It’s that time of year — when a single, contracted word lurks in the shadows of every weather forecast, like a menace waiting to be unleashed. It's just three simple syllables, but it's enough to conjure images of monumental snowdrifts, ferocious winds, and destructive tides along the the coast — shaking even the heartiest of New Englanders to their cores.
You know this one. Say it with me: "Nor'easter."
But what exactly is a nor’easter? And what — if anything — distinguishes it from other storms?
"It’s a storm that occurs, typically, on the East Coast of North America," said Bill Leatham, a meteorologist with the National Weather Service in Boston. "And the winds over the coastal area ... come out of the Northeast."
Indeed, when it comes to why we call a nor’easter a nor’easter, it really is that simple. The winds come out of the Northeast. But it’s why they do — and what happens when they do — that has earned the nor’easter its legendary status.
Nor’easters are a type of storm known as an extra tropical cyclone. They are distinct from tropical cyclones — think summer thunderstorms and systems that develop into hurricanes — which originate in or around the tropics, and are powered by heat.
Extra tropical cyclones, on the other hand, are powered by "strong differences in temperature," said Judah Cohen, a climatologist with a PhD in atmospheric sciences and a research affiliate with the Massachusetts Institute of Technology.
"The thing that they feed off is temperature gradient," Cohen said.
Here in New England, we’re uniquely positioned to provide just that — so uniquely positioned that we’re one of only two places where these kinds of storms regularly form.
"One is along the East Coast of the United States, and the other is the East Coast of Asia," Cohen said.
Cold air from the Arctic comes down into our region, through Canada, from our West and North. Meanwhile, just off the coast to our East, the gulf stream provides a steady stream of warm waters — and warm air above it — from the Gulf of Mexico.
"So that’s a very large temperature differential over a relatively short distance," Cohen said. "And that temperature differential gives you the energy for these storms."
This also explains why, in general, we don’t experience nor'easters in the warmer summer months, when temperatures inland are more in line with the warm air off the coast.
As a cyclone, a nor’easter spins in a counter-clockwise direction, which means the northeast corner of the storm is — for us — the danger zone.
“It’s very difficult to get snow, especially a disruptive snow, without being in the northeast quadrant of the storm,” Cohen said.
That northeast quadrant, he said, provides all the ingredients for the classic New England winter event.
"We have the easterly component that brings in the moisture off the ocean," Cohen said. "We also have that northern component that feeds in low-level cold air from Canada that supports snowfall versus rainfall."
And yes, some nor’easters do bring rain instead of snow.
"You can get a nor'easter and it's just not sufficiently cold," Cohen said. "If it's in the mid-30s, most people consider that cold, but it's not cold enough to snow."
But we’re headed into the heart of winter, when cold air rules. In fact, Cohen said nearly all of our large winter snow events are in fact due to nor’easters.
"It's very hard in Boston for it to snow on westerly winds," he said. "So if we get a northwesterly wind, a southwesterly wind, it's very hard to snow even if it's cold."
Southern winds tend to be warmer and dryer. Southeastern and eastern winds can bring precipitation and flooding, but they also tend to be warm, which usually means rain.
"If you get late enough in the season you can get, maybe, like six inches of snow on a southeast wind because the ocean temperatures are much colder," said Cohen. "But you really have to wait for March for that to happen."
William Shakespeare once asked, "What’s in a name?" Well, when it comes to nor’easters, it appears the answer is: everything you need to know.