In reality, though, the wind typically shifts speed and direction as it flows up or down slopes, between hills or down into valleys, turning where terrain features turn. In urban areas, wind flowing around large buildings forms eddies and changes direction and speed, significantly altering a cloud's shape and movement. Buildings may block and divert the wind. Air flowing past large obstructions such as buildings forms into turbulent eddies, just as eddies form immediately downstream of a boulder or bridge piling in a river. Air flowing across an urban landscape composed of many buildings breaks up into irregular patterns of eddies of various sizes, speeds, and strengths.
Winds blowing through city streets can speed up, slow down, and markedly change direction.
In fact, wind blowing past an obstruction such as a building sometimes can completely reverse direction. A cloud encountering the irregular air flow typical within a city may even be broken up into several clouds, each moving in a different direction. Within the complex terrain of a downtown area, concentrations of gas can become very patchy for several reasons. Heavy gases can be trapped within low areas, such as stairwells, sunken terraces, empty swimming pools, and construction ditches and pits. Sometimes, wind eddies forming behind buildings can trap and recirculate gas, building pollutant concentrations in the lee of buildings to levels much higher than expected.