An urban canyon (also known as a street canyon) is a place where the street is flanked by buildings on both sides creating a canyon-like environment, evolved etymologically from the Canyon of Heroes in Manhattan. Such human-built canyons are made when streets separate dense blocks of structures, especially skyscrapers. Other examples include the Magnificent Mile in Chicago, Los Angeles' Wilshire Boulevard corridor, Toronto's Financial District, and Hong Kong's Kowloon and Central districts.
Urban canyons affect various local conditions, including temperature, wind, light, air quality, and radio reception, including satellite navigation signals.
Geometry and classification
Ideally an urban canyon is a relatively narrow street with tall, continuous buildings on both sides of the road. But now the term urban canyon is used more broadly, and the geometrical details of the street canyon are used to categorize them. The most important geometrical detail about a street canyon is the ratio of the canyon height (H) to canyon width (W), H/W, which is defined as the aspect ratio. The value of the aspect ratio can be used to classify street canyons as follows:
Regular canyon – aspect ratio ≈ 1 and no major openings on the canyon walls
Avenue canyon – aspect ratio < 0.5
Deep canyon – aspect ratio ≈ 2
A sub-classification of each of the above can be done depending on the distance between two major intersections along the street, defined as the length (L) of the street canyon:
Short canyon – L/H ≈ 3
Medium canyon – L/H ≈ 5
Long canyon – L/H ≈ 7
Another classification is based on the symmetry of the canyon:
Symmetric (or even) canyon – the buildings that make the canyon have approximately the same height; Asymmetric canyon – the buildings that make the canyon have significant height differences.
Another specific type is:
the step-up canyon – a street canyon where the height of the upwind building is less than the height of the downwind building.
The effect of a street canyon on local wind and air quality can greatly differ in different canyon geometries and this will be discussed in detail in sections below.
Other important factors taken into account in studies of urban canyons are the air volume, orientation of the canyon (north–south, east–west etc.) and the sky view factor. The air volume of the street canyon is the air contained within the buildings on either side which act as walls, the street which is the bottom boundary, and an imaginary upper boundary at roof level called the 'lid' of the canyon.
The sky view factor (SVF) denotes the ratio between radiation received by a planar surface and that from the entire hemispheric radiating environment and is calculated as the fraction of sky visible from the ground up. SVF is a dimensionless value that ranges from 0 to 1. A SVF of 1 means that the sky is completely visible, for example, in a flat terrain. When a location has buildings and trees, it will cause the SVF to decrease proportionally.
Effects
The modification of the characteristics of the atmospheric boundary layer by the presence of a street canyon is called the street canyon effect. As mentioned earlier, street canyons affect temperature, wind speed and wind direction and consequently the air quality within the canyon.
Temperature
Urban canyons contribute to the urban heat island effect. The temperature inside the canyon can be elevated 2–4 °C. Studies of temperature phenomena consider irradiance, angle of incidence, surface albedo, emissivity, temperature and SVF. For a high SVF, the urban canyons cool quickly, because more sky is available to absorb the heat retained by the buildings. With a low SVF, 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 to have 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.