1.1. A Short History

One of the earliest SVF calculations were made with the help of digital cameras mounted with Fisheye lens. The masks were then delineated manually and the SVF is calculated with Steyn’s method [1]. Other raster-based calculations were later developed, using pixel counting methods [2] [3]. To overcome this somewhat manual and time-consuming method [4]

1.2. Sky View Factor and Radiation

A Sky View Factor (SVFs) represents the ratio at a point in space between the visible sky and a hemisphere centered over the analyzed location (Oke 1981) :

At a point where the SVF = O, the entire sky is blocked from view by obstacles. If there is an obstacle blocking parts of the sky view, the SVF will be greater than 0.

• for SVF = 0 :

  • there is no short-wave reflection
  • there is no long-wave nocturnal interference

According to Oke [5], cited by many papers including Hammerle et al. [6], the energy budget (in terms of flux) can in this case be considered ideal

Where Q_* is the total energy balance, Q_Gis the energy stored in soil and/or buildings, Q_His the sensible heat (air temperature) and Q_Eis the latent heat (evaporated water).

• for SVF > 0 :

  • incoming day-time short-wave reflection increases during the day
  • outgoing night-time long-wave radiation is reduced
  • incoming night-time long-wave radiation is increased
  • altered soil heat flux

Because of its influence on energy balances, the SVF holds an importance in various fields of climatology (urban [7], forest [8], human biometeorology [9]…) but its most widespread use concerns the heat island phenomena.

1.3. Sky View Factor and Heat Island Phenomena

One of the earliest studies demonstrating the link between sky view factor and local urban temperatures was made by Yamashita et al. [10] (1985).

Footnotes

View Factors reduce the information given by sky maps and sunny sky maps to a single numerical value, allowing for a more traditional cartography of a discretisized space. Sky View Factors represent the ratio of visible sky at a point, which we could find the Sky View Factor alternatively by attributing to a Sky Map Disc’s center the ratio between the disc’s area and the area representing visual obstacles. Similarily, Sun View Factors can be found by comparing the number of points of a sun path not blocked by obstacles with the total amount of points in the sun path.