Urban Street Canyons – Wind
As with heat, studies of wind in urban canyons include both modeling and observation. Wind in urban street canyons is important for dispersing heat and pollutants.
Understanding wind in these canyons relies on basic principles of fluid dynamics. Anne Whiston Spirn describes some of the primary causes of air movement in canyons in her 1986 report Air Quality at the Street-Level: Strategies for Urban Design. Below are three examples of how canyon geometry can affect airflow.
Channelization effect: An ideal situation for flushing out pollutants, but one that can lead to pedestrian discomfort, the channelization effect occurs when the wind is moving parallel to the canyon’s orientation. The severity of this effect is primarily a function of the wind’s speed and the total length, the average width, and the average height of the canyon.
Venturi effect: The speed of wind increases when it funnels through small openings. This is again uncomfortable for pedestrians, but an excellent way to disperse pollutants. The severity of the Venturi effect is a function of width, length, height, and size of openings in the canyon.
Bar effect: The Bar effect occurs when air flows over a street canyon at a 45-degree angle. On the opposite side (leeward) the wind speeds up. This effect depends on the width, length, and average height of the sides and size of the openings in the canyon.
A combination of those effects described above and others can lead to different flow patterns depending on the aspect ratio of the canyon. For example, the aspect ratio can determine the difference between isolated roughness flow, wake interference flow, and skimming flow as illustrated below.
From Oke, T. R. "Street Design and Urban Canopy Layer Climate." Energy and Buildings, 11 (1988)
From Oke, T. R. "Street Design and Urban Canopy Layer Climate." Energy and Buildings, 11 (1988)
Y. Nakamura and T. R. Oke’s 1988 study of an urban canyon in Kyoto, Japan provides a classic example of how urban climatologists study wind in urban microclimates. Measuring horizontal wind speed, surface temperatures, air temperatures, and solar radiation, Nakamura and Oke looked at an east-west canyon in Kyoto with an aspect ratio of 1.06 and a sky view factor of 0.43 in an attempt to link patterns in the microclimate to the air boundary layers above.
One thing to note is again the importance of heat in these wind studies, as convection currents (caused by heavier colder air sinking and displacing less dense warmer air) can drive wind currents. Isotherms (charts showing areas of equal temperature) quickly tell a story of which areas receive the most direct sunlight throughout the day (in this case the northern corner of the canyon).
General conclusions from this study include a link between the wind direction above and within the canyon (with parallel wind decreased by a factor of two-thirds) and that large surface temperature differences can occur inside the canyon—especially for surfaces receiving direct sunlight.
