Hurricane Ian Categories Explained: Florida Braces for Devastating Impact

Hurricane Ian is set to batter Florida with sustained winds of 155 miles per hour on Wednesday as it makes its way towards land, bringing with it life-threatening winds and flooding.

As of noon ET, the hurricane had developed maximum sustained wind speeds of 155 miles per hour, making it a Category 4 storm on the verge of a Category 5—the most severe type. At that time, several storm surge warnings were in effect, meaning there is a danger of catastrophic flooding from rising water moving inland from the coastline.

A public advisory from the National Hurricane Center (NHC) at 11 a.m. ET said that catastrophic storm surges of 12 to 18 feet above ground level plus "destructive waves" were expected along the southwest Florida coastline from Englewood to Bonita Beach, and that residents there should "urgently follow evacuation orders."

A map of areas forecasted to be at risk is available at Hurricanes.gov.

Storm categories are measured on the Saffir-Simpson Hurricane Wind Scale, which gives storms a rating from 1 to 5 based on sustained wind speed. It does not take into account other destructive factors like storm surge, which is historically the leading cause of hurricane-related deaths in the U.S.

Category 1

A Category 1 hurricane will have sustained winds between 74 and 95 miles per hour. Under such conditions, well-constructed homes may have roof damage, some trees will fall and power outages lasting as long as several days will likely result due to power line damage.

Category 2

A Category 2 hurricane will have sustained winds between 96 and 110 miles per hour. A Category 2 storm would cause major roof and siding damage to homes and fallen trees would block many roads. Near-total power loss would be expected to last several days to weeks.

Category 3

A Category 3 hurricane is the first of the major types, with wind speeds of between 111 and 129 miles per hour. Such winds would cause major damage to well-built homes, and electricity and water would be unavailable for several days to weeks.

Category 4

A Category 4 hurricane, in which wind speeds reach between 130 and 156 miles per hour, will cause damage described as "catastrophic" according to the NHC's description of the Saffir-Simpson scale. Well-built homes could lose most of their roof structures and some exterior walls. Most trees will be uprooted, power outages will last weeks to possibly months, and most of the area will be uninhabitable for weeks or months.

Category 5

A Category 5 hurricane is the most severe type with wind speeds of 157 miles per hour or higher. A high percentage of framed homes will be destroyed with total roof failure and wall collapse. Again, most of the area will be uninhabitable for weeks or months.

Categories 3 to 5 are described as major hurricanes, capable of causing significant loss of life due to their wind strengths alone, the NHC said.

The Saffir-Simpson scale does not address the potential for other hurricane-related impacts such as storm surges, rainfall-induced floods, or tornadoes, which require people to take protective action including evacuating from areas vulnerable to storm surges.

Hurricane Ian has grown stronger over the ocean, with the water being an abundant source of moisture to feed growing storm clouds. Warm sea surface temperatures are also a factor driving the hurricane's strength, as the evaporation of seawater pumps water into the lower atmosphere, heating the air and driving further suction and faster wind speeds.

As such, hurricanes tend to lose their strength when they move over land with wind speed approximately halving in the first 24 hours according to HurricaneScience.org.

Hurricane Harvey was a Category 4 storm that hit the U.S. on August 25, 2017, about 30 miles northeast of Corpus Christi, Texas. It dropped 52 inches of rain in the Houston area before weakening to a tropical storm. It caused $148.8 billion worth of damage, adjusted for 2022, making it the second costliest storm on record. The storm also took the lives of over 100 people, either directly from the storm or indirectly by other factors related to the storm, such as car accidents, electrocution or being unable to access medical services.

Hurricane Ida, another Category 4 storm that made landfall in Louisiana on August 29, 2021, caused $78.7 billion worth of damage. As of September 9 that year, the media had reported 91 deaths from multiple states, according to the Centers for Disease Control and Prevention (CDC).

The above chart from Statista shows the strength of the hurricanes that have made landfall in the U.S. since 1851.