What "Once In A Hundred Years" Really Means

Explaining a Commonly Misunderstood Weather Phrase

January 2, 2020
Hurricane (Once in a Hundred Years)

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From time to time, when a major weather event occurs, I see it described as a “once in a hundred years” or even “once in a thousand years” event. But what does this weather phrase really mean?

Weather phrases such as “once in a hundred” years are commonly misunderstood. Usually, this refers to a particular storm that brings heavy rain or snow.

  • For example, in 2017, Hurricane Harvey brought exceptional rain amounts to the Houston, Texas, area—including 60.5 inches in the city of Nederland— while 30 inches or more fell on an area near the Texas coast the size of the state of Maryland. The 60.5 inches in Nederland was a record for a single storm in the continental United States that created an unprecedented “once in a thousand years” flood event. No rain event of that magnitude has ever happened in the history of the United States.

But there have been several other exceptional rainfalls in Texas brought about by tropical storms in recent decades. The 48 inches of rain in Medina from Hurricane Amelia in 1978; the 45 inches that Claudette dumped on Alvin in 1979; the 43.15 inches brought by Imelda to Jefferson County in 2019; and the 40.68 inches that fell from Allison in Jefferson County in 2001 are also among the 10 highest rainfall amounts from tropical cyclones in the United States since 1950. Three of the other top 10 events occurred in Hawaii, with one each in Florida and Puerto Rico.

“Once in a Hundred Years”

All of these events have been characterized as at least “once in a hundred years” events, including all five that hit Texas in the past 41 years.

This raises two important questions: 

  1. How do we know that an event is this extreme?  
  2. How can we have five “once in a hundred years” events in Texas in less than half a century?

The answer to the first question is that we have excellent precipitation records from about the past 30 years based on rain-gauge, Doppler radar, and satellite measurements. We have pretty good records from about the past 100 years, mostly with rain gauges. Before that, the records are much more speculative, with some data from weather instruments but most of the weather information on individual events coming from personal diaries, with climatological information coming mostly from indirect measurements of things like the thickness of tree rings.

The answer to the second question is more complicated. Instead of imagining a “once in a hundred years” event happening exactly once every 100 years, think about it as having a 1% chance of happening in any given year. And once such an event occurs, the chance of it occurring again in the following year is still 1%.

Very simply, the United States is large enough, with enough different locations and weather events, that an apparent anomaly like the Texas rainfall will occur over 1% of the country each year. This means that events of this nature will happen somewhere in the United States each year, despite the tiny chance of one occurring in any one location. And Texas is a large state with plenty of room for extreme events in different parts of the state.

Two other factors for consideration: 

  1. Because we do not have good weather records going back for more than a century or two, these events may happen more often than we expect. 
  2. As climate changes, whether naturally or because of humans, particular events can become more or less frequent.

“A 100-Year Flood”

Similarly, you may have also heard the phrase “a 100-year flood,” especially around hurricane season. Perhaps you heard this phrase two years in a row or even two months in a row!

What does this mean? In the weather world, it’s about probability of the event happening, not the timing. It does NOT mean that a 100-year flood should only happen once in a hundred years. 

As with “once in a hundred years,” it’s a statistical way of describing a weather event has a 1% chance of happening in any given year. 

Now you know! 

If you enjoy these “weather whys,” find out what the phrase, “A Chance of Rain” really means!

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