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The dominant type of weather disturbance of middle and high latitudes is the wave cyclone, a vortex that repeatedly forms, intensifies, and dissolves along the frontal zone between cold and warm air masses. The Norwegian meteorologist, J. Bjerknes, at the time of World War I, recognized the existence of atmospheric fronts and developed his wave theory of cyclones.
The term front, used by Bjerknes, was particularly apt because of the resemblance of this feature to the fighting fronts in Western Europe, then active. The masses of cold polar air meet in conflict with warm, moist air from the subtropical regions. Instead of mixing freely, these unlike air masses remain clearly defined, but interact along the polar front in great whorls whose structure is not unlike the form of an ocean wave seen in cross section.
2. Main article
A meteorological situation favorable to the formation of a wave cyclone is shown by a surface weather map. A trough of low pressure lies between two large cyclones; one is made up of a cold, dry, polar air mass, the other of a warm, moist, maritime air mass. Airflow is in opposite directions on the two sides of the front, setting up a shear zone, and producing an unstable situation.
3. References
At the start of the cycle the polar front is a smooth boundary along which air is moving in opposite directions. The polar front shows a bulge, or wave, beginning to form. Cold air is turned in southerly direction, warm air in a northerly direction, as if each would invade the domain of the other.
The wavelike disturbance along the polar front has deepened and intensified. Cold air is now actively pushing southward along a cold front; warm air is actively moving northeastward along a warm front. Each front is convex in the direction of motion. The zone of precipitation is now considerable, but wider along the warm front than along the cold front. In a still later stage the more rapidly moving cold front has reduced the zone of warm air to a narrow sector. The cold front has overtaken the warm front, producing an occluded front and forcing the warm air mass off the ground, isolating it from the parent region of warm air to the south. The source of moisture and energy thus cut off, the cyclonic storm gradually dies out and the polar front is reestablished as originally.
Wave cyclones commonly form in succession to travel in a chain across the north Atlantic and north Pacific oceans.
As each cyclone moves northeastward it deepens and occludes, becoming an intense upper-air vortex. For this reason, cyclones arriving at the western coasts of North America and Europe are typically occluded.
In the southern hemisphere, storm tracks are more nearly along a single lane, following the parallels of latitude. This appears to be the result of uniform ocean surface throughout the middle latitudes, only the southern tip of South America breaking the monotonous oceanic expanse. Furthermore, the polar-centered ice cap of Antarctica provides a centralized source of polar air.
