cooling rates occurred within a few hours of sunset, but rural maxima were larger and preceded (by 1–2 h) the urban maxima. Daily variations in nocturnal mean UHI intensity exhibited significant positive correlations with cloud-base height, atmospheric stability, NO2 concentration, and total solar radiation and significant negative correlations with relative humidity, wind speed, and cloud cover. When optimal weather for UHI development was present, UHIs were more intense on weekdays than on weekends. During the 40-yr period, an appreciable positive trend in UHI intensity occurred. These results support the notion that weather, air pollution, and urban form change can significantly modulate UHI intensities. Similarities and differences between the Charlotte UHI and those observed in similar cities are discussed.