This is the official peak week for hurricanes, and it seems fitting that Hurricane Florence, which seems destined to be a historic hurricane, is headed toward the east coast today. More than 1.5 million people have been told to evacuate, and like Hurricane Harvey, it is slow moving and expected to drop up to 40 inches of rain, creating devastating floods.

If you've been watching the news, you've seen the description of the storm vary from a Category 4 to a Category 3, back to a Category 4, and currently to a Category 2. But what do those numbers mean? Generally, the categories are a tropical depression, tropical storm, severe tropical storm, typhoon, very strong typhoon, and violent typhoon, rated on the Saffir-Simpson scale for hurricanes from category one to category five.

The scale used for typhoons is different and was developed by the Japanese Meteorological Agency, The China Meteorological Administration and the Hong Kong Observatory.

There are other scales used in the Indian Ocean, South-Western Indian Ocean and waters surrounding Australia and Fiji. The scales are relatively similar across territories with minor variations.

Related: Hurricane Florence – A triple threat?

Category 1

In general, the stronger the hurricane the more damage that is caused, but that's not always the case. A weaker hurricane may cause more damage in a populated area than a stronger hurricane causes in a less populated, open area. The more construction there is, the more likely there will be damage.

With a category 1 hurricane, the storm surge is around 4–5 feet. Storm surge is an increase in the ocean's level and can be in excess of several feet. It's often the most damaging feature of a hurricane, especially when it occurs during high tide in low-lying areas. Storm surge is often responsible for most hurricane deaths because people can't escape the rising water. A category 1 hurricane causes some damage to trees and shrubbery and unanchored mobile homes.

Hurricane Katrina in 2005 was at different times a category 1 hurricane and a category 3. It caused severe flooding due to the low-lying areas it inundated. Hurricane Agnes in 1972 was category 1. Agnes caused $11 billion in damages.

Category 2

Category 2 hurricanes have storm surge between 6 and 8 feet. A category 2 hurricane can cause major damage to mobile homes, damage buildings' roofs, and blow trees down. Blowing trees cause damage to property as they fall onto houses or are blown through windows. Coastal roads and low escape routes may be flooded 2 to 4 hours before the storm. Two feet of water can float most cars, even SUVs. Six inches of rapidly moving water can knock people off their feet. Roofs, windows and doors may be damaged.

Hurricane David in 1979 was a category 2 hurricane and caused $320 million in damage.

Related: Prepare for flooding from Hurricane Florence with this checklist

Category 3

Category 3 hurricanes have storm surges of 9–12 feet, destroy mobile homes, down large trees and damage small buildings. To put that into perspective, the standard ceiling height is eight feet. Suspended ceilings are generally seven and a half feet, while tall ceilings are nine to 10 feet.

Signs will be down, and structural damage often occurs to small buildings and homes. Serious flooding at the coast, as well as waves and debris, can damage large and destroy small structures. Roads are often cut off 3–5 hours before the storm, and flat terrain less than 5 feet above sea level can be flooded inland 8 miles or more. Piers and marinas will be damaged and flooded. Evacuations of low-lying island and shoreline residents may be put in force.

Hurricanes Ivan and Jeanne in 2004, and Katrina, Wilma and Dennis in 2005 were category 3 hurricanes. Jeanne caused $8 billion, Wilma $20 billion, Ivan $19 billion and Dennis $25 – $30 billion in damages. Katrina was the costliest hurricane on record at $105 billion.

Category 4

Category 4 hurricanes have storm surges of 13–18 feet. They completely destroy mobile homes, and the lower floors of structures near the shore are susceptible to flooding. Trees and all signs are likely to come down, and structural damage is often severe with complete failure of some roofs. Flat terrain less than 10 feet above sea level can be flooded 6 miles inland and there will be major beach erosion.

All residences within 500 yards and single-story homes on low ground within two miles will be evacuated. Roads will be cut off 3-5 hours before the storm. A storm surge going through a beach house can easily fill the first floor with water. Hurricane Charley in 2004 was a category 4 hurricane, as was Hurricane Hugo in 1989 and Hazel in 1954. Charley caused $15 billion and Hugo $9 billion in damages. Hurricane Maria was a category 4 when it struck Puerto Rico, and it's the third most costly hurricane at $90 billion in damages. It devastated the island so badly that many are still trying to recover.

Hurricane Harvey ranks second costliest at $125 billion. It struck Texas as a category 4 hurricane, and it was slow moving, which allowed it to drop a historic 60 inches of rain in some areas. Many areas that weren't considered flood zones were under water and many people are still displaced a year later.

Related: The 'whys' behind lack of flood insurance coverage

Category 5

Category 5 hurricanes are the strongest with a storm surge of 18 feet or higher, and only one is on record: Hurricane Andrew in 1992, which caused $45 billion in damage. A category 5 hurricane causes extensive damage to homes and industrial buildings, blows away small buildings, and significantly damages the lower floors of structures within 1,640 feet of shore and less than 15 feet above sea level. Major erosion occurs, and people are required to evacuate within 5-10 miles of the shore. Roads again will be cut off 3-5 hours before the storm, and major damage to lower floors of all structures less than 15 feet above sea level and within 500 yards of shore is likely to occur.

Hurricane Irma in 2017 was a category 5 when it destroyed St. Thomas and St. John, and it was a category 4 when it made landfall in Florida. Its damage total was $50 billion.

Small but mighty

A list of the 30 costliest hurricanes can be found at Weather Underground's website.

Hurricanes are very damaging and deadly; unfortunately, there are often many deaths from hurricanes. Many people underestimate the strength and power of hurricanes and don't realize the amount of damage even a small storm can cause.

Another factor is how well the buildings and grounds have been maintained. Trees with dead limbs break apart easily and can damage nearby structures, and older roofs are more susceptible to having shingles blown off and other damage. However even the most well-built, well-maintained building is will be damaged in a category 5 hurricane.

Christine G. Barlow, CPCU, is the managing editor of FC&S Online, the authority on insurance coverage interpretation and analysis for the P&C industry. It's the resource agents, brokers, risk managers, underwriters, and adjusters rely on to research commercial and personal lines coverage issues. She can be reached at [email protected].

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