Life Cycle of a Wave Cyclone    

 (a)  Initial (polar front) state
    -- Low pressure trough along stationary front
       with higher pressure to either side.
    -- Cold air to the north, warm air to the south.
    -- Winds parallel to front but in opposite
       directions on opposite sides, leading to wind
       shear.
    -- Somewhere along such a stretch of stationary
       front a kink-like instability forms.  Such an
       instability tends to strengthen if it forms
       beneath a region of divergence aloft.  In such
       circumstances the growing instability
       develops the frontal wave structure seen in
       frame (b).		  (b) Frontal wave or incipient cyclone
    -- Low pressure develops at the kink assisted by
       divergence aloft.
    -- The stationary front begins to rotate around
       the low pressure, becoming a cold front to the
       west of the low and a warm front to the east.  
    -- Frontal uplift behind the cold front and
       overrunning ahead of the warm front
       generates a narrow precipitation band
       (green).
    -- The system usually moves to the east or
       northeast, pushed by winds aloft.
    -- The system grows in size and strength.  The
       low pressure deepens and becomes a fully
       developed open wave as in frame (c).		  (c) Open wave
     --Deep central low pressure and associated cold
       and warm fronts may be associated with the
       now fully developed wave cyclone.
     --Precipitation may occur in a broad band ahead
       of the warm front and in a narrow, but more
       intense band trailing the cold front as well as
       in a broad region north of the pressure center.
     --A warm sector, usually dry with partly cloudy
       skies, occurs between the two fronts.
     --Energy sources are potential energy converted
       to kinetic energy in ascending warm air and
       descending cold air.  Latent heat is released in
       condensation. The system continues to
       strengthen.
 (d) Mature wave cyclone, maximum intensity
    --The cold front begins to overtake the warm
       front near the system's center forming a point
       of occlusion.  The formation of this point
       usually coincides with the system's maximum
       strength.
     --Precipitation is widespread and probably
       most intense near the triple point.
     --The most common direction of motion for the
       wave cyclone is toward the northeast.		  (e) Weakening wave cyclone
     --The triple point migrates outward and as it
       does so the warm sector shrinks and a
       growing occluded front replaces the merging
       warm and cold fronts.
     --Precipitation remains widespread, with the
       most intense precipitation occurring near the
       triple point.
     --Deprived of its supply of warm air, the low
       pressure tends to weaken and fill in.
     --When upper-level conditions are correct, a
       new low pressure center ("secondary low")
       may form at the triple point.		  (f) Dying wave cyclone
     --Deprived of its sources of energy, the wave
       cyclone dissipates and loses its identity.  The
       typical lifetime for the entire cycle described
       here is roughly a week with considerable
       variations from storm to storm.  A typical
       migration speed for a wave cyclone is about 25
       knots.  Such a system typically traverses a few
       to several thousand miles during its lifetime.
     --The polar front reforms, detached from the
       dying system.