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Extratropical cyclones lie somewhere in between tropical cyclones and mid-latitude cyclones, drawing a portion of their energy through the evaporation and condensation of ocean water, and some through horizontal temperature gradients in the atmosphere. They move out of the extratropical regions towards the polar regions, bringing precipitation in the form of rain or drizzle. Nearly always called by the term low-pressure cells in North America, they often form quickly along cold fronts that have stagnated after moving into an area where warm, moist air exists. The warm, moist air is less dense, therefore it overruns the more dense cold air at and behind the cold front. A cyclonic motion is imparted to the ascending air, naturally, forming a shallow cyclone. Usually, a part of the cold front will develop into a warm front, giving the frontal zone a shape (when drawn on a weather chart) that is called a "wave". An old name for such a system is "warm wave" (in the United States).
Formation
In the beginning, the sea-level pressure is not very low; typically, it is about 1009 mbar (hPa). Intensification or "deepening" of the cyclone progresses slowly. Often, the sea-level pressure in the low-pressure cell fails to drop below 1000 mbar (hPa). Rapidly falling atmospheric pressure is very rare or non-existent, even after several days have passed and the storm has moved to higher latitudes.
Relation to tropical cyclones
Tropical cyclones often transit into extratropical cyclones at the end of their tropical existence, usually between 40° and 50° North and South latitude. In an extratropical transition, poleward displacement of the cyclone occurs and the cyclone's primary energy source converts from the release of latent heat of condensation to a baroclinic process. On rare occasions, an extratropical cyclone can transition into a tropical cyclone if it enters warmer waters and a more favorable environment.Effects
Extratropical cyclones are generally mild storms with surface winds of 7-15 knots. The band of precipitation that is associated with the warm front is often extensive. They tend to move along a predictable path at a moderate rate of progress. They are much more common in the United States than in Europe which is located above the arid Sahara desert. The dry, arid air of the Sahara desert is not conducive to the formation of extratropical cyclones.
While most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or tropical storm force. For instance, the 1959 Mexico Hurricane had sustained winds of 140 knots (160 mph, 260 km/h) as it became extratropical shortly before landfall. In 1954, Hurricane Hazel became extratropical over North Carolina while a strong Category 3 storm. The Columbus Day Storm of 1962 caused heavy damage well north in Oregon and Washington states with widespread damage equivalent to a Category 3 or higher hurricane. More recently, Hurricane Wilma in 2005 began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical while still a Category 1 storm).
See also
"Extratropical cyclone"