The 50-Year Wind
Okay, let’s say I’m gonna build a `shop’ near, say, Moscow (Latah County), Idaho, Zip 83843. The big worries there are snow, in winter, and wind, any time of year. For wind, let’s go to https://hazards.atcouncil.org, … `Wind’, and then the zip code. I’m gonna pick `MRI 50-Year’. MRI stands for Mean Recurrence Interval. It gives 83 mph (miles per hour) for the town of Moscow. `MRI 50-Year’ means that, on the average, a wind with speed 83 mph will occur (`return’) once in 50 years. Precisely, it means that on any given (particular) year, there is a 1-in-50 (1 year in 50 years, 2 percent) chance that a wind of 83 mph (or greater) will occur. It does not mean that an 83-mph wind will occur in 50 years. If we sit around waiting for 50 years, it might occur, or it might not, or it could occur even more than once. The exact probability that we will get this wind at some time during the next 50 years, is, in fact (statistical calculation), 64% (not an absolute, but more probable than not). Nor does it mean that if one year goes by without the 83-mph wind, that in the next year there will be greater chance. It is simply, based on the information and science to date, the wind that we would expect once in 50 years … the `50-Year Wind’ … perhaps the `once-in-a-lifetime’ wind. It could happen next year, in year 50, or, not at all. Or, if we’re willing to wait, it might not happen for a hundred years. I’m going to `design’ the shop to resist a 50-mph wind, and assume the risk, 2 percent each year, that it could hit my building. If, on the other hand, I decide I don’t like that risk, I can pick a different MRI, or some other amount of `acceptable’ wind speed.