Extratropical Cyclone
Extratropical Cyclone

Extratropical cyclones arise through a process called cyclogenesis, in which cold and warm air masses interact in an unstable environment. Colder air to the north and warmer air to the south flow toward each other, in opposite directions, creating an area of low pressure between them. This sets up a cyclonic wave, lowering central pressure further and increasing wind speeds. Warm air rises and then condenses, producing precipitation and releasing energy in the form of latent heat. Kinetic energy is also produced from the motion of the rising air, as well as from the convergence of surface air toward the central area of low pressure.

Cyclogenesis occurs throughout the extratropical regions. Because extratropical cyclones form where cold and warm air masses come into contact with each other, however, storm formation is most favorable in the mid latitudes (between 35 and 60 degrees latitude) of both the Pacific, near the Asian coast, and the Atlantic, near Greenland and the North American coasts. Storms affecting Europe typically originate to the east of North America or Greenland and subsequently move eastward across Europe.

Extratropical storm systems are typically comprised of multiple areas of relatively low and high pressure, the locations of which can change quickly and frequently. This is in sharp contrast to the relatively simple, symmetric structure that is observed for tropical storm systems in low latitudes. As an extratropical system evolves, the interaction of these low and high areas creates changes in the horizontal pressure gradient field. The windfield associated with such a system is driven as a response to the evolving pressure gradient field.

Extratropical cyclones typically do not achieve the intensity, in terms of wind speeds, that tropical cyclones achieve. However, extratropical cyclones can affect, at a single time, an area of tens of thousands of square miles and can subject individual locations to high winds for up to several days.

While cyclogenesis occurs during every month of the year, formation into significant extratropical storms almost always occurs during the late fall, winter, or early spring. These systems are, therefore, often referred to simply as "winter storms".


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