Does a 1,000-year rain really happen every 1,000 years?
In short: Not really. The 1,000-year – or 100-year or 500-year – terminology is really a shorthand way of talking about statistical probability and isn’t related to how often a certain event is expected to happen.
A bit more: The frequently used weather terms, liberally applied in situations like South Carolina’s record rainfall and devastating aftermath, are misleading misnomers.
“We’re trying to convey the probability of that happening in any given year,” said Jim Kolva, hydrologist with the U.S. Geological Survey’s Office of Surface Water.
Answer: A 1,000-year event is one that, based on historical data, has about a 0.1 percent chance of happening. Likewise, a 100-year event has a 1 percent chance of happening; a 500-year event, a 0.2 percent chance.
“It’s better to think of it as a chance of something occurring than it is the number of times in any given period that it would occur,” said Joshua Palmer, meteorologist with the National Weather Service in Greer.
“It does not mean that in the next 999 years, you won’t get a storm like you got,” earlier this month, said Kolva. “There is no immunity that comes with a natural disaster like this.”
Terms like 100-year or 1,000-year likely originated as an effort to make the probabilities more understandable, something a little simpler to digest than jargon about percentages, statistics and confidence intervals. But they may be contributing to some misapprehensions.
“It’s common for people to misunderstand the terminology, which frankly is probably not very good terminology,” Palmer said.
Gov. Nikki Haley said in a news conference, “We haven’t seen this level of rain in the Lowcountry in 1,000 years.” And media reports commonly referenced this “1,000-year rain.”
Comedian and South Carolina native Stephen Colbert skewered the comments with a quip about “surviving Kiawah Indian meteorologists.”
But in truth the “1,000-year” probabilities are based on a limited amount of historical data. Rainfall measurements began locally in the late 1800s, Palmer said. Streamflows have been charted only for the past 100 years or less.
An informational brochure from the USGS highlighted that floods and other weather phenomena occur irregularly and unpredictably.
“If we had 1,000 years of streamflow data, we would expect to see about 10 floods of equal or great magnitude than the ‘100-year flood.’ These floods would not occur at 100-year intervals. In one part of the 1,000-year record, it could be 15 or fewer years between ‘100-year floods,’ whereas in other parts, it could be 150 years or more between ‘100-year floods,’” it said.
“There’s nothing to say that Columbia or the state of South Carolina is in the clear now for 10, 20, 30, 40, 1,000 years,” Palmer said. “That’s certainly not the way to look at it.”