Soon after your local meteorologist utters something about a nor’easter, batteries and other essentials start flying off store shelves. For many, the word basically means huge storm—and rightly so. Nor’easters have earned their reputation for wreaking havoc on land. But what qualifies a storm as a nor’easter?

As the National Weather Service explains, nor’easters occur on the East Coast of the U.S. The name, which people have been using at least since the 19th century, doesn’t just symbolize the region where nor’easters hit; it actually refers to where the wind comes from. When warm air from the Southwest meets cold air heading south from Canada’s polar region, it creates a strong wind—called a polar jet stream—that moves east across North America.

One half of the equation for a nor'easter.National Oceanic and Atmospheric Administration

Meanwhile, the Gulf Stream—a warm, powerful current—flows up the East Coast, heating both the water and air above it along the way. If that warm air (heading northeast) collides with the polar jet stream (also heading east), the temperature difference between them creates a low pressure system; in short, precipitation starts to form. This usually happens somewhere between Georgia and New Jersey, and the wind cyclone gains strength as it drifts farther north. Whether your city is pelted with rain or blanketed in snow simply depends on how cold it is. Bostonians might be shoveling their driveways at the same time Baltimoreans are steering clear of flooded roads.

A cyclone, but not a hurricane.National Oceanic and Atmospheric Adminstration

Though nor’easters might look a lot like hurricanes from above, they’re not the same type of cyclone. The word hurricane refers specifically to a cyclone that forms in the tropical waters of the Atlantic Ocean (or, more rarely, the eastern Pacific Ocean). The warm water heats the air above it, which can form storm clouds and eventually become a hurricane if it collides with warm air moving west from Africa.

And if you’re not sure whether the bad weather headed your way is a hurricane or nor’easter, you can always call it a cyclone (or just a “huge storm”).

[h/t National Weather Service]