You’ve probably heard hurricanes described as “a Category 3 storm,” “a hurricane of Category 4 strength” “downgraded to a Category 2 hurricane,” etc. Perhaps you’ve wondered what these various categories mean, where this terminology comes from.
These “categories” are references to what’s called the “Saffir-Simpson Hurricane Wind Scale.” This is a system that was devised to provide some expectation of the level of damage to be expected from winds of a given strength. It was developed in 1971 by civil engineer Herbert Saffir, and Director of the National Hurricane Center Bob Simpson.
Which category a hurricane falls into at a given time is a function of its highest sustained surface wind speed (meaning at least one minute of wind at the standard meteorological observation height of 10 meters over unobstructed exposure). The categories are as follows:
Category 1: 74-95 MPH
Category 2: 96-110 MPH
Category 3: 111-130 MPH
Category 4: 131-155 MPH
Category 5: 156 MPH and above
The force of the wind, and the potential damage it can cause, goes up by roughly four times as it increases one category. So a Category 2 hurricane can do about four times as much damage as a Category 1 hurricane, a Category 3 hurricane can do about four times as much damage as a Category 2 hurricane (and about sixteen times as much as a Category 1 hurricane), etc.
As you may recall from hurricane coverage, hurricanes pick up strength over water, and lose strength over land. And it happens very quickly. Typically, the winds even a half mile inland are a full category less than the winds at sea. Even a little further inland a hurricane usually soon drops to a tropical storm at most.
But not always. Hurricane winds can occasionally remain longer in a higher category as a storm goes over land, plus even if they diminish, lower level hurricanes and tropical storms can still do plenty of damage and cause serious flooding inland.
Which leads to another point about the Saffir-Simpson scale. It is based solely on wind speed, and projects only the expected damage from wind. It does not address potential damage from storm surge waves washing ashore, rainfall-induced floods, spawned tornadoes, or anything else. (It used to. The scale started off as the Saffir-Simpson Hurricane Scale and it attempted to take into account other factors such as storm surge, but it was later changed.)
Also, even wind damage will not be a function solely of the wind itself, but also of what it’s potentially damaging. A Category 3 hurricane will do a different amount of damage to a city with strict building codes than one without. A Category 1 hurricane will stand a better chance of breaking windows that have not been properly shuttered than those that have, etc.
This page found on the website for the National Hurricane Center of the National Oceanic and Atmospheric Administration provides an excellent summary of each of the five categories of the Saffir-Simpson scale and what damage to expect from each to different types of structures. It is an excellent starting point to learn more about how to interpret this tool.