The All-Peril Cat Five

Why the Saffir-Simpson Hurricane Intensity Scale had five levels we don’t know. The digits on a hand? Better than three, but lower resolution than the dozen rungs for wind speeds or earthquake intensity? Whatever the reason it seems to work.

In the late 1960s, Herbert Saffir, a Florida building engineer, was sent by the United Nations to study the hurricane vulnerability of low-cost housing in the Caribbean. He realized something was needed to rank hurricane destructiveness. Saffir had some “Richter envy” from observing the ease with which seismologists now communicated with the public. In 1971, he contacted Robert Simpson, head of the National Hurricane Center to help link damage levels with wind speeds.

Seeing the opportunity to communicate evacuation warnings, Simpson also added details around the height of advancing storm surges. Better information was clearly needed, after the loss of life in Hurricane Camille on the Mississippi coast in 1969.

The Saffir-Simpson Hurricane Intensity Scale was publicized more widely in 1973, and after 1974 it became the standard means of communicating hurricane intensity.

In parallel with Saffir’s work, in 1971 Tetsuya Fujita, the tireless expert on U.S. tornadoes, and Alan Pearson, head of the National Severe Storms Forecasting Center, created a five-grade scale for assessing the severity of tornado wind speeds from the level of damage.

Hurricane Katrina as it strengthened to a Cat 5 storm over the Gulf of Mexico – picture taken on August 28, 2005. Image credit: Flickr/NASA Goddard Space Flight Center

It’s Bad. It’s a Category 5.

For both the hurricane and tornado scales, the top rank “Category 5” has a hallowed, iconic status. In the Atlantic basin, a hurricane only makes landfall at Cat 5 intensity about once a decade. When the scale was first launched there were only two U.S. landfalling Cat 5 storms: the 1935 Labor Day Hurricane in the Florida Keys and Hurricane Camille in 1969, later to be joined by Hurricane Andrew in 1992. However, at any one location around the Atlantic the return period of Cat 5 wind speeds is a minimum of two hundred years and, out of the tropics, more likely 500 or 1000 years. (The probabilities are even lower for two tornadoes to strike the same location at F-5 wind speeds.)

In consequence we get to the core philosophical significance of Cat 5. That this brings wind speeds so rare that it does not make economic sense to design ordinary buildings to withstand them. In any cost benefit analysis, the additional costs of deep foundations and concrete bunker construction would not be worth the investment. (For the short forecast time of the tornado compared with the hurricane this is also a life safety issue).

Extending Cat 5 Beyond Hurricanes

So, if this is our definition of Cat 5, maybe there is a case for extending the concept to other perils? In fact, this idea was born out of a conversation with Roy Wright, head of the Insurance Institute for Business and Home Safety (IBHS), who described the total immolation of the city of Paradise in California in the November wildfire last year, as being “Cat 5”.

The “Cat 5 wildfire” is one that leads to total destruction of a town: a fire so fast and so intense, that it overwhelms the defenses and more or less consumes everything. In Paradise only a few structures survived, including the concrete-walled and metal-roofed Starbucks, protected by its “defensible space” car parks.

We can find “Cat 5” in situations of ultra-liquefaction, when buildings sink into the ground, or are split in half by lateral movements, as in Christchurch in 2011. We could characterize the scene as “Cat 5” where in September 2018, massive liquefaction outside the city of Palu on Sulawesi, Indonesia, caused buildings to slide sideways at the speed of horses, folding like card-houses on their occupants. There is unlikely to be anything that could be done, at any reasonable cost, to build to withstand these motions and forces. These situations are simply Cat 5.

How about a “Cat 5 storm surge”? We have the situation on the Mississippi State Coast in Hurricane Katrina, when buildings for five blocks inland were reduced to slab. Here we had a Cat 5 storm surge and water height associated with a Cat 3 hurricane landfall. Yet the U.S. Base Flood Elevation for new builds is set at the “100-year” return period flood hazard. A house built to this standard could expect to be wrecked by the ”500-year” storm.

There was little that could withstand the 2011 “Cat 5 tsunami” along the Tōhoku coast of Japan. Or even a bull’s eye direct hit “Cat 5 earthquake” impact in Christchurch New Zealand in 2011.

“Cat 5” deserves to extend its range across the perils. “Cat 5” provides the language to describe the situation where a catastrophe is highly disastrous but at the same time not unforeseen. “Cat 5” helps communicate around the problem: that when we annualize the costs of extremely rare events, it may well not be worth the extra cost to build to withstand them.

Which brings us back to California. The recurrence interval of Cat 5 fire conditions in the city of Paradise, California or in the northern suburbs of Santa Rosa, may be significantly less than 500 years. High land prices have become a risk factor. Houses are placed so close that if one catches fire, the rest are destroyed in a chain reaction. A “one-in-five-hundred year” total loss from wind, flood or earthquake can be handled by insurance. But not a “one-in-fifty year” total loss for wildfire.

Chief Research Officer, RMS
Robert Muir-Wood works to enhance approaches to natural catastrophe modeling, identify models for new areas of risk, and explore expanded applications for catastrophe modeling. Recently, he has been focusing on identifying the potential locations and consequences of magnitude 9 earthquakes worldwide. In 2012, as part of Mexico's presidency of the G20, he helped promote government usage of catastrophe models for managing national disaster risks. Robert has more than 20 years of experience developing probabilistic catastrophe models. He was lead author for the 2007 IPCC 4th Assessment Report and 2011 IPCC Special Report on Extremes, is a member of the Climate Risk and Insurance Working Group for the Geneva Association, and is vice-chair of the OECD panel on the Financial Consequences of Large Scale Catastrophes. He is the author of six books, as well as numerous papers and articles in scientific and industry publications. He holds a degree in natural sciences and a PhD in Earth sciences, both from Cambridge University.

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