When a forecaster calls a storm a Category 4, they are telling you one precise thing: how fast its strongest sustained winds are blowing. The Saffir-Simpson scale runs from Category 1 to Category 5, and every step is a wind speed and only a wind speed. It is a clean, useful number. It is also famous for giving people the wrong idea, because the thing most likely to kill you in a hurricane, the water, is nowhere in the number at all.
Before Category 1
A storm earns a name before it earns a category. When its sustained winds reach 39 miles an hour it becomes a tropical storm. Below that it is a tropical depression. Only when the sustained winds cross 74 miles an hour does it become a hurricane and step onto the Saffir-Simpson scale at Category 1. So the scale begins where the hurricane begins, and everything under it is still a storm worth respecting.
The five categories
Category 1 is sustained wind from 74 to 95 miles an hour. Dangerous winds, damage to roofs, siding and trees, and power outages that can last days.
Category 2 is 96 to 110 miles an hour. Extensive damage, major roof and siding loss, many trees snapped or uprooted, near total power loss for a time.
Category 3 is 111 to 129 miles an hour. This is the first step that forecasters call a major hurricane. Devastating damage, well built homes losing roof decking and gables, water and power out for days or weeks.
Category 4 is 130 to 156 miles an hour. Catastrophic damage, severe loss of exterior walls and roofs, trees and power poles down everywhere, whole areas left unlivable for weeks or months.
Category 5 is 157 miles an hour or higher. Catastrophic. A large share of framed homes destroyed, and the struck area uninhabitable for a long time.
What the number leaves out
The category measures wind. It does not measure storm surge, the wall of sea the storm pushes onto the coast. It does not measure rainfall, which can flood places hundreds of miles from the shore. It does not measure the storm's size or the tornadoes it spins up on landfall. Many of the deadliest hurricanes in American history came ashore at a middling category and killed through water, not wind. A slow, soaking Category 1 sitting over a river basin can take more lives than a fast Category 4 that blows through dry.
Why it matters
The scale is a good tool used badly when the category becomes the whole story. Read the number for what it is, a wind rating, and then go find the hazards it ignores. A low category is not permission to relax, and a high category is not the only thing that should move you. The storm is always more than its number.
The category is the headline. You need the whole page.
A single number on the news cannot tell you what a storm will do to your stretch of coast. Magna Earth tracks intensity, wind arrival and surge against your exact location and tells you when each hazard reaches you, so you are reading your own risk instead of a label.
Read the storm over your coast — from $14/mo