To measure a hurricane's intensity, meteorologists use what's called the Saffir-Simpson Scale. The scale was devised in the early 1970s by Herbert Saffir, a consulting engineer, and Robert Simpson, then the director of the National Hurricane Center. They combined structural engineering and meteorology to quantify the level of damage to expect from a hurricane.

A tropical storm becomes a hurricane when its winds reach 74 mph, and that's when the Saffir-Simpson scale kicks in. The scale runs from 1 to 5, with 1 being a minimal hurricane and 5 the strongest. Meteorologists call these rankings "categories."

Categories 3, 4, and 5 are called major hurricanes, and they have the potential to inflict extensive damage. Only two hurricanes on record have ever made landfall in the United States with Category 5 intensity: Camille in 1969 and a hurricane that struck the Florida Keys on Labor Day in 1935.