Venturi effect
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 is seen when winds funnel through small openings, further enhancing the acceleration of winds. Both these effects are explained by the Bernoulli's principle. 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.