How to Make a Nor’easter

As 2010’s first big snowfall swirled up the coast, BU Today staff mused about a few basic questions — for example, “What exactly is snow?” To find out, we talked with Bruce Anderson, a College of Arts & Sciences associate professor of geography and environment and coauthor with Alan Strahler, a CAS professor of geography and environment, of Visualizing Weather and Climate (Wiley, 2008).

BU Today: What is a nor’easter?
Anderson: A nor’easter is a storm that comes up the coast. Generally our biggest storms are associated with low pressure, and because the circulation around low pressure is counterclockwise, the resulting winds bring cold moist air off the northern Atlantic. Because the air is cold, we end up getting a heavy snowfall. If that cold air were coming off of Canada, it would be relatively dry, but because it comes off the water, it is laden with moisture.

Meteorologists always refer to wind direction by the direction the wind is coming from, so a nor’easter comes from the northeast.

Can you recall some famous nor’easters?
The Blizzard of ’78 was a nor’easter. The “Perfect Storm” in 1991 was a nor’easter. The storm that is called the Storm of the Century, which happened in 1993, was a nor’easter.

Why are nor’easters so serious?
Because the other thing that happens is they have strong winds. The strength of the winds depends on the difference between the low pressure in the center of the storm and the surrounding air masses over the ocean and continent. If the difference is great enough, it can sustain hurricane-force winds.

What exactly is snow?
Snow is ice crystals on a crystalline growth. What happens is individual water molecules attach to each other one by one. If the individual molecules coalesce, they form a nice snowflake, but that generally doesn’t happen. You also get accumulation by freezing water that forms things like sleet, and if you combine the two you get misshapen crystals. So there are different kinds of snow, as well as hail, sleet, and freezing rain.

Why do we sometimes have snow when it’s above freezing?
Because as you go up in the atmosphere it gets colder. So while the surface temperature may be above freezing, it is below freezing up there. The snow doesn’t always melt by the time it lands.

Is snow good for anything?
Sure. It’s one of the great natural reservoirs of water. We don’t use it so much here in the East, but out West most of the water resources accumulate in the form of snow. When it melts in the summer, it is a major source of water. Also, it can protect plants and animals from cold. It can provide animals with a natural insulation.

Why have the Olympic sites in British Columbia had so little snow this year, while places like Washington, D.C., are getting clobbered?
It’s all related. Storm systems generally travel along the jet stream, which is a fast-moving river of air about 10 to 15 kilometers above the surface. That’s where we put airplanes when we want them to go faster, and the jet stream can generate an additional 200 miles per hour. The jet stream is also called the storm track. In some years, particularly when there is excess energy in the Pacific, when there is El Niño, there is a global shift in the jet stream. It tends to shift south, toward the equator, and it tends to intensify. So storms that normally would track to Washington State or British Columbia are hitting California.

This year the jet stream is taking storms across the Southwest into the southeastern United States and sometimes into the Gulf of Mexico, where they reintensify. They move up the Eastern seaboard, and they hit places like Virginia and Washington, D.C. In a normal year they would arc up over Canada and come back down on us.

Art Jahnke can be reached at jahnke@bu.edu.