Dear Cecil:

You may have heard the line, "tornadoes are God's way of telling us there are too many mobile homes." Why is it that mobile homes seem to get damaged by twisters with such regularity?

Is it that people who live in mobile homes know when they've been hit, while the rest of us think it was just high winds that blew off those shingles?

Or could it be that twisters are deflected by the updrafts in cities with more substantial buildings? --Dave G., Evanston, Illinois

Cecil replies:

I'm not sure I follow you, Dave. Are you suggesting somebody in a non-mobile home could live through a tornado and NOT REALIZE IT? If so, be assured that a tornado is not the sort of delicate phenomenon that easily eludes notice.

Nonetheless, you're quite right in noting that mobile homes seem to sustain an inordinate amount of damage from twisters. In late March of 1984 tornadoes in the Carolinas killed 60 people, 23 of whom, or 38 percent, were mobile-home dwellers--an unusually high proportion by any standard.

One of the principal reasons for this, of course, is that mobile homes are generally of fragile construction. Few can endure winds of more than 100 MPH, which would be generated by a tornado of only moderate destructive power.

(By the same token, southern states suffer much greater loss of life during tornadoes than northern states, due to the flimsier construction of their buildings.)

In addition, however, there is some indication that larger cities tend to be tornado-resistant, at least in the sense that they get fewer twisters than would seem statistically likely.

I have been communing on this topic with Theodore Fujita, a professor of meteorology at the University of Chicago and one of the world's leading authorities on tornadoes. He says a big city can whittle down the intensity of a tornado by one or two levels on the "Fujita scale" (and guess who THAT's named after).

Thus an F0 (40-72 MPH winds, light damage) or F1 (73-112 MPH, moderate damage) tornado might be reduced to a mere thunderstorm upon encountering a town of consequence.

Professor Fujita thinks there are a couple reasons for this. One is that big cities contain steel or reinforced-concrete high-rise structures that can withstand tornado-strength winds, and these slow the twister down by sheer force of friction.

The warm updrafts that cities create also tend to interfere with tornado formation somehow.

Professor Fujita's reasoning on this is a bit difficult to follow--they don't call this guy "Mr. Tornado" for nothing--but in general we can say that tornadoes need an inrush of cool air at ground level, which cities do not provide.

Lest you be too comforted by this, urbanite that you are, bear in mind that tornadoes are perfectly capable of ignoring all the preceding and pounding quite sizable towns into rubble.

You may recall the sad tale of Kalamazoo, Michigan, much of which was reduced to ruins by tornadoes a few years ago. Going further back, the city of St. Louis has suffered some of the worst tornado disasters in history, including one in 1896 that killed 306 people, and another in 1927 that killed 79.

And I'm sure I don't need to remind you that your hometown of Evanston, Illinois, is believed to have been struck by a tornado a few years ago (although, perhaps significantly, it caused little damage). So don't think you can't be flattened just because you haven't been flattened yet.

--CECIL ADAMS