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Hurricane Ian neared landfall Wednesday in Florida, where it is projected to bring “catastrophic” storm surges, winds and flooding to the coastal areas in its path.

Residents were braced for an extremely dangerous storm, which battered Cuba and could hit the Florida peninsula as one of the strongest hurricanes to strike the U.S. in years.

Ian has reached maximum sustained winds of 155 miles per hour, according to the National Hurricane Center. That makes it a strong category 4 hurricane—on the cusp of category 5.

But what do meteorologists and news anchors mean when they talk about hurricane categories, like category 1, category 3 or category 5?

Most people simply want to know how much water and wind to expect, and what a hurricane will mean for their safety. Hurricane categories can’t predict everything about a storm, but categories do give an indication of how a hurricane will affect people and property in its path.

Here’s what you need to know about hurricane categories.

Where do hurricane categories come from?

Hurricanes are measured on the Saffir-Simpson Hurricane Wind Scale, which runs from Category 1 up to Category 5. According to The Weather Channel, the scale was developed in the 1970s by Miami engineer Herbert Saffir and Robert Simpson, a meteorologist who was director of the National Hurricane Center.

The hurricane category scale has evolved over time, but in its current version, it only measures the wind speeds produced by a hurricane. This means the different hurricane category designations won’t tell you about the flooding or tornadoes that might accompany hurricanes, but they can give you a sense of how strong a hurricane might be and what kind of threat they present.

See how the National Oceanic and Atmospheric Administration breaks down the hurricane category scale here.

Category 1 hurricane: Very dangerous winds will produce some damage

In a Category 1 hurricane, winds range from 74 to 95 mph. Falling debris could strike people, livestock and pets, and older mobile homes could be destroyed. Protected glass windows will generally make it through the hurricane without major damage. Frame homes, apartments and shopping centers may experience some damage, and snapped power lines could result in short-term power outages.

Hurricane Dolly in 2008 was an example of a Category 1 hurricane that whipped through southern Texas. Hurricane Sandy was also a Category 1 hurricane when it hit Jamaica in 2012, but it weakened before reaching the east coast of the United States.

Category 2 hurricane: Extremely dangerous winds will cause extensive damage

Winds range between 96 and 110 mph during a Category 2 hurricane. There is a bigger risk of injury or death to people, livestock and pets from flying debris. Older mobile homes will likely be destroyed, and debris can ruin newer mobile homes, too. Frame homes, apartment buildings and shopping centers may see major roof and siding damage, and many trees will be uprooted. Residents should expect near total power loss after a Category 2 hurricane, with outages lasting anywhere from a few days to a few weeks.

Hurricane Frances in 2004 was a Category 2 hurricane when it hit the west coast of Florida.

Category 3 hurricane: Devastating damage will occur

In a Category 3 hurricane, winds range from 111 to 129 mph. There is a high risk of injury or death to people, livestock and pets from flying and falling debris. Nearly all older mobile homes will be destroyed, and most new ones will experience significant damage. Even well-built frame homes, apartments and industrial buildings will likely experience major damage, and the storm will uproot many trees that may block roads. Electricity and water will likely be unavailable for several days to a few weeks after the storm.

Hurricane Wilma was a Category 3 hurricane when it reached Florida’s coast in 2005, bringing initial winds of close to 120 mph. Hurricane Katrina was also a Category 3 hurricane when it made landfall in Louisiana, but it also brought high levels of storm surge that levees in Louisiana and New Orleans couldn’t hold up against, which is part of what made it so destructive.

Prior to making landfall as a Category 3 hurricane, Hurricane Irma was a Category 5 hurricane—the second-strongest Atlantic hurricane on record.

Category 4 hurricane: Catastrophic damage will occur

During a Category 4 hurricane, winds range from 130 to 156 mph. At these speeds, falling and flying debris poses a very high risk of injury or death to people, pets and livestock. Again, most mobile homes will be destroyed, even newer ones. Some frame homes may totally collapse, while well-built homes will likely see severe damage to their roofs, and apartment buildings can experience damage to upper floors.

A Category 4 hurricane will blow out most windows on high-rise buildings, uproot most trees and will likely down many power lines. Power outages can last for weeks or even months after storms of this level. Water shortages are also common in the aftermath of Category 4 hurricanes, potentially making the affected area uninhabitable for weeks or months.

Hurricane Harvey made landfall as a Category 4 hurricane in Texas in August.

Category 5 hurricane: Catastrophic damage will occur

In a Category 5 hurricane, the highest category hurricane, winds are 157 mph or higher. People, livestock and pets can be in danger from flying debris, even indoors. Most mobile homes will be completely destroyed, and a high percentage of frame homes will be destroyed. Commercial buildings with wood roofs will experience severe damage, metal buildings may collapse and high-rise windows will nearly all be blown out.

A Category 5 hurricane is likely to uproot most trees and ruin most power poles. And like with Category 4 hurricanes, power outages will likely last for weeks to months. People should expect long-term water shortages.

Only three hurricanes have made landfall as a Category 5 hurricane since 1924. One of the most famous hurricanes in recent decades, Hurricane Andrew back in 1992, was a Category 5 hurricane when it hit Florida. Hurricane Camille was a Category 5 when it hit in 1969, as was 1935’s “Labor Day” hurricane.

Category 6 hurricane

There is no such thing as a Category 6 hurricane. When Hurricane Irma was headed toward the coast of southern Florida in August, it had maximum wind speeds of 185 mph, according to the New York Times. But the Saffir-Simpson scale only goes up to 5.

Some people have been talking about creating a Category 6 hurricane designation, but a Category 5 already means near total destruction. So while there is a measurement difference between 157 mph winds and 200 mph winds, there may not be much practical difference in terms of destructive force between a Category 5 hurricane and what might be labeled as a Category 6 hurricane.

The problem with hurricane categories

There is a key problem with how hurricane categories are measured: The Saffir-Simpson scale only takes into account a storm’s maximum sustained windspeed, and disregards other threats, like expected rainfall or storm surge. Even a category 1 hurricane or a tropical storm can bring serious damage and risk to life and limb, but people in their path may underestimate the danger they pose because of how they’re categorized. An alternative to the Saffir-Simpson Scale is AccuWeather’s “RealImpact Scale,” which takes other metrics besides wind speed into account.

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