Categories and storm surge: what the scale measures

The Saffir-Simpson Hurricane Wind Scale rates a hurricane by its maximum one-minute sustained wind:

Categories 3, 4 and 5 are called major hurricanes. The NHC describes the damage in words: Category 1 brings "very dangerous winds [that] will produce some damage", Category 2 "extremely dangerous winds [that] will cause extensive damage", Category 3 "devastating damage", and Categories 4 and 5 "catastrophic damage".

What the scale leaves out. The scale is based on wind only. It says nothing about:

• Storm surge: an abnormal rise of sea water pushed onto the coast by the storm, over and above the normal tide. It depends on the storm's size, speed, angle of approach and on the shape of the coast and sea floor, not only on its category.
• Rainfall flooding: a slow-moving storm can drop huge amounts of rain. Hurricane Harvey in 2017 was a major hurricane at landfall but became dangerous mainly because it stalled over Texas, with over 60 inches (1.5 m) of rain in places.
• Size: a wide storm can bring damaging winds and surge over a long stretch of coast.
• Tornadoes spawned in the outer bands.

A storm's danger can be far higher or lower than its category suggests.

Water is the main killer. A study by NHC hurricane specialist Edward Rappaport (2014) looked at deaths directly caused by United States tropical cyclones from 1963 to 2012. About half were caused by storm surge and more than a quarter by rainfall flooding. Wind caused less than a tenth. The numbers come from one study and one country, but the message agrees with the NHC's advice: the biggest threat to life from a hurricane is usually water, not the wind.

Wind damage also grows quickly with speed. The force of the wind on a wall goes up with the square of the wind speed, so doubling the wind makes the force four times larger.