Microclimatic Variations in a Downtown Urban Environment

A built-up urban area contains a complexity of different microclimates created by differing directions of orientation with respect to direct beam solar radiation and by the relative amount of exposure to the open sky. This study investigates the microclimatic variations between the north, south, east, and west exposures of a downtown block in a city of modest size and compares these with a more open residential area. On a sunny day the downtown southern exposure received the most solar energy, had the most favorable net radiation balance and the warmest air temperatures. The northern exposure ranked lowest in all elements. The western exposure had a warmer radiation and air temperature regime than the eastern exposure because the wider street and lower buildings allowed a greater penetration of solar radiation. The downtown areas received less solar radiation than the residential section but had more favorable longwave radiation balances. Daytime air temperatures were higher in downtown areas exposed to direct sunlight but lower in shaded locations. Cloud cover reduced the differences between exposures, and the amount of open sky, rather than the direction of exposure, became the most important variable in controlling the surface microclimate.