What is the hurricane "cone of uncertainty", and why do so many people misread it?

The cone of uncertainty is a map of where a hurricane's centre might go, not of how big the storm is or where it will do damage. The National Hurricane Center draws it by placing circles along the forecast track at 12, 24, 36, 48, 60, 72, 96 and 120 hours and enclosing them. Each circle is sized so that two-thirds of historical official forecast errors fall inside it. That means the cone is built to catch the centre of the storm about two-thirds of the time, so one time in three the centre can pass outside it.

People misread the cone in two ways. The first is to treat it as the storm's size or the area of danger. It describes only the probable track of the centre. The NHC says storm surge and large waves pose the greatest threat to life and property along the coast, and hurricanes may dump as much as 300 mm of rainfall in 24 hours over large areas, so the dangerous zone is much wider than the line. The second is to read the widening cone as a growing storm: it widens because the future path becomes increasingly uncertain.

The cone is also getting smaller over the years: over the past decade storms travelled within their projected areas two-thirds of the time, and the cones have shrunk thanks to improved methods. Its nicknames, including the Error Cone, the Cone of Probability and even the Cone of Death, show how many ways people have read it.