Wind Shadow - A.W. Spirn

Excerpt from Spirn, Anne Whiston. Air Quality at the Street-Level: Strategies for Urban Design. Cambridge: Harvard Graduate School of Design. (1986)



Wind Shadow

Principle
Wind speed and direction are altered as wind flows around an obstacle, creating updrafts, downdrafts, and swirling eddies, zones of accelerated wind speeds and zones of reduced air circulation (wind shadows).

The form of an obstacle, its density, and its placement relative to wind direction all influence the pattern of air flow around it.

Variables
The form of an obstacle, its density, and its placement relative to wind direction all influence the pattern of air flow around it.

• Dimensions (height, width, length) influence the size of the wind shadow at ground level and the volume of air within it. The higher and wider the building, the larger the wind shadow it casts. A solid, rectangular obstacle casts a wind shadow approximately four times its height.
  
  
• Shape (in plan and profile) An obstacle of pyramidal shape tends to create a smaller wind shadow and less corner effect than one of rectangular shape.
  
  
• Orientation (in relation to wind direction)
  
  
• Porosity (including size and location of openings) The denser the building material, the more pronounced the wind shadow. A semi-porous obstacle will produce a wind shadow that is less pronounced, but which extends over a greater area and permits some air circulation.
  
  
• Surface Roughness A multi-faceted or deeply indented facade may slow winds slightly through increased friction.