| Despite its relevance to many aspects of urban climate and to several practical questions, urban dew has largely been ignored. Here, simple observations an out-of-doors scale model, and numerical simulation are used to investigate patterns of dewfall and surface moisture (dew + guttation) in urban environments. Observations and modelling were undertaken in Vancouver, B.C., primarily during the summers of 1993 and 1996.;Surveys at several scales (0.02--25 km) show that the main controls on dew are weather, location and site configuration (geometry and surface materials). Weather effects are discussed using an empirical factor, FW . Maximum dew accumulation (up to ∼ 0.2 mm per night) is seen on nights with moist air and high FW , i.e., cloudless conditions with light winds. Favoured sites are those with high Ysky and surfaces which cool rapidly after sunset, e.g., grass and well insulated roofs.;A 1/8-scale model is designed, constructed, and run at an out-of-doors site to study dew patterns in an urban residential landscape which consists of house lots, a street and an open grassed park. The Internal Thermal Mass (ITM) approach is used to scale the thermal inertia of buildings. The model is validated using data from full-scale sites in Vancouver. Patterns in the model agree with those seen at the full-scale, i.e., dew distribution is governed by weather, site geometry and substrate conditions. Correlation is shown between Ysky and surface moisture accumulation.;The feasibility of using a numerical model to simulate urban dew is investigated using a modified version of a rural dew model. Results for simple isolated surfaces---a deciduous tree leaf and an asphalt shingle roof---show promise, especially for built surfaces. |
