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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 A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...
-like environment, evolved etymologically from the
Canyon of Heroes Broadway () is a road in the U.S. state of New York. Broadway runs from State Street at Bowling Green for through the borough of Manhattan and through the Bronx, exiting north from New York City to run an additional through the Westcheste ...
in
Manhattan Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...
. Such human-built canyons are made when streets separate dense blocks of structures, especially
skyscraper A skyscraper is a tall continuously habitable building having multiple floors. Modern sources currently define skyscrapers as being at least or in height, though there is no universally accepted definition. Skyscrapers are very tall high-ris ...
s. Other examples include the
Magnificent Mile The Magnificent Mile, sometimes referred to as The Mag Mile, is an upscale section of Chicago's Michigan Avenue, running from the Chicago River to Oak Street in the Near North Side. The district is located within downtown, and one block ...
in Chicago, Los Angeles'
Wilshire Boulevard Wilshire Boulevard is a prominent boulevard in the Los Angeles area of Southern California, extending from Ocean Avenue in the city of Santa Monica east to Grand Avenue in the Financial District of downtown Los Angeles. One of the principal ...
corridor, Toronto's
Financial District A financial district is usually a central area in a city where financial services firms such as banks, insurance companies and other related finance corporations have their head offices. In major cities, financial districts are often home to s ...
, and Hong Kong's
Kowloon Kowloon () is an urban area in Hong Kong comprising the Kowloon Peninsula and New Kowloon. With a population of 2,019,533 and a population density of in 2006, it is the most populous area in Hong Kong, compared with Hong Kong Island and t ...
and
Central Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...
districts. Urban canyons affect various local conditions, including temperature, wind, light, air quality, and radio reception, including
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
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 Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The de ...
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 In meteorology, the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Ear ...
by the presence of a street canyon is called the street canyon effect. As mentioned earlier, street canyons affect
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
,
wind speed In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer. Wind speed ...
and
wind direction Wind direction is generally reported by the direction from which it originates. For example, a ''north'' or ''northerly'' wind blows from the north to the south. The exceptions are onshore winds (blowing onto the shore from the water) and offsho ...
and consequently the
air quality Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...
within the canyon.


Temperature

Urban canyons contribute to the
urban heat island An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
effect. The temperature inside the canyon can be elevated 2–4 °C. Studies of temperature phenomena consider
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
, 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.


Wind

Street canyons can modify both the speed and the direction of winds. The vertical wind velocity approaches zero at the roof level of the canyon. Shear production and dissipation are high at the roof level and a strong thin shear layer is created at the building height.
Turbulence kinetic energy In fluid dynamics, turbulence kinetic energy (TKE) is the mean kinetic energy per unit mass associated with eddies in turbulent flow. Physically, the turbulence kinetic energy is characterised by measured root-mean-square (RMS) velocity fluctuat ...
is higher near the downwind building than near the upwind building because of stronger wind shears. The resulting flow patterns inside the canyon depend on the wind direction with respect to the street orientation direction.


Wind parallel to canyon

When the roof level/background wind direction is parallel to the street, a channelization effect is seen where winds tend to be channeled and accelerated through the canyon. Where the street width is non-uniform, a
Venturi effect The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...
is seen when winds funnel through small openings, further enhancing the acceleration of winds. Both these effects are explained by the
Bernoulli's principle In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematici ...
. The along-street wind and transport can be significantly different for short and long canyons as the corner vortices have a stronger influence in short canyons.


Wind perpendicular to canyon

When the roof level/background wind direction is perpendicular to the street, a vertically rotating wind flow is created with a centered primary vortex inside street canyons. Based on the aspect ratio, different flow regimes are defined in street canyons. In the increasing order of aspect ratio these flow regimes are: isolated roughness flow, wake interference flow and skimming flow. The total number of vortices made and their intensities depends on many factors. Numerical model studies done for isolated street canyons have shown that the number of vortexes made increases with increasing aspect ratio of the canyon. But there is a critical value of the ambient wind speed, above which the number and pattern of vortexes become independent of the aspect ratio. Numerical and wind tunnel studies have shown that for symmetric canyons with an aspect ratio = 0.5, a ground level secondary vortex can be seen near the leeward side building wall. For symmetric canyons with aspect ratio ≥ 1.4, a weaker ground level secondary vortex can be seen near the windward side building wall and for aspect ratio ≥ 2 secondary vortexes are seen right below the primary vortex. In asymmetric and step-up canyons the formation of secondary vortexes can be more common. Wind tunnel studies have shown that, in a step-up canyon where the upwind building is shorter, a
stagnation point In fluid dynamics, a stagnation point is a point in a flow field where the local velocity of the fluid is zero.Clancy, L.J. (1975), ''Aerodynamics'', Pitman Publishing Limited, London. A plentiful, albeit surprising, example of such points seem ...
can be identified on the windward face of the taller building. The region below this stagnation point is called the interaction region, since all streamlines in this region are deflected downward into the street canyon. The characteristics of the vortex flow patterns inside the canyon strongly depend the height ratio of buildings on either side of the canyon. For a downwind building height Hd to upwind building height Hu ratio of 3, a single primary vortex has been observed. But for Hd/Hu=1.67, counter-rotating vortexes can occupy the whole depth of the canyon. Other factors that have an effect on the strength of this re-circulation flow are traffic induced turbulence and roof shapes of the buildings. Physical model studies have shown that two way traffics increases the turbulence in the lower half of the canyon and that pitched roof on either side of the canyon shifts the main area of turbulent production downstream and reduced the intensity of the re-circulation flow inside the canyon. Under these perpendicular wind conditions, mainly at the street level, at each end of the canyon, horizontally rotating corner/end vortexes are made. The horizontal extent of these corner vortexes are different at each end of a canyon and this leads complex surface level wind patterns at intersections. Field experiments have further shown that corner vortexes can extend in the whole depth of canyon but with changing horizontal extent with height. The structure of the neighboring area of a street canyon; for instance a series of street canyons, add more complexity to flow field. All the above-mentioned results are for situations with no heating effects. A numerical model study have shown that, when a surface in a street canyons heats up, it changes the characteristics of the vortex flow. And heating of different surfaces; upwind wall, downwind wall, canyon floor, changes the vortex flow in different manners.


Air quality

The modification of temperature and wind by the presence of a street canyon, consequently affect the air quality inside the street canyon. When the direction of the mean wind is parallel to the street, the channelization and Venturi effects described above, increases the dispersion of pollutants inside the street canyon. This often act to 'flush out' the air pollutants and increase the air quality inside the street canyon. But in instances where air pollutant sources are present upwind, channeling winds could transport pollutants to downwind locations far from the source and contribute to bad quality at the downwind locations. When the mean wind direction is perpendicular to the street, the vortex flow formed inside the canyon acts to confine air flow, reduce dispersion of pollutants and increase the pollution concentrations inside the street canyon. Pollution from local source inside the canyon and also pollution entrained in to the canyon from the mean wind flow, is carried by the vortex flow and is re-circulated within the canyon. In urban environments, vehicle tail pipe emissions are the major source of many air pollutants such as
ultrafine particles Ultrafine particles (UFPs) are particulate matter of nanoscale size (less than 0.1 μm or 100 nm in diameter). Regulations do not exist for this size class of ambient air pollution particles, which are far smaller than the regulated PM10 and ...
, fine particles,
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
,
NOx In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tropos ...
. These pollution plumes created on the street, at the surface level, are pushed toward the lee side of the canyon by the vortex flow, making the surface level pollution concentrations much higher at the
leeward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side of the street compared to the
windward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side. Secondary vortexes in the lower portion of the canyon can further act to stagnate pollutants on the side walks; specially on the leeward side. One field study has reported ultrafine particle concentrations four times higher on the
leeward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side sidewalk compared to the
windward Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...
side.


GPS signal reception

When using GPS receivers in street canyons with tall buildings, the shadowing and multipath effects may contribute to poor GPS signal reception.{{cite book, last=MISRA, P., P. ENGE, title=Global Positioning System: Signals, Measurements, and Performance, Second Edition, date=2006, publisher=Ganga-Jamuna Press, location=Lincoln (MA), U.S.A


See also

*
Manhattanhenge Manhattanhenge, also inaccurately called the Manhattan Solstice, is an event during which the sunset, setting sun or the sunrise, rising sun is aligned with the east–west streets of the main street grid of Manhattan, New York City. The astroph ...
*
Manhattanization ''Manhattanization'' is a neologism coined to describe the construction of many tall or densely situated buildings, which transforms the appearance and character of a city to what is similar to Manhattan, the most densely populated borough of New ...
* Setback *
Wind engineering Wind engineering is a subset of mechanical engineering, structural engineering, meteorology, and applied physics that analyzes the effects of wind in the natural and the built environment and studies the possible damage, inconvenience or benefits w ...


References

Urban geography Canyons and gorges