How are hurricanes measured?
Hurricanes are measured based on windspeed and barometric pressure. Because they are storms of long duration, they can be assigned a number on the Saffir-Simpson Scale that changes as the storm intensifies and diminishes. Hurricanes begin as a tropical depression that intensifies into a tropical storm. Sometimes tropical depressions and tropical storms never develop into hurricanes. This is especially true at the beginning and end of the season.
Tables courtesy of NASA