4.3- Extratropical Cyclones
Instructor's Video: Atmospheric Disturbances Links to an external site.
Probably the most significant weather disturbances in the midlatitudes are extratropical cyclones. Extratropical means that they form outside of the tropics and cyclone, denotes low pressure. Extratropical cyclones are found between 35° and 70° in both hemispheres. They are found in the band of the westerlies and their general movement is toward the east. They are called depressions or wave cyclones in other parts of the world and meteorologists have started naming them in the U.S. and in Europe, a convention that used to apply only to tropical cyclones. Maybe they thought it would boost TV viewership during a storm!
A mature extratropical cyclone is a vast low pressure cell - 1000 miles in diameter. In the northern hemisphere, the flow is converging and counterclockwise. These cyclones follow a series of stages to becoming fully mature. They originate along the front, where air masses of different temperatures are moving parallel to the front in opposite directions. You can imagine this happening where the cold polar easterlies are blowing north of the front and the warmer westerlies are south of the front. This opposing air flow eventually sets up a counterclockwise rotation. If you have trouble visualizing this, put a pencil between your palms and you move your top hand to the left and the pencil will rotate counterclockwise.
Eventually, the front takes the shape of a wave with a warm front to the east and a cold front to the west. The cold front moves faster than the warm front and as it progresses it becomes what looks like a tall, breaking wave. Ultimately the storm dissipates because the cold front overtakes the warm front, forcing more and more warm air aloft. When the cold front catches up with the warm front, warm air is not longer in contact with the surface and an occluded front forms, killing the storm.
At any given time, there are 6 to 15 extratropical cyclones in the Northern Hemisphere and 6 to 15 in the Southern Hemisphere. They move at about 20 to 30 miles per hour and take three days to cross the U.S. and 3 or 4 days to reach maturity. They are faster moving and better developed in the winter months. Most enter the U.S. in the Pacific Northwest and leave through New England.
Extra Credit Opportunity
Take a picture of the weather page of the Press Democrat and identify whether there is an extratropical cyclone or not. Post your photo and comment in the extra credit discussion.