As wind flows between buildings the mass of the gas is compressed with subsequent increase in velocity to wind speeds that may be several times the speed of the wind on lee side of the buildings. In addition to creating increased wind velocity turbulence is also likely to occur on the leeward side of the building. It may require a distance of 7-10 times, (depending on the wind speed and amount of compression), the width of the compressing building before the effect of compression on air flow is minimized.
Wind Flow Over Tall Buildings
When wind hits a tall building it will be deflected upward as well as around the building. This can create turbulence and backflow of wind near the windward surface and vertical deflection of the wind up the windward side of the building. The upward motion of the wind can create a substantial decrease in horizontal wind speed at the top of the building, a good reason to be suspicious of any wind instrument located on a building top. The wind immediately to the leeward side of the building will be turbulent and significantly reduced in speed until it is measured at a distance that is 7-30 times the height of the building on the leeward side.
Point to Ponder: When making wind measurements on or near buildings for rooftop Helicopter landing pads, where should you position the measurement instruments? Should you place a wind instrument on a rooftop edge or at its center?