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Rapid methods to estimate sky‐view factors applied to urban areas

A common parameter used to characterize the geometry of urban canyons is the sky‐view factor (ψs). Here, two simple alternatives are presented that can be used both objectively and rapidly to estimate ψs. The first method uses a Nikon CoolPix 950 digital camera fitted with a Nikon FC‐E8 fisheye lens. The second method involves a LI‐COR LAI‐2000 Plant Canopy Analyzer to measure automatically diffuse non‐interceptance (DIFN) light using a fisheye optical sensor. Through a series of intercomparisons for urban canyons of known geometry, the digital camera is shown to provide accurate estimates of ψs. The LAI‐2000 also performs well, although it tends to over‐estimate ψs (for the conditions considered here, the mean absolute error is 0.04), and has a more restricted set of sky conditions under which it performs well (ideally, uniform overcast skies). For both methods, data collection and post‐processing is rapid, and storage of data is straightforward. Thus, mobile data collection is possible which allows detailed information on the spatial variability of ψs in urban areas to be determined. An example of such an application for a small US city, Bloomington, IN, is presented. Copyright © 2001 Royal Meteorological Society