As Hurricane Florence barrels toward the United States, news coverage is full of terms that often create confusion, like “landfall.” Here is a glossary.

Landfall

The term does not refer to when the storm meets land; rather, it refers to when the calm center of the storm (or its “eye,” see below) crosses the shoreline. It is an important marker of the storm’s progression.

The high winds and rain of Florence are expected to do heavy damage before that point, or even if there is no landfall. The storm’s diameter was 310 miles Wednesday, making it wider than either North or South Carolina.

As of early Thursday, the eye of the storm was expected to arrive at the Carolina coast by Friday. But tropical-storm-force winds, extending 175 miles from the center, were expected to arrive on land by Thursday morning.

The storm can make a “direct hit” on a particular location, an “indirect hit” or a “strike,” depending on its spatial relationship to the storm’s track and the strength of winds and tides. An expanded list of definitions from the National Hurricane Center can be found at www.nhc.noaa.gov.

Eye and Eyewall

The eye is the roughly circular area of relatively light winds in the middle of a storm. It can range from a 20- to 40-mile diameter of clear skies.

“Inside a well-defined eye, the winds drop down sharply, sometimes almost to nothing,” said Dennis Feltgen, a meteorologist for the National Hurricane Center.

“Skies clear, the sun comes out,” he added. “And it feels like, ‘Wow, the whole thing’s over.’ But it’s not. It’s a very eerie silence.”

Something fierce lurks nearby. That’s the “eyewall,” a ring of cumulonimbus clouds also known as a “wall cloud.” It contains the strongest winds of a hurricane.

Storm surge

The National Hurricane Center has predicted a life-threatening storm surge on the low-lying coasts of North and South Carolina.

When heavy winds push water toward the shore, the water level rises well above the regular tide. To measure the storm surge, meteorologists compare the height of the sea level during the storm with the predicted astronomical tide.

The storm surge can be incredibly dangerous. As winds whip around, the mass of seawater rushes onto land, resulting in devastating flooding, especially when it coincides with high tide.

Feltgen said about half of the deaths in a hurricane can be attributed to storm surge, while another quarter generally come from inland flooding, from heavy rainfall and rising rivers.

Flood plain

The low-lying land areas that are likely to be inundated when a river or lake overflows, or as the storm surge washes ashore.

Maps of flood plains are used to plan evacuations and assess flooding risk. A 50-year flood plain is an area that experts think has a 1-in-50 chance of flooding in any given year; a 100-year flood plain has a 1-in-100 chance. The maps can often be contentious because they affect how much people pay for insurance and where they are allowed to build.

Hurricane, typhoon, cyclone

All three terms refer to tropical cyclones — low-pressure circular storm systems with winds greater than 74 mph that form over warm waters. But they are used in different parts of the world, and they also have different seasons.

Categories

As of Thursday morning, Florence had gone from a Category 4 to a Category 2 storm, although it could strengthen again.

The system, known as the Saffir-Simpson scale, is based on wind speed and largely meant to help predict structural damage. The most dangerous is a Category 5 storm, which means winds fast enough to cause extreme, catastrophic damage.

The Waffle House Index

Look for this term once the storm passes. It is a simple test that some — including disaster management officials at FEMA — use to gauge damage, particularly in the South, where the Waffle House diner chain is ubiquitous.

As FEMA explained in a blog post last year, looking at data on which diners are closed, or open with full or limited menus, gives a good sense of how the surrounding neighborhood has fared.