Principle
Low pressure systems that travel across the Northern Hemisphere
(better known as mid-latitude cyclones and more loosely known
as lows, cyclones or storm systems) typically develop, intensify
and dissipate over periods of 3 to 4 days. In the process,
they travel thousands of miles, disrupt "normal" life
as they go, are responsible for much of the precipitation
that falls, and can encompass areas as large as the central
United States. They are much more common and more intense
during the winter season than during the summer season.
High
pressure systems, on the other hand, tend to be of two types:
either the semi permanent systems that occupy the central
portions of the Pacific and Atlantic Oceans, or the large
systems that represent the large domes of cold (cool) air
that invade southern latitudes following the passage of lows. |
Straight-Line,
Frictionless Flow
- Straight-line, frictionless flow has the following
characteristics:
- The wind is stronger when isobars are closer
together.
- The wind is parallel to the isobars and both
the speed and direction of the wind are constant.
- The wind blows with lower pressure to the left
in the Northern Hemisphere and to the right in
the Southern Hemisphere.
- This type of flow is usually a good approximation
(to about 90%) of the upper-level winds.
- Meteorologists call this type of wind the "geostrophic
wind".
The forces at work are the
horizontal pressure gradient force and the Coriolis
force.
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Curved,
Frictionless Flow
- We see that there rarely is straight-line flow
in the atmosphere. The curvature of isobars indicates
that we must also consider the centrifugal force.
- Curved, frictionless flow has the following characteristics:
- The wind is stronger when isobars are closer
together.
- The wind is parallel to the isobars.
- The wind blows with lower pressure to the
left in the Northern Hemisphere and to the
right in the Southern Hemisphere.
- Around a trough or low pressure system, the
wind speed is weaker than it would have been
if it were blowing in a straight line (called
subgeostrophic).
- Around a ridge or high pressure system, the
wind speed is stronger than it would have been
if it were blowing in a straight line (called
supergeostrophic).
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Curved
Flow with Friction
- Friction slows motion (always opposite to the motion).
- Friction is important only in the lowest 1 kilometer
of the atmosphere.
- The wind direction changes such that the flow is across
the isobars (30-50s usually) toward lower pressure.
Hence, near the surface, air
converges into the center of a low and diverges away
from the center of a high. Because this convergence
or divergence is on the surface and air cannot go
down through the ground, we have vertical motion
above the surface.
In the case of low-level convergence,
the air moves into the center of a surface low pressure
system and is forced to rise. When the rising air
hits the bottom of the stratosphere, it diverges
outward (it cannot go upward any longer).
In
the case of low-level divergence, the air is evacuated
away from the center of a high pressure system. The
space that is evacuated is filled with air from above
(that is, sinking motion is induced). To "refill" the
column with air, convergence occurs just below the
stratosphere.
- Low pressure systems are marked by low-level convergence,
upper-level divergence, rising motion, and clouds. High
pressure systems are marked by low-level divergence, upper-level
convergence, sinking motion, and clear skies.
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Oklahoma's
Mid-Latitude Cyclones
- In winter, mid-latitude cyclones frequently develop
across the southern United States just east of the
Rocky Mountains. Thus, western Oklahoma is often the
birthing grounds for storm systems.
- In summer, low pressure systems rarely develop across
the southern United States. Instead, the zone of active
cyclogenesis (i.e., the development of cyclones) shifts
northward to near the US/Canadian border.
- If a low pressure area were to move eastward along
the Kansas and Oklahoma border, winds over Oklahoma
would begin as light southerly, strengthen, gradually
become southwesterly, then westerly before becoming
strong northerly.
- If a low pressure area were to move eastward along
the Texas and Oklahoma border, winds over Oklahoma
would be light east to northeast, would become stronger
northeasterly, gradually shift to the north before
becoming strong northwesterly.
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