Sky View Factor Analysis

SVF is a dimensionless number between 0 and 1 giving the fraction of the sky hemisphere visible from a point. A value of 1 means completely open sky, as in a flat field; a value near 0 means almost all the sky is blocked, as at the bottom of a deep, narrow street. Because it is cosine-weighted, sky directly overhead counts for more than sky near the horizon, matching how a horizontal surface exchanges radiation with the sky. In practice SVF is read as 'how open to the sky is this spot', and lower values flag places that cool poorly at night.

At night, surfaces lose heat mainly by radiating longwave energy to the cold sky. A point with a low sky view factor — hemmed in by tall buildings — can radiate to only a small slice of sky, and much of what it loses is offset by longwave radiation it receives back from surrounding warm walls. So dense, deep-canyon streets cool far less after sunset than open areas, which is a primary geometric cause of the nocturnal urban heat island. Oke's 1981 work established this quantitative link between canyon geometry, SVF, and heat-island intensity.

There are two main routes. Computationally, SVF is derived from a three-dimensional building model or a digital surface model by ray-casting or shadow-volume methods that find the horizon obstruction angle in every direction around each point, then integrating the open sky. Empirically, it is measured in the field from an upward-pointing fisheye (hemispherical) photograph, which is classified into sky and non-sky pixels and integrated. Both should account for vegetation carefully, since foliage partly transmits radiation and treating trees as solid obstructions biases the result.