Winter Storm Pax: Anatomy of an Extra-Tropical Cyclone


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The National Weather Service forecast for Valentine's Day, 2014. Pax has reached the Gulf of Maine and the cold front (blue) has overtaken the warm front (red) to form an occluded front (purple).

The National Weather Service forecast for Valentine’s Day, 2014. Pax has reached the Gulf of Maine and the cold front (blue) has overtaken the warm front (red) to form an occluded front (purple). Image courtesy of the National Weather Service

You’d think a storm by any other name would work the same. But though the end results are wind and precipitation, tropical and extra-tropical cyclones are totally different animals.

Winter storm Pax is now transitioning from a ripple in the jet stream to a full-fledged nor’easter.

The Energy Budget of the Atmosphere

Ultimately all the energy in the atmosphere is derived from the sun, but entirely different mechanisms fuel tropical and extra-tropical systems.

Tropical systems depend on latent heat bound up in water molecules that have been torn from the ocean surface by solar radiation. Extra-tropical systems sustain themselves on the potential energy of abutting cold and warm air masses.

The Source of Atmospheric Energy: The Angle of the Sun

The sun strikes the ground at an oblique angle at the poles and more directly at the equator. The resulting energy absorbed per unit of area is greater at the equator. If Mother Nature didn’t do something to adjust her energy balance sheet, the equator would get unbearably hot and the poles frighteningly cold. The habitable zone would be very narrow. Fortunately for us humans, She has figured out a way to moderate the imbalance.

Mechanics of an Extra-Tropical Cyclone

Where cold air and warm air lie side-by-side, the potential energy is high. Warm air on top of cold would be a lower energy state. When a ripple occurs along the polar front, a spinning motion can begin: the warm air rides over the cold and the cold air wedges under the warm. In a vigorous storm, the cold front catches up to the warm front. The resulting boundary is called an occluded front; the warm air has been entirely squeezed upward, lowering the potential energy.

The Development of Pax

Winter Storm Pax began as a number of ripples on the polar front, each bringing its round of precipitation due to the rising air. The warm air rose when it pushed up over the cold air or the cold air physically forced the warm air upward by wedging underneath. Since the warm air was moisture-laden due to its origin in the Gulf of Mexico, it didn’t take much lifting to produce rain or snow. Where the surface air was below freezing, an ice storm was the result. Eventually one of the ripples was strong enough to become a closed low-pressure circulation. As the storm reaches the Atlantic coast, the warm water will add further fuel to the storm.

Pax’s Effect on the Energy Balance

Many storms this winter have simply rippled along the polar front, producing significant precipitation but not lowering the potential energy much. Pax appears to be a game-changer, mixing the warm and cold air masses to reduce potential energy. Hopefully this will allow Mother Nature to take a break from her production of endless storms in order to re-charge the potential energy — or maybe even shift her attention to another part of the northern hemisphere. No one will mind waiting a while for the next winter storm, which the Weather Channel will call Quintus.

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