Answers: Storm Winds

By Jack Williams ©2015

Q: Why doesn’t an extratropical cyclone continue to rotate around a 360-degree axis, like a hurricane?  I’ve always wondered why a cold front begins to the northwest of the low center and then dies out on the northeast side. Why doesn’t it continue to just rotate?  How does the Coriolis effect play into all of this?

Jeff, Pensacola, Florida

A:  The answer begins with the fact that a hurricane–like any tropical cyclone–is embedded in a mass of warm, humid air while at least two air masses are parts of an extratropical cyclone.

A hurricane draws almost all of its energy from the latent heat released as water vapor rises, cools, and condenses into cloud and rain drops and then turns into ice crystals high in the storm. A deep layer of warm humid air over a warm ocean supplies the water vapor a hurricane needs.

The NOAA satellite image of Katrina below gives you a pretty good view of what goes on. Winds carrying warm, humid air spirals in toward the nearly clear eye, as shown by the bands of thunderstorms spiraling into the eye. Some of the air rises to form the bands of thunderstorms with the most air rising in the ring of thunderstorms right around the eye–the eye wall.

A satellite image of Hurricane Katrina on August 29, 2005, the day it hit Louisiana and Mississippi, clearly shows the storm's structure. NOAA image.

A satellite image of Hurricane Katrina on August 29, 2005, the day it hit Louisiana and Mississippi, clearly shows the storm's structure. NOAA image.

Since a tropical cyclone needs a steady supply of warm, humid air, it weakens and dies when it moves over water that’s cooler than approximately 80 degrees F, or over land. Cold, dry air aloft, which  can be pulled into the storm, can also weaken the storm.

As the bands of thunderstorms in the image show, a water molecule could make a complete circle around the eye before rising in the eyewall.

Chapter 10 of The AMS Weather Book is all about tropical cyclones with the focus on hurricanes, which are tropical cyclones over the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or the Pacific Ocean east of the International Date Line. The Sources for Chapter 10 page of the book’s supplemental Web site has links to more information on hurricanes.

An extratropical cyclone, on the other hand, is a much more complex phenomena involving masses of cold and warm air. In the Northern Hemisphere, the cold air is moving generally toward the south or the southeast. The warm air is generally moving toward the north. Most of the energy for extratropical storms comes from the differing densities of the contrasting air masses.

NWS-ET-matureThe image here from the National Weather Service’s Jetstream–Online School for Weather, is from a page showing the birth, growth, and death of an idealized extratropical cyclone.

The air masses involved in a tropical cyclone form when air stays over a part of the earth’s surface for several days. For example, air that stays over the snow or ice of the Arctic becomes very cold and dry.  Air that stays over a tropical ocean for several days becomes very warm and humid while an air mass that forms over a desert is very warm and dry,

Since density differences, with the heavier cold air, sliding under and pushing up lighter warm air, supplies most of the storm’s energy, the storm grows weaker when the cold air warms up as it moves over warmer land or water. In a storm that begins with the cold front–the boundary where cold air is replacing warm air–going west from the storm center, the cold air will normally have lost its punch by the time air rotates 90 degrees around the low so the cold front is running roughly north-south.

In a Northern Hemisphere extratropical cyclone the winds are generally counterclockwise around the low center, but as the image above shows, they are not swirling around and into the center. Instead, are pushing the cold air to the southeast and the warm air north.

The entire sequence of a storm’s life that has the image above will give you a better idea of what happens. Chapter 5 of the AMS Weather Book has a great deal more on extratropical cyclones, including a three-dimensional view of an East Coast storm on page 109 showing upper-level winds as well as the surface winds like those in the illustration above.

As for the Coriolis force, which is caused by earth’s rotation, it combines with other forces to cause winds to flow generally counterclockwise around low pressure and clockwise around high pressure in the Northern Hemisphere, and in the opposite directions in the Southern Hemisphere.  Chapter 3 of the AMS Weather Book describes these forces in some detail.

The Explorations: The Coriolis force page on the AMS Weather Book’s supplemental Web site has more information, including why the Coriolis force does not cause water to swirl down drains in different directions in the two hemispheres.

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