Jeff Masters Weather Blog

Hurricanes are changing with the climate. Our words about them may need to change, too. » Yale Climate Connections

On Friday, Aug. 5, 2016, the National Hurricane Center identified an ill-defined area of low pressure drifting westward along Florida’s sweeping Big Bend, where the state’s panhandle arcs into its long-tailed peninsula. A tropical disturbance in August over the Gulf of Mexico typically garners the wary eye of hurricane forecasters and coastal residents alike, but by Sunday, Aug. 7, the disorganized disturbance was moving inland, adrift over the Deep South. Everyone knows tropical cyclones don’t form over land, or at least they shouldn’t.

In the coming days, the nameless, unclassified low-pressure system of tropical origins would blanket parishes of southern Louisiana in biblical rains, enough in the span of a few days to have filled Florida’s Lake Okeechobee — the largest freshwater lake in the Southeastern U.S. — four times over. The ensuing floods triggered the most expansive federal response to a disaster since Superstorm Sandy. And at a cost of over $12 billion, it was the most expensive disaster to strike Louisiana in over a decade.

It wasn’t a tropical storm. It wasn’t even a tropical depression. But it certainly felt like one.

Outdated names and thresholds

The lack of a name for this catastrophe became its own topic of discussion in the ensuing days, raising a key question: Could arbitrary definitions from yesteryear be hindering how we perceive and respond to the tropical threats of today?

Modern-day hurricane messaging rests on an aging foundation. Although scientists have fine-tuned their forecasts, dramatically slicing hurricane track errors in half since the days of Hurricane Andrew, and more recently enlisting social science teams to tailor-make forecast graphics, our language and terminology are miring communications in the past.

We can see the generation gap in conspicuous areas like the names we give to tropical storms and hurricanes. Once-popular names like Arlene, Beryl, Debby, and Wilfred still pepper the six rotating lists — but according to the Social Security Administration, the popularity of these names peaked in 1934, 1920, 1959, and 1917, respectively. Of the 126 names included on the original six Atlantic lists introduced in 1979, over half remain.

But personifying storms with names from generations past is a benign symptom compared to widely used definitions that carry over from bygone centuries. Take, for example, tropical storms — tropical cyclones whose maximum sustained winds reach 39 mph. It’s at this stage that storms attain a name.

You may wonder why the threshold was set at 39 mph instead of 40 mph, especially since the accuracy of instruments only allows forecasters to estimate tropical cyclone winds to the nearest 5 mph. That’s because the tropical storm threshold is derived from a primitive scale developed in 1805 by Royal Navy officer Francis Beaufort and expanded in 1926 to estimate wind speed from observed sea conditions. At Beaufort force 8 — gale conditions, signaling a strong wind or stiff breeze — ocean behavior suggests winds of at least 39 mph. By the 1950s, gale-force winds came to define the term “tropical storm.”

Although there’s nothing scientifically significant about 39 mph winds, the threshold has outsized societal implications. When forecasters christen a tropical storm with a name, special attention is given to it by the public and press. Financial triggers like higher-dollar named-storm deductibles kick in, leaving many homeowners to pay more out-of-pocket for damages before their insurance pays. Television stations replay the ubiquitous forecast cone and update viewers on the storm’s strength. What Sir Francis Beaufort advanced over 200 years ago for mariners has unknowingly seeped into the crevices of modern-day hurricane communications without many — including meteorologists — understanding how or why the arcane threshold is still used today.

Similarly, the term for entry-level tropical cyclones, “tropical depressions,” was conceived as a byproduct of jargon past. Though mostly obsolete today in U.S. meteorology, the word “depression” was common parlance among early 20th-century weather forecasters in describing a variety of low-pressure systems. By the early 1960s, the adjective “tropical” was added to distinguish warm-core depressions in the tropics from midlatitude or wintertime lows. The moniker stuck, despite its anachronous and often confusing connotation: Today, most folks think of depression as a physical or psychological condition rather than a meteorological phenomenon.

Adapting the message for a changing climate

The climate is changing and communicating with the public about extreme weather grows increasingly difficult if our language and definitions don’t change with it. Climate and weather are inextricably linked — when one changes, so does the other. Climate “normals,” like the average high or low temperature on a given date, are updated every 10 years (using 30-year rolling averages), but we tend to adjust our words more gradually.

A study published last summer by a who’s who of hurricane experts found tropical activity in the Atlantic, including landfalls, is starting earlier and earlier due to warming oceans, to the tune of five days per decade. Last March, the World Meteorological Organization began discussions on potentially shifting back the start date of the Atlantic hurricane season from June 1 to May 15 but demurred, pushing any decision to future years.

In one sense, it’s easier to change what already exists or to recalibrate a definition that’s measurable than it is to introduce new terminology. With hurricane season, scientists can choose to adjust the bookends if the averages change.

The incidence of rapid intensification — when a tropical cyclone strengthens by 35 mph or more in 24 hours — is growing more common due to climate change, according to numerous studies. Taken to an extreme, if everything is rapidly strengthening, then nothing is rapidly strengthening — and the definition loses meaning. So scientists might reevaluate the definition of this threshold to ensure that it remains statistically consistent over time.

Water, water everywhere

But what about tropical cyclones moving more slowly and delivering more intense rainfall — as much as 15% more from climate change? What language do we adjust? Most deaths in tropical cyclones — nearly 90% — result from water. This is true for high-end hurricanes like Ian, when catastrophic storm surge scours the shoreline, but it’s especially true for low-end tropical depressions and tropical storms, which can bring deadly flooding from catastrophic rainfall.

Three of the five wettest tropical cyclones to impact the mainland U.S. were tropical storms with winds of 50 mph or less, not hurricanes. Even Harvey in 2017 — the worst rainstorm in U.S. history — was largely a 40-50 mph tropical storm, not a hurricane, when its heaviest rains fell over southeastern Texas. Post-tropical cyclones like Ida in 2021, which killed dozens and left over $20 billion in damage from widespread flooding across the densely populated northeastern U.S., can also be extremely dangerous, but reduced winds and a “post” qualifier may give the impression that the danger has passed.

For systems that don’t achieve tropical depression status, it’s easy for the public to assume no threat. Even thunderstorms don’t get a severe warning until winds reach 58 mph. The very label “tropical storm” or “tropical depression” implies something about wind. Water is killing when winds aren’t the primary threat, but where are the words?

Words for the future

A team of government scientists concluded that human-caused climate change increased the odds of the 2016 deluge in southern Louisiana that August by an astounding 40%. But even had the nameless storm earned a name in our current system, the naming would’ve said something about the wind and nothing about the water.

Named storms make headlines, but when winds aren’t the major hazard, they can be a distraction and a reason for audiences to dismiss the message. “It’s only a tropical depression” or “It’s just a tropical storm” are counterproductive messages when high water threatens. Strong winds matter for damage potential over land, but not as much on the low end as some may assume.

In fact, forensic engineers investigating wind damage in the aftermath of storms usually find little structural damage from winds under 50 mph. Instead, at lower winds, it’s often water intrusion from heavy rainfall that causes more damage to buildings. Additionally, maximum winds are estimated over unobstructed surfaces, such as open water, so even 50 mph tropical storms might not clear 40 mph in dense, tree-lined suburban corridors.

The 1805 Beaufort scale was never intended to be used over land. If destructive winds on the coast and inland are the threat, perhaps it’s not unreasonable to raise the threshold for named tropical storms from 40 mph to 50 mph. But if water, not wind, is the major threat during early development, perhaps tropical depressions should be reclassified as tropical monsoons. And if a tropical disturbance poses a high risk of destructive overland rainfall, perhaps it merits a stronger label.

As for the names we use for tropical storms and hurricanes, there are signs of progress in the new “overflow” list that the World Meteorological Organization recently adopted for use whenever an Atlantic season has run through all of the names on its standard list, as occurred in 2005 and 2020. The new list includes Braylen, Deshawn, Kenzie, Makayla, and other names that will sound much more familiar to contemporary ears.

Our language isn’t a complement to the forecast; it’s fundamental to it. More people and infrastructure are settling along the vulnerable Atlantic and Gulf Coasts than ever, with tropical cyclones responsible for over $1 trillion in losses and nearly 7,000 deaths in the U.S. since 1980. To re-imagine hurricane messaging opens a lane for science to drive society forward to better meet the needs of a perilous coastline. Rethinking our legacy words and definitions isn’t an abandonment of the past, but a recognition of the changing climate on the weather extremes of today and into the future.

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