Saffir-Simpson Hurricane Scale

The Saffir–Simpson Hurricane Scale, also referred to as the Saffir–Simpson Hurricane Wind Scale, is a classification used for some Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms. The scale divides hurricanes into five categories distinguished by the intensities of their sustained winds. To be classified as a hurricane, a tropical cyclone must have maximum sustained winds of at least 74 mph (74 mph; 74 mph). The highest classification in the scale, Category 5, is reserved for storms with winds exceeding 155 mph (155 mph; 155 mph).

The classifications are intended primarily for use in measuring the potential damage and flooding a hurricane will cause upon landfall, although they have been criticized as being too simple. Officially, the Saffir–Simpson Hurricane Scale is used only to describe hurricanes forming in the Atlantic Ocean and northern Pacific Ocean east of the International Date Line. Other areas use different scales to label these storms, which are called "cyclones" or "typhoons", depending on the area.

History
The scale was developed in 1971 by civil engineer Herbert Saffir and meteorologist Bob Simpson, who at the time was director of the U.S. National Hurricane Center (NHC). The scale was introduced to the general public in 1973, and saw widespread use after Neil Frank replaced Simpson at the helm of the NHC in 1974.

The initial scale was developed by Saffir, a structural engineer, who in 1969 went on commission for the United Nations to study low-cost housing in hurricane-prone areas. While performing the study, Saffir realized there was no simple scale for describing the likely effects of a hurricane. Mirroring the utility of the Richter magnitude scale in describing earthquakes, he devised a 1–5 scale based on wind speed that showed expected damage to structures. Saffir gave the scale to the NHC, and Simpson added the effects of storm surge and flooding. However, in 2009, the NHC made moves to eliminate pressure and storm surge ranges from the categories, transforming it into a pure wind scale. The new scale became operational on May 15, 2010. The scale does not take into account rainfall or location, which means a Category 2 hurricane which hits a major city will likely do far more cumulative damage than a Category 5 hurricane that hits a rural area.

The NHC decided that for its 2010 hurricane season, it would use the experimental Saffir–Simpson Hurricane Wind Scale (SSHWS), which would be based on the SSHS, but exclude flood ranges and storm surge estimations. The agency cited various hurricanes as reasons for removing the "scientifically inaccurate" information, including Hurricane Katrina and Hurricane Ike which both had stronger than estimated storm surge and Hurricane Charley which had weaker than estimated storm surge.

Categories
The scale separates hurricanes into five different categories based on wind. The U.S. National Hurricane Center classifies hurricanes of Category 3 and above as major hurricanes (although all hurricanes can be very dangerous). Most weather agencies use the definition for sustained winds recommended by the World Meteorological Organization (WMO), which specifies measuring winds at a height of 33 ft for 10 minutes, and then taking the average. By contrast, the U.S. National Weather Service defines sustained winds as average winds over a period of one minute, measured at the same 33 ft height. Central pressure and storm surge values are approximate and often dependant on other factors, such as the size of the storm and the location. Intensity of example hurricanes is from both the time of landfall and the maximum intensity. As a result, it is not uncommon for a pressure to be significantly higher or lower than expected for a specific category. Generally, large storms with very large radii of maximum winds have the lowest pressures relative to its intensity.

The scale is roughly logarithmic in wind speed, and the top wind speed for Category "c" (c=1 to 4) can be expressed as $$83\times 10^{c/15}$$ miles per hour rounded to the nearest multiple of 5.

The five categories are, in order of increasing intensity:

Category 1
Category 1 storms usually cause no significant structural damage to building structures; however, they can topple unanchored mobile homes, as well as uproot or snap trees. Poorly attached roof shingles or tiles can blow off. Coastal flooding and pier damage are often associated with Category 1 storms.

Examples of storms of this intensity include: Hurricane Alice (1954), Danny (1985), Jerry (1989), Ismael (1995), Claudette (2003), Gaston (2004) and Humberto (2007).

Category 2
Storms of Category 2 are strong enough that they can lift a house, and inflict damage upon poorly constructed doors and windows. Vegetation, poorly constructed signs, and piers can receive considerable damage. Mobile homes, whether anchored or not, are typically damaged, and many manufactured homes also suffer structural damage. Small craft in unprotected anchorages may break their moorings.

Hurricanes that peaked at Category 2 intensity, and made landfall at that intensity, include Diana (1990), Erin (1995), Alma (1996), Marty (2003), Juan (2003), and Tomas (2010).

Category 3
Tropical cyclones of Category 3 and higher are described as major hurricanes in the Atlantic or Eastern Pacific basins. These storms can cause some structural damage to small residences and utility buildings, particularly those of wood frame or manufactured materials with minor curtainwall failures. Buildings that lack a solid foundation, such as mobile homes, are usually destroyed, and gable-end roofs are peeled off. Manufactured homes usually sustain severe and irreparable damage. Flooding near the coast destroys smaller structures, while larger structures are struck by floating debris. Additionally, terrain may be flooded well inland.

Examples of storms of this intensity include Carol (1954), Alma (1966), Alicia (1983), Fran (1996), Isidore (2002), Jeanne (2004), Lane (2006), Bertha (2008), Karl (2010).

Category 4
Category 4 hurricanes tend to produce more extensive curtainwall failures, with some complete roof structural failure on small residences. Heavy, irreparable damage and near complete destruction of gas station canopies and other wide span overhang type structures are common. Mobile and manufactured homes are leveled. These storms cause extensive beach erosion, while terrain may be flooded far inland.

The Galveston Hurricane of 1900, the deadliest natural disaster to hit the United States, peaked at an intensity that corresponds to a modern-day Category 4 storm. Other examples of storms at this intensity are Hazel (1954), Carmen (1974), Iniki (1992), Luis (1995), Iris (2001), Charley (2004), Igor (2010).

Category 5
Category 5 is the highest category a tropical cyclone can obtain in the Saffir-Simpson scale. These storms cause complete roof failure on many residences and industrial buildings, and some complete building failures with small utility buildings blown over or away. Collapse of many wide-span roofs and walls, especially those with no interior supports, is common. Very heavy and irreparable damage to many wood frame structures and total destruction to mobile/manufactured homes is prevalent. Only a few types of structures are capable of surviving intact, and only if located at least 3 to 5 mi inland. They include office, condominium and apartment buildings and hotels that are of solid concrete or steel frame construction, public multi-story concrete parking garages, and residences that are made of either reinforced brick or concrete/cement block and have hipped roofs with slopes of no less than 35 degrees from horizontal and no overhangs of any kind, and if the windows are either made of hurricane resistant safety glass or covered with shutters.

The storm's flooding causes major damage to the lower floors of all structures near the shoreline, and many coastal structures can be completely flattened or washed away by the storm surge. Storm surge damage can occur up to four city blocks inland, with flooding, depending on terrain, reaching six to seven blocks inland. Massive evacuation of residential areas may be required if the hurricane threatens populated areas.

Storms of this intensity can be severely damaging. Historical examples that reached the Category 5 status and made landfall as such, include the Labor Day Hurricane of 1935, the 1959 Mexico Hurricane, Camille in 1969, Gilbert in 1988, Andrew in 1992, Dean, and Felix (both in 2007).

Criticism
Some scientists, including Kerry Emanuel and Lakshmi Kantha, have criticized the scale as being too simplistic, indicating that the scale does not take into account the physical size of a storm, nor the amount of precipitation it produces. Additionally, they and others point out that the Saffir-Simpson scale, unlike the Richter scale used to measure earthquakes, is not open-ended, and is quantized into a small number of categories. Proposed replacement classifications include the Hurricane Intensity Index, which is based on the dynamic pressure caused by a storm's winds, and the Hurricane Hazard Index, which bases itself on surface wind speeds, the radius of maximum winds of the storm, and its translational velocity. Both of these scales are continuous, akin to the Richter scale; however, neither of these scales have been used by officials.

Category 6
After the series of powerful storm systems of the 2005 Atlantic hurricane season, a few newspaper columnists and scientists brought up the suggestion of introducing Category 6, and they have suggested pegging Category 6 to storms with winds greater than 174 or; 150 –. Only a few storms in history have reached into this hypothetical category. Many of these storms were West Pacific super typhoons, most notably Typhoon Tip in 1979 with sustained winds of 190 mph.

According to Robert Simpson, there is no reason for a Category 6 on the Saffir-Simpson Scale because it is designed to measure the potential damage of a hurricane to manmade structures. If the wind speed of the hurricane is above 155 mph, then the damage to a building will be "serious no matter how well it's engineered".