Armed with howling winds and punishing rain, Hurricane Florence laid siege to Lumberton, North Carolina earlier this month. To a town where more than a third of residents live below the poverty line, the storm brought all manner of mayhem. It made quick work of a temporary dam, sending floodwaters rushing into the poorest neighborhoods, low-lying areas in the south and west of the city.

Eight miles away in Wilmington, North Carolina, the storm also left homes and businesses under water. But Wilmington is not like Lumberton. In Wilmington, the poorest communities sit further inland from the river. This fact is crucial to understanding the impact of Hurricane Florence in the two cities.

Calculating the human toll of a hurricane isn’t just a matter of gauging where it rained the hardest. One must also determine who has the means to recover from a storm and, more importantly, who doesn’t.

Researchers have long understood from case studies that some people are more vulnerable to natural disasters — the poor, the infirm and the elderly, for example — but it was difficult to account for these facts in disaster planning. So, in 2003, University of South Carolina geographer Susan Cutter developed a method for encapsulating all these factors in a single number, the Social Vulnerability Index. The index uses census data on income, education, age and other factors to chart the relative vulnerability of different communities.

Disaster planners can compare maps of social vulnerability with maps of physical vulnerability — flood maps, for examples — to prepare an effective, tailored and just response to natural disasters. In Lumberton, North Carolina, for example, the most socially vulnerable communities also see the greatest risk of flooding. For that reason, they require the most rigorous protections.

Poverty is a central factor in social vulnerability, as people who are struggling financially might not have the means to prepare for a disaster, or to escape when one strikes. “You have got to have transportation. You have got to have money to fill up your gas tank. You have got to know where you’re going and, when you get there, how you’re going to eat and sleep,” said Nakisa Glover, founder of Sol Nation, a North Carolina-based climate justice group. “If you don’t have the financial means leading up to a natural disaster to be prepared, it exacerbates the issue.”

Poverty isn’t the only factor, however. “Frequently vulnerable groups include people in poverty, but also the very old and the very young, racial and ethnic minorities, renters and the disabled, just to name a few,” said Eric Tate, a professor of geography at the University of Iowa, who studies environmental hazards. “The principle is that vulnerability is multidimensional. It’s not just the poor.”

The elderly are also at greater risk, for example, as they are more likely to need medical care. In the aftermath of a hurricane, a wheelchair-bound retiree may find himself unable to reach a pharmacy at the precise moment that he runs out of life-saving medicine.

People with a poor knowledge of English or a limited education are also vulnerable. As Tate explained, it helps to speak English in the lead-up to a disaster to understand warnings from authorities and directions on how to prepare. After a disaster, when survivors are applying for assistance, a more educated person might find it easier to navigate the government bureaucracy.

One’s housing situation is also factor. Families that live in mobile homes are more vulnerable than those in permanent homes, which are more resilient in the face of a storm. Similarly, renters are more vulnerable than homeowners, as recovery efforts tend to favor the latter.

Notably, the index also weighs race and ethnicity. “We have a long history of racial discrimination in this country and that can be born out in heightened exposure to hazards,” Tate said, explaining that racial discrimination can be a factor in recovery efforts, as seen after Hurricane Katrina.

Tate said that officials tend to focus on flood-prone areas with the most highly valued property, but that’s not where the most vulnerable people tend to live. It’s areas like southwest Lumberton that are most likely to suffer in a disaster. “Maybe that’s where we should be spending most of our time and effort as opposed to just the places that are physically vulnerable,” Tate said.

“You can use it before disasters, when you’re planning, to try to understand where resources are going to be needed the most,” he said, “You can use it in the aftermath of disasters to help guide decisions in recovery — where resources should be focused, how much should be devoted to one place versus another — with the idea that your dollar may go farther if you’re helping more vulnerable populations.”

Analysis from the Brookings Institute finds that hurricanes hit poor communities the hardest, as these communities tend to lie in more flood-prone areas. The same goes for communities of color, which are often situated in vulnerable areas as a result of discriminatory housing practices. “It goes back to the history of redlining across America,” Glover said. “If you’ve been impacted by unfair housing practices, you really don’t have a choice.”

Climate change will ramp up threats to these communities in the years ahead. Florence, a storm made measurably more severe by climate change, offers a preview of things to come. The flooding of southwest Lumberton, for example, highlights the need to guard vulnerable communities against future natural disasters.

“Folks are fighting for their lives today,” Glover said. “They don’t have the benefit and the luxury and the privilege to say, ‘I’m doing this for future generations.’ They are fighting for their very survival today.”

Jeremy Deaton writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture. You can follow him @deaton_jeremy.