ARTICLE INDEX:
The forward motion of an extratropical cyclone generally ranges from 9-20m/s, but can reach as high as 40m/s. Both these average and extreme values greatly exceed the forward speeds of low-latitude hurricanes. The storm's wind field thus becomes highly asymmetrical, with damaging winds generally restricted to the south or right-hand side of the track
As an extratropical cyclone travels eastward toward the European coastline, it may interact with a trough of low pressure high in the atmosphere. This interaction can cause the storm to intensify rapidly, as happened during the October 1987 windstorm.
European windstorms may produce wind gusts of more than 67m/s in exposed coastal areas and the mountains. Inland, wind gusts may reach about 45m/s. Sustained wind speeds in the most intense Atlantic hurricanes can exceed 70m/s, with wind gusts exceeding 90m/s.
Structure
Extratropical cyclones differ structurally from tropical cyclones in that they have fronts, discrete boundaries between air masses of different temperature. The highest surface winds and heaviest precipitation often occur along these frontal boundaries. There are generally two fronts: one where warm air overrides cold air to the northeast of the center, another where cold air wedges beneath warm air to the southwest (this is where the highest surface winds generally occur).
Because the lower tropical atmosphere is typically of a uniform temperature, tropical cyclones have no fronts. Instead, their winds and precipitation are concentrated in the ring of intense thunderstorms surrounding the eye, and in the spiral rain bands that feed those thunderstorms. Another structural difference between extratropical and tropical cyclones is that the highest winds in the former occur high in the atmosphere, whereas the highest winds in a tropical cyclone occur near the surface.
Shape and Size
Extratropical cyclones change shape during their development. When mature, their cloud patterns are often "comma" shaped, with a warm front and a cold front radiating from an area of low-pressure at the storm center. These storms grow in size during their life-cycle, with individual storms reaching a range of maximum sizes, 200-2000 km in greatest dimension. The size of an extratropical cyclone can best be defined by the swath of its damaging winds. This is usually found to the right of the storm track and is typically 150-500 km wide.
Tropical cyclones typically exhibit a more symmetric, concentric shape. The size of a tropical cyclone can best be defined as the radius of the area experiencing winds greater than or equal to hurricane force (= 33 m/s). This is typically ~140km, although values range widely, from 1100 km in Typhoon Tip (western Pacific, 1979) to 50 km in Tropical Cyclone Tracy (Darwin, Australia, 1974). (4)
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