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Understand air flow patterns of the midlatitudes in the upper troposphere. Learn about how these air flow patterns affect the development and movement of cyclones on the surface.

Air Flow in the Midlatitudes

In March of 1993, a huge midlatitude cyclone brought intense cold air and severe thunderstorms to the United States.

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These storms and cold air drastically affected the East Coast and brought snow as far south as the Florida panhandle. Many Southern states received as much as 10 inches of snow and dangerous flooding and tornadoes killed hundreds in Northwest Florida. Almost half the population of the U.S. was affected by this massive storm and it became known as the ’93 Super Storm.

So, how does weather this devastating develop?A midlatitude cyclone is a large-scale, low pressure system that travels eastward around the planet between 30 and 60 degrees latitude. In this lesson, you will learn how it is affected by air movement patterns of Rossby waves high in the atmosphere, including divergence and meridional flow. Midlatitude cyclones are crucial to day-to-day weather changes and to bringing rain and storms to much of the planet. The polar front is the boundary between the cold winds from the poles and the warm winds from the midlatitudes. When these winds meet at the polar front, they cause midlatitude cyclones at the surface of the planet.

The warm midlatitude winds at the surface tend to blow from west to east but are slowed down by friction. However, high up in the atmosphere, there is almost no friction from the surface, so winds blow very persistently from west to east. In fact, near the polar front, there is an area of very fast wind speed high in the atmosphere that is called the polar front jet stream. The pattern of the flow of this jet stream is what is important in affecting our weather and one pattern that is particularly important is called a Rossby wave.

Meridional Flow of Rossby Waves

Sometimes the polar front jet stream flows in a straight path from west to east around the planet and this is called zonal flow.

But the jet stream is not always in a zonal flow. Instead, there are often changes in latitude that are up-and-down wavelike movements in the jet stream. This is called meridional flow.

So it wanders quite a bit north and south and the waves can be quite large, dipping far down into lower latitudes.These large waves in the jet stream are called Rossby waves, and they are a product of meridional flow. At any time, there are usually three to six Rossby waves within the jet stream in each hemisphere. Since the jet stream separates cold polar air from warm midlatitude air, each oscillation of a Rossby wave moves cold air down toward the equator and warm air up toward the poles.

This brings severe, frequent weather changes to the midlatitudes. In the United States, a big Rossby wave can bring down enough cold air for a severe cold spell that covers half the country. So how does this type of air flow, high up in the atmosphere, affect the weather we have here on the surface?

Divergence in Rossby Waves

Rossby waves in the polar front jet stream have a very real effect on major weather disturbances in the midlatitudes. If you remember, the jet stream is high up in the atmosphere, but when Rossby waves develop, they can be such a powerful phenomenon that surface weather is drastically affected. As a Rossby wave dips down, it creates a trough. To the east of the trough, or downstream in the flow of the jet stream, divergence occurs high in the atmosphere at the jet stream.

Divergence just means that air flow is spreading away from the jet stream and, as it does, it pulls air upward from the surface, creating instability. It is sort of like all the air pulls away, or diverts, from the jet stream, leaving a sort of hole. This hole must be filled, so air from the surface is pulled upward to do that. This leads to instability and the development of a midlatitude cyclone that forms a large storm system on the surface.

But how exactly does divergence and air flow in Rossby waves influence a cyclone from beginning to end?

The Life Cycle of Midlatitude Cyclones

The polar front jet stream and Rossby waves directly influence the life cycle, development, and trajectory that a midlatitude cyclone goes through. At the polar front and the jet stream, winds blow right past each other in a straight line during zonal flow. Cold polar air and warm midlatitude air are separated and flowing right past each other. This is the beginning of formation of a midlatitude cyclone. However, certain conditions cause the straight, smooth polar front to develop a kink or wave at the surface, usually where there is a sharp temperature difference. While that is happening at the surface, in the upper atmosphere, winds are diverging in the Rossby waves to the east of a trough.

Both factors combined lead to further development of the midlatitude cyclone.The wave or kink at the surface deepens and causes a low pressure area. The warm air from the tropics and the cold air from the poles begins to flow counterclockwise around the low pressure, forming a cyclone. This starts the mature stage of cyclone development. During this mature stage, a very distinct cold front forms at the front edge of the cold polar air where there is heavy precipitation and cloud formation. Also, a well-formed warm front develops at the front edge of the warm midlatitude air that leads to light precipitation and cloud development.

During this time, there is a sector of warm air wedged between cold air on either side.The mature stage will not last forever, though. Mainly because the cold front and air behind it is moving faster than the warm air. While the cyclone is in full development, the entire storm, including the cold and warm fronts, are moving to the northeast along the path of the jet stream. Eventually, the faster moving cold front catches up to the warm front pinching off the sector of warm air.

This continues until the warm air is pushed completely off of the surface of Earth and a short period of heavy precipitation stops. Finally, storm activity slows down and dissipates. Eventually, the cyclone gets completely cut off from the polar front and the life cycle of the cyclone ends.From beginning to end, the life cycle of the cyclone is driven by the divergence of air at the jet stream in the upper atmosphere.

The path that the storm follows, including the cold and warm fronts, is to the northeast in the direction that the jet stream is flowing.

Lesson Summary

A midlatitude cyclone is a large-scale, low pressure system that is an important phenomenon that brings storms to much of the planet. The polar front is the boundary between the cold winds from the poles and the warm winds from the midlatitudes and is where midlatitude cyclones form. High up in the atmosphere, winds blow very persistently from west to east in an area of very fast wind speed that is called the polar front jet stream. When the jet stream flows in a straight path from west to east around the planet, it is called zonal flow. When air flow in the jet stream changes in latitude in up-and-down wavelike movements, it is called meridional flow. These waves in the jet stream are called Rossby waves and they move cold air down toward the equator and warm air up toward the poles.

As a Rossby wave dips down, it creates a trough. To the east of the trough, divergence, or air flow spreading away from the jet stream, occurs. This leads to the formation of a midlatitude cyclone.

In the first stage of the cyclone, cold polar air and warm midlatitude air are separated and flow right past each other. Conditions cause the polar front to develop a kink or wave at the surface, while air is diverging from the jet stream, and these lead to advancement of cyclone formation. The wave deepens and causes a low pressure area where air begins to flow counterclockwise around that low pressure to form a cyclone. This starts the mature stage of cyclone development. Then a cold front forms at the front edge of the cold polar air, and a warm front develops at the front edge of the warm midlatitude air.

During this mature stage, the entire storm, including the cold and warm fronts, are moving to the northeast along the path of the jet stream. Then the cold front catches the warm front, forcing all warm air up into the atmosphere and ending the life of the cyclone. From begin to end, the life cycle of the cyclone is driven by the divergence of air at the jet stream in the upper atmosphere.

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