Urban Street Canyon

Many urban climatologists see the urban street canyon, defined as the space above the street and between the buildings, as the basic urban unit. By studying climate in these canyons, they hope to extrapolate to the city as a whole and to find general properties of urban climate applicable to a variety of cities.

Oke “latched onto” the street canyon early in his career. He describes it as appealing to researchers given its universality: “Everywhere has streets. Everywhere has buildings . . . If you could describe the channel—the way the wind goes down, the way the sun comes in, where the pollution is released in the bottom from vehicles—you’re a large way there. If you can describe how roofs behave, the difference between east-west and north-south canyons, and the general intersections, you have cities. You can build up the pattern that characterizes Marrakech, New York, or Los Angeles. You have the building blocks. That’s the way I always approach it.”

Though models of these canyons are growing increasingly complex, many researchers focus primarily on canyon shape, which they find has a greater impact on microclimate than the materials the buildings and streets are made from.

Two important metrics for measuring street canyon climate include the aspect ratio and sky view factor.

Height/Width Ratio or Aspect Ratio: The aspect ratio is the ratio of the mean height of the buildings to the width of the street. The ratio is especially crucial when it comes to determining the type of airflow in the canyon.

From Oke, T. R. "Street Design and Urban Canopy Layer." Energy and Buildings. 11 (1988)

Sky View Factor: The sky view factor is best visualized as a “fish-eye” photograph of the sky from the street. The value of the sky view factor ranges from zero to one. When obstacles block the sky, the factor goes to zero. When the sky is completely visible, the factor is one. The sky view factor is helpful, for example, in determining the amount of heat a canyon will retain at night. In this case, because the sky is cooler than buildings warmed over the day, the sky can be thought of as a giant heat sink. The more sky exposed, the more quickly the street canyon will cool.

SVF = 0.12

Aspect Ratio = 2.6

From Bourbia, F. "Impact of street design on urban microclimate." Renewable Energy (2009)