Thayer Watkins
Silicon Valley
& Tornado Alley

The Dynamics of Northeasters
and Other Extratropical Cyclones

Northeasters are cyclones which form in the Atlantic off the coast of North America in the vicinity of 30°N. They are low pressure systems and the winds blow in a counterclockwise direction. These systems move generally to the northeast and this will be explained below. When the system moves just off the coast the winds in the coastal area, which is to the left of the center, is coming from the northeast. The name northeaster comes from the direction of the wind rather than the direction of movement of the system. The spelling nor'easter is an affectation adopted by the media rather than a dialect pronunciation by northeasterners.

The Reason for the Polar-Easterly
Movement of Cyclonic Systems

A cyclone has angular momenta of two types. First there is the angular momentum that comes from the circulation about its center, call it the spin angular momentum. Second it has as merely a mass rotating with the Earth an angular momentum with respect to the Earth's axis of rotation, call this the terrestrian angular momentum.

The turning of the spin angular momentum with the rotation of the Earth is a forced precession. Almost everyone has seen a gyroscope precess. When one end of a gyroscope is placed upon a pivot point the weight of the gyroscope exerts a torque upon it and the gyroscope then precesses. If a gyroscope is forced to precess there is created a torque which would tend to line up the gyroscope's spin axis with the axis of the precession. The spin angular momentum of the cyclone, as result of its being on the Earth's surface and its verticality being maintained, is forced to precess and as a result it experiences a force that tends to move it towards a pole. As the cyclone moves toward a pole it gets closer to the axis of rotation of the Earth and in order for its terrestrian angular momentum to be preserved it travels faster in an easterly direction. The combination of the forces toward the pole and to the east gives cyclones in the Northern Hemisphere a northeasterly direction and in the Southern Hemisphere a southeasterly direction.

For the mathematical physics of the effect of forced precession see Hurricane Paths

Hurricane, typhoons and other tropical cyclones have high wind speeds and consequently relatively high levels of spin angular momentum. They therefore move more quickly. Tornadoes likewise have high levels of angular momentum compared to their mass and also move generally northeast in the Northern Hemisphere. Extratropical cyclones like northeasters have lower wind speeds, say 60 mph, than the 150 mph wind speeds of the tropical cyclones or the 200 mph wind speeds of the tornadoes and therefore their forward motion is much slower. The prolonged exposure to the weaker winds of the northeaster may create as much damage as the stronger winds of a hurricane.

The general path of a cyclone, either tropical or extratropical, is westerly then with a recurvature toward a pole and movement to the east. While this is the general path the short term behavior can be very erratic. It can be influence by local topography and regional pressure patterns. But ultimately the polar-easterly path prevails. The situation is like the trajectory of a dry leaf falling from a tree. The leaf may be buffeted to the right and to left; it may even rise a bit; but ultimately the inexorable force of gravity will prevail. For cyclones the inexorable force due to their forced precession with the rotation of the Earth will ultimately prevail if the they last long enough.

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