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Those Hurricane Maps Don’t Mean What You Think They Mean

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Opinion

Those Hurricane Maps Don’t Mean What You Think They Mean

We use hurricane forecasts to warn people. Why do we misinterpret them so often?

The National Hurricane Center publishes graphics like this one so the public can plan ahead for tropical storms and hurricanes. However, many people who see these maps read them wrong.

Studies show that some people misinterpret the map as indicating the hurricane getting bigger over time. Others think it shows areas under threat. Research by Hurakan, a University of Miami team I’m a part of, suggests 40 percent of people wouldn’t feel threatened if they lived just outside of the cone.

People who are inside the cone, but far from the center, tend to prepare less than those closer to the central line. All of these interpretations of the map are incorrect.

These misunderstandings have real-world consequences. As Hurricane Irma approached Florida’s east coast in 2017, people near the cone’s center began preparing and even evacuating.

When Irma was a few days away, the central path turned gradually to the west coast of Florida, and some people in that region hadn’t done enough preparation for the storm.

How to interpret the National Hurricane Center’s map, then? The cone actually represents a range of possible positions and paths for the storm’s center.

The dots in the middle of the cone correspond to the N.H.C.’s forecast of where the hurricane’s center could be in the following five days. But there’s a good chance that the actual center of the storm won’t end up being at those positions.

To create the cone, the N.H.C. surrounds each estimated position of the storm center with circles of increasing size. These circles represent uncertainty.

This means that the storm center may end up being anywhere inside those circles — and even outside of them. The uncertainty circles grow over time because it’s easier to predict what will happen one day from now than five days from now.

Finally, a curve connects the circles. The result is what is popularly known as the “cone of uncertainty.”

The National Hurricane Center says cones will contain the path of the storm center only 60 to 70 percent of the time. In other words, one out of three times we experience a storm like this, its center will be outside the boundaries of the cone.

Hurricanes are also hundreds of miles wide, and the cone shows only the possible path of the storm’s center. Heavy rain, storm surges, flooding, wind and other hazards may affect areas outside the cone.

The cone, when presented on its own, doesn’t give us much information about a hurricane’s dangers. The N.H.C. designs other graphics, including this one showing areas that may be affected by strong winds. But these don’t receive nearly as much attention as the cone.

The cone graphic is deceptively simple. That becomes a liability if people believe they’re out of harm’s way when they aren’t. As with many charts, it’s risky to assume we can interpret a hurricane map correctly with just a glance. Graphics like these need to be read closely and carefully. Only then can we grasp what they're really saying.

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