Water temperatures have to at least be 80F or higher and the air must be cool the higher you go. Also winds from each side must be coming toward each other at the same speed so it will force the air upward. The coriolis force needed to create the spin.
1969 hurricane Camille. It was estimated that it was 1.421 billion in damage and it was recorded as the largest single act of destruction in US history. Hurricane Camille was categorized as level 5 which is catastrophic. Hurricane Galveston, Hugo, and Anderson are others that were devastating.
How is the event measured? The hurricane hunters which are apart of the national oceanic and atmosphere administration fly over a hurricane a few times to get calculations on the storm. Also they drop instruments into the storm to get better readings and to determine where the storm is heading