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Mid-latitude cyclones form along a boundary separating polar
air from warmer air to the south. |
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These
cyclones are large-scale systems that typically travels eastward over
greart distance and bring precipitations over wide areas. |
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Lasting
a week or more. |
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Bjerknes,
the founder of the Bergen school of meteorology, developed polar front
theory during WWI to describe the formation, growth, and dissipation of
mid-latitude cyclones. |
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Cyclogenesis |
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Mature
Cyclone |
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Occlusion |
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Cyclogenesis typically begins along the polar
front but may initiate elsewhere, such as in the lee of mountains. |
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Minor perturbations occur along the boundary
separating colder polar easterlies from warmer westerlies. |
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A low pressure area forms and due to the
counterclockwise flow (N.H.) colder air migrates equatorward behind a
developing cold front. |
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Warmer air moves poleward along a developing
warm front (east of the system). |
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Clouds and precipitation occur in association
with converging winds of the low pressure center and along the developing
fronts. |
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Well-developed fronts circulating about a deep
low pressure center characterize a mature mid-latitude cyclone. |
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Heavy precipitation stems from cumulus
development in association with the cold front. |
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Lighter precipitation is associated with stratus
clouds of the warm front. |
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Isobars close the low and are typically kinked
in relation to the fronts due to steep temperature gradients. |
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When the
cold front joins the warm front, closing off the warm sector, surface
temperature differences are minimized. |
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The system is in occlusion, the end of the
system’s life cycle. |
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Carl Rossby athematically expressed
relationships between mid-latitude cyclones and the upper air during WWII. |
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Mid-latitude cyclones are a large-scale waves
(now called Rossby waves) that grow from the “baroclinic” instabiloity
associated with the north-south temperature differences in middle
latitudes. |
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Temperature differences between the equator and
poles |
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The rate of rotation of the Earth. |
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The
rotation of a fluid (such as air and water) is referred to as its
vorticity. |
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Earth vorticity is a function solely of
latitude. |
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The higher the latitude, the greater the
vorticity. |
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Earth vorticity is zero at the equator. |
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Rossby
waves are produced from the conservation of absolute vorticity. |
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As an
air parcle moves northward or southward over different latitudes, it
experiences changes in Earth (planetary) vorticity. |
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In order to conserve the absolute vorticity, the
air has to rotate to produce relative vorticity. |
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The
rotation due to the relative vorticity bring the air back to where it was. |
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Decreasing vorticity in the zone between a
trough and ridge leads to upper air convergence and sinking motions through
the atmosphere, which supports surface high pressure areas. |
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Increasing vorticity in the zone between a ridge
and trough leads to upper air divergence and rising motions through the
atmosphere, which supports surface low pressure areas. |
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Upper
air troughs develop behind surface cold fronts with the vertical pressure
differences proportional to horizontal temperature and pressure
differences. |
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This is due to density considerations associated
with the cold air. |
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Such interactions also relate to warm fronts and
the upper atmosphere. |
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April 16 - The northeasterly movement of the
storm system is seen through a comparison of weather maps over a 24-hour
period |
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Occlusion occurs as the low moves over the
northern Great Lakes |
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In the upper air, the trough has increased in
amplitude and strength and become oriented northwest to southeast |
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Isobars have closed about the low, initiating a
cutoff low |
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April 17 - Continual movement towards the
northeast is apparent, although system movement has lessened |
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The occlusion is now sweeping northeastward of
the low, bringing snowfall to regions to the east |
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In the upper air, continued deepening is
occurring in association with the more robust cutoff low |
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April 18 -The system has moved over the
northwestern Atlantic Ocean, but evidence persists on the continent in the
form of widespread precipitation |
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The upper atmosphere also shows evidence of the
system, with an elongated trough pattern |
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The
movement of surface systems can be predicted by the 500 mb pattern. |
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The surface systems move in about the same
direction as the 500 mb flow, at about 1/2 the speed. |
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Upper-level winds are about twice as strong in
winter than summer. |
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This results in stronger pressure gradients (and
winds), resulting in stronger and more rapidly moving surface cyclones. |
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