Urban canyon

Ideally an urban canyon is a relatively narrow street with tall, continuous buildings on both sides of the road.

For a high SVF, the urban canyons cool quickly, because more sky is available to absorb the heat retained by the buildings.

A study done by Nunez and Oke investigated the energy exchanges in an urban canyon in mid-latitudes in fine summer weather.

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.

Based on the aspect ratio, different flow regimes are defined in street canyons.

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.

[8] 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.

[10] Other factors that have an effect on the strength of this re-circulation flow are traffic induced turbulence and roof shapes of the buildings.

[11] 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.

A numerical model study have shown that, when a surface in a street canyons heats up, it changes the characteristics of the vortex flow.

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.

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 leewardside of the street compared to the windward side.

Near the equinoxes , sunrises and sunsets align with east-west urban street canyons, an event known as Chicagohenge (pictured). In other cities, similar phenomena with names such as Manhattanhenge occur at various times of the year.
The comparison of (a) isolated roughness flow and (b) skimming flow regimes in a street canyon (after Oke, 1988)
In the skimming flow regime, the wind vortex made inside a street canyon, when mean wind direction is perpendicular to the street (after Oke, 1988)