The Impact of Sky View Factor on Outdoor Thermal Comfort; Case Study: Tehran Urban Parks

According to the nature of open public space, it’s involved a huge part of urban activities. One of the most important principles in design is considering thermal comfort in order to improve the quality of space and increase user’s satisfaction. The general aim of this research is controlling environmental elements affected on thermal comfort to increase satisfaction. This research seeks to determine the relationship between the thermal environment and “sky view factor”. The “sky view factor” represents an estimation of the visible area of the sky from an earth viewpoint, being defined as the ratio between the total amounts of radiation received from a plan surface and that available from the whole radiant environment. There are several techniques for calculating of the “sky view factor”: using surveying techniques, digital camera with fish-eye lens, signals from GPS receivers or lately thermal fish-eye image. The photographic technique is particularly well suited for urban open space, where vegetation is present. Late research has identified that “sky view factor” has a multiple effect on open space climate. In these researches, the “sky view factor” has been shown to be well correlated with surface temperature but not with air temperature. Nevertheless, the mean “sky view factor” for an area has been demonstrated to have a good correlation with the urban heat island. This study was conducted in Iran, Tehran, in five parks area (Mellat, Saei, Laleh, Shahr and Besat). Field surveys were performed from October11 to 28, 2010 and have taken place between 10 a.m. and 5 p.m., on weekdays and weekends. During the period of fieldwork, meteorological parameters such as air temperature, relative humidity, wind velocity, and globe temperature were measured. The Mean radiant temperature was calculated using mentioned parameters. Photographs were taken in the urban parks’ area at ground level shooting upwards using a fisheye lens. Utilizing image analysis and in processing software, “Sky view factor” was computed for each photograph. The main conclusions from this paper can be summarized as follows. • “Sky view factor” analysis is a useful and effective tool for the landscape architecture and urban climatologist conducting studies. • There is a relatively strong relationship between “sky view factor” and Mean radiant temperature during the day. The Mean radiant temperature has the strongest influence on thermo physiological comfort indexes. • Reducing the temperature of the “sky view factor” is the most effective means to achieve outdoor thermal comfort.