The Saffir-Simpson Hurricane Wind Scale (SSHWS), initially known as the Saffir-Simpson Hurricane Scale (SSHS), is a classification system that measures and categorizes the strength of hurricanes based on their sustained wind speeds. Specifically, it applies to storms in the North Atlantic Ocean and the Northeast Pacific Ocean. This scale enables meteorologists to predict potential damage and flooding caused by hurricanes as they make landfall. Ultimately, it helps forecasters communicate the potential danger of storms in advance of their arrival.
Civil engineer Herbert Saffir and meteorologist Robert Simpson developed the scale in the early 1970s. The SSHWS uses wind speed to categorize hurricanes into five distinct categories, ranging from Category 1 (least intense) to Category 5 (most intense).
- The Saffir-Simpson Hurricane Wind Scale runs from 1 to 5.
- The scale uses the maximum sustained wind speed within the storm.
- A Category 1 hurricane is the weakest, while a Category 5 hurricane is the strongest.
- Tropical depressions and tropical storms are not part of the scale, but they are also determined by wind speed.
Difference Between Tropical Depressions, Tropical Storms, and Hurricanes
Understanding the difference between a tropical depression, tropical storm, and hurricane makes it easier to understand the Saffir-Simpson scale. All of these storms are tropical cyclones, but their maximum sustained wind speed differs.
- Tropical Depression: A tropical depression forms when a low-pressure area is accompanied by thunderstorms that produce a circular wind flow with maximum sustained winds below 39 miles per hour (mph) or less than 63 kilometers per hour (km/h) or 34 knots. They do not typically cause significant damage but can evolve into more severe weather phenomena.
- Tropical Storm: A tropical depression is upgraded to a tropical storm when its sustained wind speeds range between 39-73 mph (63-118 km/h or 34-63 knots). These storms are assigned names by meteorological agencies and can cause minor damage.
- Hurricane: A tropical storm escalates to a hurricane when sustained wind speeds exceed 74 mph (119 km/h or 64 knots). Hurricanes can cause substantial damage, with intensity increasing according to the SSHWS categories.
The Five Categories of the Saffir-Simpson Hurricane Wind Scale
The SSHWS categorizes hurricanes into five categories based on the 1-minute maximum sustained winds, as follows:
- Category 1: This is a hurricane with wind speeds of 74-95 mph (119-153 km/h or 64-82 knots). Although it’s the least intense, a Category 1 hurricane can cause significant damage to unanchored mobile homes and landscaping and causes some coastal flooding. Hurricane Dolly (2008) was a Category 1 hurricane when it made landfall in South Texas.
- Category 2: This category consists of hurricanes with wind speeds of 96-110 mph (154-177 km/h or 83-95 knots). These storms can cause extensive damage to roofs, doors, and windows, as well as near-total power loss. Hurricane Frances (2004) made landfall in Florida as a Category 2 storm.
- Category 3: A Category 3 hurricane has wind speeds of 111-129 mph (178-208 km/h or 96-112 knots). Known as a “major” hurricane, it can cause devastating damage, including the removal of roofs and exterior walls, and the destruction of mobile homes. Hurricane Katrina (2005) was a Category 3 hurricane at its second landfall near the Louisiana-Mississippi border.
- Category 4: These hurricanes, with wind speeds of 130-156 mph (209-251 km/h or 113-136 knots), are capable of causing catastrophic damage. This damage includes the loss of most of a building’s roof structure and/or exterior walls. Hurricane Charley (2004) struck Florida as a Category 4 storm.
- Category 5: This is the highest category on the SSHWS, consisting of hurricanes with wind speeds exceeding 157 mph (252 km/h or 137 knots). Category 5 hurricanes cause catastrophic damage, including the total destruction of homes and significant damage to infrastructure. Hurricane Dorian (2019) was a Category 5 hurricane when it stalled over the Bahamas.
Could There Be a Category 6 Hurricane?
The Saffir-Simpson Hurricane Wind Scale currently only goes up to Category 5 for storms with sustained winds of 157 mph (252 km/h or 137 knots) or higher. This means that even if a hurricane’s winds significantly exceed this speed, it is still a Category 5 storm. The scale does not officially include a Category 6.
The reason for this is twofold. First, Category 5 storms are already incredibly destructive, capable of causing catastrophic damage. The kind of damage a hurricane can cause does not increase linearly with wind speed, but rather grows exponentially. Therefore, once a storm has reached Category 5 intensity, it causes devastating effects, regardless of whether its winds increase beyond the 157 mph threshold.
Second, hurricanes reaching and surpassing the threshold for Category 6 (if it existed) are relatively rare, and therefore, expanding the scale might not provide meaningful additional information for the majority of storms.
However, due to climate change and the increasing intensity of hurricanes, some scientists and meteorologists have started discussions about whether a Category 6 should be added to the scale. The debate is ongoing and reflects broader discussions about how we understand and categorize extreme weather events in a changing climate. Since the categories run in roughly 20 mph increments, a Category 6 hurricane might have sustained winds greater than 175 or 180 mph.
Criticisms of the Saffir-Simpson Scale and Alternatives
The SSHWS is widely used, but it faces criticisms. The main critique is that it does not take into account other vital factors like storm size, rainfall totals, and speed of forward motion, which significantly contribute to the overall destructive power of the storm and subsequent flooding.
In response to these limitations, other scales have been proposed. Examples of alternatives include the Integrated Kinetic Energy index, which considers the storm’s total energy, and the Hurricane Severity Index, which includes size and intensity.
History of the Saffir-Simpson Scale
Herbert Saffir, a structural engineer, developed the initial scale in 1969 to estimate the impact of hurricanes on buildings and infrastructure. In 1971, Robert Simpson, then-director of the U.S. National Hurricane Center, added information about storm surge and flooding to create the Saffir-Simpson Hurricane Scale. The term “wind scale” was added in 2010 when the National Hurricane Center removed storm surge, flooding, and barometric pressure from the criteria, leaving wind speed as the sole metric.
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