Cities are less windy than the surrounding countryside. The average wind speed is up to 30 percent lower inside the city than it is outside and there are 5 percent to 20 percent more calm days. This is due to the friction caused by buildings. Wind blowing in from outside is deflected this way and that, so there are many eddies and the wind inside the city is gusty, but each of the structures it blows against absorbs some of its energy. The buildings slow the wind.
There are exceptions, however. Buildings block and deflect the wind quite effectively, but only provided that they are aligned at an angle to the wind direction. If they are aligned approximately in the same direction as the wind, the wind speed will increase. The effect is called funneling and it happens most often in urban canyons.
As the moving air approaches, the buildings guide it into the spaces between them—along the streets. The street confines the air into a narrower space than it occupied outside, but it must leave the canyon at the same rate as the air above it and to the sides, which was not funneled along it. If the air did not accelerate, air would accumulate on one side of town and there would be less on the opposite side. This means that the air must move faster along the street than it does elsewhere. The wind accelerates, just as wind accelerates when it is funneled through a natural canyon.
Heat islands and urban domes
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