What Hurricane Categories Really Mean

Hurricane Irma reached a maximum sustained wind speed of 185 miles per hour (297 kph), making it the strongest hurricane to ever form in the Atlantic Ocean. NOAA

When hurricane season arrives each year on June 1, phrases such as "storm surge," "wind speed," and "eyewall" suddenly become part of the summer lexicon in the United States. But probably the most important words to know about a hurricane are those that describe its power — and those include whether it's a Category 1 or a Category 5. The variance between the strengths of these two storms could mean the difference between life and death.

Meteorologists rank hurricanes from one to five based on the Saffir-Simpson scale. The scale is a yardstick that takes into account a hurricane's wind speed, storm surge and air pressure. The scale begins with a Category 1, the least powerful and dangerous hurricane, and moves towards its climax at Category 5 — the most catastrophic. But how did the Saffir-Simpson scale come to be, and what does it mean? We'll tell you in a bit. Let's first look at what a hurricane is, how it forms and why we need to be aware of its destructive power.

What Is a Hurricane?

Hurricanes are among the most violent storms on the planet and are born as tropical disturbances near the equator, where warm water, moist air and the rotational movement of Earth create winds, clouds and thunderstorms. As these disturbances move across the ocean, warm moist air rises and is replaced by cooler air. The cooler air condenses and falls toward the ocean surface only to be warmed again. The cycle repeats itself, and tropical disturbances gain strength and spin faster as wind picks up speed and more moist, warm air is pulled upward.

When wind speed reaches 25 to 38 miles (45 to 61 kilometers) per hour, the tropical disturbance morphs into a tropical depression, which turns into a tropical storm when winds reach 39 miles (62 kilometers) per hour. Tropical storms are like giant wet vacs on steroids — they suck a generous amount of moisture and heat from the ocean water, adding more fuel to the growing storm. An eye begins to form as the winds turn counterclockwise at a tremendous speed. A hurricane is born when winds are clocked at 74 miles (119 kilometers) per hour.

As the storm pushes across the ocean, it gathers speed and strength. Low air pressure forces ocean water into a huge mound near the eye, which could create a devastating storm surge when the wall of water reaches land. The more heat and moisture a hurricane consumes, the more powerful the storm becomes. That's where the Saffir-Simpson scale comes in.

What Is the Saffir-Simpson Scale?

The one through five scale was created when Robert H. Simpson was director of the National Hurricane Center in 1969 during the time Hurricane Camille blew through the Caribbean and into the warm waters of the Gulf of Mexico. Its winds were clocked at 190 miles (306 kilometers) per hour as it struck Mississippi. The official death toll from wind, storm surge and rain was 256.

"I wasn't able to communicate with people, like the Office of Emergency Planning, the American Red Cross, the Salvation Army and all the state agencies that were preparing for the storm," Simpson lamented in the book Hurricane Watch: Forecasting the Deadliest Storms on Earth. "I couldn't get a handle on the storm to answer their question: 'what kind of resources must we put in this particular area to do our job?' I needed something to give them a handle on it, so they would need to know what resources they needed to deal with the storm."

In 1971, Herbert S. Saffir was working as an engineer in Florida preparing a report for the United Nations on building codes that could withstand the onslaught of high speed winds. He eventually came up with a table that outlined the damage to buildings wind can cause at various speeds. He worked up five categories of hurricanes based on damage each could cause. In 1972, Simpson took Saffir's numbers and correlated them with storm surge estimates and barometric pressure. The result was the Saffir-Simpson scale. By 1975, the Saffir-Simpson scale was in widespread use. Local, state and federal officials, not to mention the public at large, now had an easy-to-read-and-understand chart that outlined a hurricane's impact.

What Do Hurricane Categories Mean?

According to the Saffir-Simpson scale:

Category 1 storms have sustained winds of 74 to 95 miles (119 to 153 kilometers) per hour. These are dangerous winds that could damage roofs, snap tree branches and uproot some trees. Power outages can occur. The storm surge can range between 4 to 5 feet (1.2 to 1.5 meters).
Category 2 storms have sustained winds of between 96 to 110 miles (154 to 177 kilometers). Winds at this speed can cause extensive damage, including power loss, downed streets, and major roof and siding damage to well-constructed frame houses. The storm surge could reach up to 8 feet (2.4 meters).
Category 3 storms have sustained winds of 111 to 129 miles (178 to 208 kilometers) per hour. Damage to homes can be major. It's possible that electricity and water will be unavailable for weeks after the storm passes. The storm surge can range between 9 and 12 feet (2.7 and 3.6 meters).
Category 4 storms have sustained winds of 130 to 156 miles (209 to 251 kilometers) per hour. Property damage can be extensive. Most trees will snap or be uprooted. Power could be out for months and most of the area devastated by the storm will be uninhabitable for weeks or months. The storm surge is between 13 and 18 feet (3.9 and 5.4 meters).
Category 5 storms have sustained winds of 157 miles (252 kilometers) per hour or higher. Category 5 hurricanes are the most catastrophic and will cause total roof and building failures, collapsed walls, and the isolation of neighborhoods due to downed trees and power lines. Storm surge with storms of this strength can reach more than 18 feet (5.4 meters).

While the Saffir-Simpson scale is a good measuring tool, it doesn't really tell the full story of a hurricane's impact. Hurricanes pack a lot of kinetic energy, and as a byproduct, a hurricane's power increases exponentially from one category to the next as wind speed increases. A Category 5 hurricane, for example, is 500 times more powerful than a Category 1. How does this relate to property damage? Compared to a Category 1, a Category 2 hurricane can generate seven times the amount of damage, while a Category 5 storm can generate 144 times the amount of destruction.