The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 rating based only on a hurricane’s maximum sustained wind speed. It estimates potential property damage from wind and does not account for other deadly hazards such as storm surge, rainfall flooding, or tornadoes. Hurricanes rated Category 3 and higher are known as major hurricanes. On the scale, Category 1 covers sustained winds of 74–95 mph, and Category 5 begins at 157 mph or higher.
Source: NOAA National Hurricane Center, Saffir-Simpson Hurricane Wind Scale
Image: Eye of Hurricane Isabel from the ISS, 13 September 2003 (NASA astronaut Ed Lu, public domain), via Wikimedia Commons.