The recent floods in the Midwest have drawn attention not
only to the state of our levees and flood-control measures, but also to the
terms we use to describe the flooding itself.
I wrote on this site on June 19 that “some are
finding it difficult to accept the idea of a second 500-year storm in 15 years,”
assuming that most readers of this site would sympathize with the irony of the
situation that the people in the inundated areas are experiencing.
A reader
promptly pointed out that such terms as “500-year flood” have always been
misleading to the public. When we refer to a 100-year storm, for example, we
don’t mean that such a storm will occur only once in a century but rather that
there is a 1% chance in any given year that a storm of that level will occur.
The reader said that the media should work to clarify the meaning of these
terms—and in fact, in the last couple of weeks, it has (see for example this
article on a popular news site, which claims some homeowners have dropped their
flood insurance after experiencing a “100-year flood,” believing that another
such flood can’t possibly happen again in their lifetime: http://www.msnbc.msn.com/id/25463476).
The problem is somewhat similar to that of a doctor
explaining to prospective parents that their child has a 25% chance of being
born with a particular genetic trait. What the doctor means to say, of course,
is that each individual child the couple has will have a 25% chance of
inheriting that trait, but for convenience’s sake, he might say instead that
“one of every four children” will have it. Some parents take that very
literally, and are confused when, say, all four of their children are born
without, or when every one of their children has the trait.
A similar sort of confusion exists when the weather report
predicts a 30% chance of rain; however, most people—accustomed as they are to
hearing that phrasing—seem to have less trouble with that idea.
What has your experience been with terms like
“100-year-flood” and “10-year-storm”? If your job involves working with the
public or passing along public information about storm and flood conditions, do
you think most people in your area have a fairly good concept of what we mean?
Or should we be explaining more clearly?