Air Movement Patterns & Their Effects on Cyclones

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  • 0:00 Air Flow in the Midlatitudes
  • 1:45 Meridional Flow of…
  • 2:51 Divergence in Rossby Waves
  • 3:52 The Life Cycle of…
  • 6:14 Lesson Summary
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Instructor: Linda Fye
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. 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.

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