RELATION OF FRONTS TO CYCLONES
There is a systemic relationship between cyclones
and fronts, in that the cyclones are usually associated
with waves along frontsprimarily cold fronts.
Cyclones come into being or intensify because pressure
falls more rapidly at one point than it does in the
surrounding area. Cyclogenesis can occur anywhere,
but in middle and high latitudes, it is most likely to
occur on a frontal trough. When a cyclone (or simply
low) develops on a front, the cyclogenesis begins at the
surface and develops gradually upward as the cyclone
deepens. The reverse also occurs; closed circulations
aloft sometime work their way downward until they
appear on the surface chart. These cyclones rarely
contain fronts and are quasi-stationary or drift slowly
westward and/or equatorward.
Every front, however, is associated with a cyclone.
Fronts move with the counterclockwise flow associated
with Northern Hemisphere cyclones and clockwise
with the flow of Southern Hemisphere cyclones. The
middle latitudes are regions where cold and warm air
masses continually interact with each other. This
interaction coincides with the location of the polar
When the polar front moves southward, it is usually
associated with the development and movement of
cyclones and with outbreaks of cold polar air. The
cyclonic circulation associated with the polar front
tends to bring polar air southward and warm moist
tropical air northward.
During the winter months, the warm airflow
usually occurs over water and the cold air moves
southward over continental areas. In summer the
situation is reversed. Large cyclones that form on the
polar front are usually followed by smaller cyclones
and are referred to as families. These smaller cyclones
tend to carry the front farther southward. In an ideal
situation these cyclones come in succession, causing
the front (in the Northern Hemisphere) to lie in a
southwest to northeast direction.
Every moving cyclone usually has two significant
properties. The discontinuity line on the forward side of
the cyclone where warm air replaces cold air is the
warm front; the discontinuity line in the rear portion of
the cyclone where cold air displaces warm air is the
The polar front is subject to cyclonic development
along it. When wind, temperature, pressure, and upper
level influences are right, waves form along the polar
front. Wave cyclones normally progress along the polar
front with an eastward component at an average rate of
25 to 30 knots, although 50 knots is not impossible,
especially in the case of stable waves. These waves may
systems with gale force winds. The development of a
significant cyclone along the polar front depends on
whether the initial wave is stable or unstable. Wave
formation is more likely to occur on slowly moving or
stationary fronts like the polar front than on rapidly
moving fronts. Certain areas are preferred localities for
wave cyclogenesis. The Rockies, the Ozarks, and the
Appalachians are examples in North America.
A stable wave is one that neither develops nor
occludes, but appears to remain in about the same state.
Stable waves usually have small amplitude, weak low
centers, and a fairly regular rate and direction of
movement. The development of a stable wave is shown
in views A, B, and C of figure 4-21. Stable waves do not
go into a growth and occlusion stage.
A. COLD AND WARM AIR FLOW
C. TYPICAL WAVE
Figure 4-21.Life cycle of a stable wave cyclone.