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An urban canyon is an artefact of an urban environment similar to a natural canyon. It is manifested by streets cutting through dense blocks of structures, especially skyscrapers that form a human-built canyon. Examples of urban canyons include the Magnificent Mile in Chicago, the Canyon of Heroes in Manhattan, and Hong Kong's Kowloon and Central districts. Urban canyons contribute to the urban heat island effect.
An urban canyon is a basic urban surface unit that can be seen repeatedly throughout urban areas. The form of the canyon consists of the geometric combination of horizontal and vertical surfaces; the building arrangement acts as the walls and the streets is the ground. The canyon air volume is the air contained within the walls, ground and 'lid' of the canyon, the lid being an imaginary boundary at roof level. Asymmetric canyons are when the buildings on one side of the street are of a different height than the buildings across from them.
Researcher T. R. Oke has studied the microclimate inside urban canyons, with the aim of determining the general properties of urban climate. Studies of urban canyons take into account the orientation of the canyon (being north-south, etc.), the height, length and width of the canyon. Studies of temperature phenomena consider illumination, angle of incidence, surface albedo, emissivity and temperature. Furthermore, the aspect ratio and the sky view factor are important measurements. Aspect ratio (or height/width ratio) is the ratio of the average height of the buildings (on both sides) to the width of the street. Sky view factor ranges from zero to one and is calculated as the amount of sky visible when viewed from the ground up. Often a 'fish-eye' photograph is taken from the street level. A sky view factor of one denotes a completely visible sky, and zero denotes a sky blocked by obstacles. The higher the sky view factor, the quicker the urban canyon will cool, because more sky is available to absorb the heat retained by the buildings. With a low sky view factor, the canyon can retain more heat during the day, creating a higher heat release at night.
A study done by Nunez and Oke investigated the energy exchanges in an urban canyon in mid-latitudes in fine summer weather. The study showed that the amount of surface energy at various times within the canyon depends on canyon geometry and orientation. Canyons with North-South orientation were found as having the floor being the most active energy site. In such a canyon, 30% of midday radiant surplus is stored in the canyon materials (the buildings). At night, the net radiant deficit (meaning the lack of solar radiation) is countered by the release of energy that was being stored in the canyon materials. This phenomenon contributes heavily to the urban heat island effect.
Urban canyons have an impact on various local conditions: