Mid-Latitude Cyclones
Mid-Latitude Cyclones:
-The general theory of cyclonic development in the mid-latitudes (South Florida to Alaska) was in place
in the early part of the 20th century. Since then, new discoveries and technologies have added additional
information.
-The basic idea of this theory, called the Polar Front Theory, is that when cold air from the north interacts
with warmer air from the south, lower pressure develops. This occurrence is often along the polar front
(polar jet stream).
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Formation:
-During this process of cyclogenisis, two differing air masses are moving along a boundary parallel to the
front that separates them, but in opposite directions.
-The front will eventually start to take on a wave-like appearance.
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Development of Cyclonic Flow:
-As the wave develops, warm air moves toward the north, where the cold air once was, and cold air
moves towards the south, where the warm air once was. As the wave deepens, the pressure gradient
increases and a low pressure center develops.
-This pressure center inhibits an inward, counterclockwise flow, and convergence.
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Development of Fronts:
-As warm air from the south continues to move north and east, it displaces cooler air that was to the north,
and thus a warm front is born.
-As the cool air from the north continues to move south and east, it overruns the warmer air that is
moving away. This is the cold front.
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Occlusion:
-As the cyclone develops, the cold front moves faster than the warm front and eventually catches up to it.
-During this period, an occluded front occurs, and the storm cyclone intensifies for a time. Once most of
the warm air is forced upward however, the temperature gradient begins to weaken and the air stabilizes.
At this point the cyclone dissipates.
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Movement of Mid-Latitude Cyclones:
-Typically these cyclones take 2-3 days to move through a region.
-The prevailing westerlies bring these storms from the west to the east.
-Depending on the position of the jet stream, a cyclone may turn to the northeast since the jet stream often
takes that course as well.
-Alternately, anticyclones (formed in high pressure areas) move southeast down with the jet stream and
can work to block the movement of cyclones in a given area (mostly the plains and mid-western North
America).
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Role of Airflow Aloft:
-Cyclones are the result of jet stream flow, and divergence aloft.
-Cyclones will initially form as a result of differing temperature/pressure air masses, but will continue to
grow as a result of divergence aloft.
-If speed divergence (intensified upper air flow) occurs, this will increase the pressure difference from the
surface to the upper air levels. This intensifies the low at the surface.
-Cyclones tend to form downstream of troughs.
-Anticyclones tend to form downstream of ridges.
-Cyclones and anticyclones will always appear in pairs, and help to balance the pressure over an extended
area.
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