| Urban microclimates are discrete regions in the urban canopy layer where values of radiative and meteorological variables deviate from their averages over larger spatial scales. The canopy extends from street-level to mean building height. The energy balance framework described here, combined with relevant site measurements, permits quantitative assessments of the mechanisms controlling surface temperatures in microclimates. There are substantial differences in the fluxes of solar and thermal radiation, sensible heat transport, evaporation, conduction, air and surface temperatures, and wind speeds across microclimates in regions that should not be assumed to be homogeneous. Anthropogenic surface alterations appear to be responsible for some of these differences.; Measurements were recorded at four locations with varying levels of urban development. The net all-wave energy flux, RNET, represents the limit to the input of energy at the surface and equals the sum of incoming solar and thermal radiation minus the sum of reflected and emitted values. Typically, over a period of 24 hours more radiation is incident on a surface than is reflected and emitted by it. This net gain would result in perpetual heating unless other fluxes dissipated the excess. Those fluxes are sensible heat transport, evaporation and conduction. Thus net radiation equals the sum of the model-derived energy fluxes, such that RNET = +/- QSENS +/- QCOND + QEVAP.; The demand for energy to heat or cool interior spaces depends on both outdoor air temperatures and the temperatures of built surfaces, especially the roofs and walls of buildings exposed to the atmosphere. The temperature of a solid surface differs from the air temperature and is a function of its properties, such as albedo, thermal conductivity, heat capacity, and availability of liquid water for evaporation. This study demonstrates how the external surroundings of a structure influence its surface temperature. The interplay of these factors produces microclimates, typified by an urban canyon and a non-canyon urban site studied here. Although these sites are in close geographic proximity, they experience different diurnal cycles in surface temperature and, hence, have different requirements for interior temperature control. |
