High and Low Pressure Systems

High and Low Pressure Systems

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.

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).

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 (30-50š 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.

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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	OK-FIRST Project, Oklahoma Climatological Survey, 100 East Boyd Street, Suite 1210, Norman, OK 73019. Copyright © 1996-2004 Oklahoma Climatological Survey. All Rights Reserved. Send comments or questions concerning OK-FIRST to okfirst@mesonet.org

	High and Low Pressure Systems

	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.

	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).

	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 (30-50š 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.

	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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