Urban structures and layout affect winds in that friction from urban structures and tall buildings cause winds to lose energy and to be diverted (respectively) ultimately leading in calm conditions and light winds. When moving through an urban area winds must pass through areas of skyscrapers, oddly shaped buildings and brick built structures. Energy is lost when the movement of air encounters friction and hinders its ability to move as fast.
Friction van be small scale such as rough surface of brick buildings or large scale such as large tall structures which wind must go around. This results in a reduction of wind speeds in urban areas. Also the more densely built up an area is the less wind has an effect as the air must be diverted around many structures and can in fact be diverted round the area altogether. In terms of layout, parallel built streets, especially long narrow streets with buildings either side can cause wind to be funnelled down these passages creating ‘wind tunnels’ where wind speeds increase greatly, this increases the average wind speed of an urban area. The same effect occurs with narrow passages/gaps between tall buildings, wind is forced through in a wind tunnel creating high speed winds. In urban areas with a less structured layout eg wide streets with building s of moderate height set out in an unparallel layout, wind is less affected whilst speed is still reduced by friction, the occurrence of wind tunnels is less as the conditions are not conducive to such an event.