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
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
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