Meteorologists joke about building trailer parks to attract tornadoes, and there is more than a grain of truth in the jest, because trailer parks tend to occupy the kind of terrain that allows tornadoes to develop.
About 80 to 90 percent of the time, tornadoes are the offspring of a thunderstorm that is rotating, spinning about a vertical axis.
For that to occur, generally speaking, the storm must move over a wide, open area to get the wind circulation that would permit a tornado to form.
Tornadoes rarely strike large metropolitan areas, though in this century large ones hit Wichita Falls, Texas, in 1979, Topeka, Kansas, in 1966 and Worcester, Massachusetts, in 1953. They have not struck cities on the scale of Chicago, New York, St. Louis or Dallas.
Tornadoes formed outside the area have a better chance of making it to an urban environment. In fact, the Wichita Falls, Topeka and Worcester tornadoes formed outside the city and reached maximum intensity as they came to the city.
In the more urbanized areas, winds are altered by urban topography. When you look at the city environment, it is not surprising that winds would become so contorted by buildings and large structures that the obstacles would interfere with the structure of the tornado and weaken or dissipate it.
The probability of any particular area of ground’s being struck by a tornado is phenomenally small, so the chances of any specific urban area’s being hit are just minuscule.
A mini-tornado or dust devil in the canyons of a city is the second cousin once removed of the tornado.
It may be caused by air flowing around a building, especially when it is breezy.
The downwind side of a building is an area of low pressure, the same thing that causes an airplane to be lifted, but in this case the system is vertical, not horizontal, and picks up dust, leaves or other debris.
The building acts as an airfoil, an impedance to wind, splitting the stream of air.
On the leeward, downwind side, the pressure is lower because the streams of air have to go a longer path around the building before meeting. On the windward side, the pressure is higher, as air is forced against that side of the building.
These very small whirls depend on the shape of the building, wind strength and the configuration of doors, because the pressure may be slightly different inside a building.
That is also why doors may be hard to open in a high wind.