Roadside restaurant chain measures fury of Hurricane Ian

As Hurricane Ian's powerful combination of winds, rain and storm surges battered Florida, the US breakfast chain Waffle House is joining the list of agencies and businesses tracking the super storm.

The roadside restaurants are known for staying open during natural disasters, and US government officials use them to measure the severity of a hurricane, tornado or other hazards on a local area.

Waffle House closed 21 of its locations in mandatory evacuation zones in Florida on Wednesday morning (Thursday AEST), ahead of Hurricane Ian making landfall, the company said.

READ MORE: 'Substantial loss of life' possible in Florida

The US Federal Emergency Management Agency (FEMA) coined the term "Waffle House Index" to gauge the effect of a natural disaster on an area.

The FEMA website says it keeps track of Waffle House menus after natural disasters.

"A limited menu means an area may not have running water or electricity, but there's gas for the stove to make bacon, eggs, and coffee: exactly what hungry, weary people need."

If a Waffle House shuts down or limits its menu after such hazards, federal officials conclude the community took a major hit.

The origins of the index can be found in comments by former FEMA director Craig Fugate in 2004.

READ MORE: Queen Elizabeth's cause of death revealed

"If you get there and the Waffle House is closed? That's really bad," he told the Wall Street Journal.

"That's where you go to work."

Waffle House is headquartered in Georgia, and has more than 2000 restaurants, most of them in southern states.

FEMA told CNN in 2019 it doesn't base any recommendations for post-storm recovery on the Waffle House Index, but the agency said consulting with businesses is an important component of extreme weather event clean-up.