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Chances are you’ve never heard of a Rossby wave, but these waves are important players in the weather patterns you observe on a daily basis. This lesson will explore Rossby waves and then we’ll explain how Rossby waves are related to cyclones.

What Is a Rossby Wave?

Today, you are going to get the unique opportunity to follow a teeny, tiny particle of air through the atmosphere. And, by following this particle around, you will learn all about Rossby waves! In order to keep our air particle separate from all of the other air particles, let’s call our particle Paul Particle.Paul is going to be traveling high in the atmosphere in a jet stream, heading east. Jet streams are superhighways of fast moving wind that form when cold polar air meets warm equatorial air.

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Paul is happily traveling with his jet stream friends when – boom – he hits a mountain. This changes his trajectory, and now Paul is heading north.Will Paul keep heading north and be subjected to cold, frigid temperatures the rest of his life? Nope! Due to the Coriolis Effect, or the deflection of air due to the rotation of the Earth, Paul is not destined to live a life at the North Pole. As the Earth rotates, everything on Earth goes along for the ride.

This includes, you, me and even Paul! But everyone isn’t spinning at the same speed. What? I know, crazy!In order to understand this, you need to imagine the Earth with an imaginary pole running through it. This imaginary pole is actually called an axis, and it is what the Earth rotates around. The Earth is a spherical shape, so its widest point is the equator, and it gets thinner as you go north or south of the equator.

Now visualize Paul at the equator. In a 24-hour span, the time it takes the Earth to rotate around its axis once, Paul is going to travel a huge distance. Now picture Paul at the North or South Pole. He won’t travel at all during that 24-hour period. So, Paul travels faster at the equator since he is covering a huge distance in that 24-hour period of time, whereas Paul travels slower at the poles, where he doesn’t really move at all in that 24-hour period.Now that you have some basic Earth rotation under your belt, let’s get back to Paul (who has been flung northward by the mountain). Paul is still rotating at the same rate he was when he was at the equator, which is faster than the rotation at his new Northern latitude.

So, what happens? Because his rotation is different than that of his current location, he gets flung south. But he actually gets flung further south than his original location, so his rotation still doesn’t match the rotation of the Earth at the latitude.Again, his rotation doesn’t match the latitude he is at so he gets flung northward again. Guess what? Yep, the new position still doesn’t match his rotation rate so he gets flung southward.This keeps happening and results in a zigzag pattern as he heads east in the jet stream, and this movement is known as Rossby Waves, or horizontal waves that are found in winds at high altitude.

In Paul’s case the winds are part of a fast-moving jet stream. And it’s not just Paul that gets flung north and south. In fact, all of Paul’s air particle buddies are part of this zigzag pattern, as you can see here. Now the focus of this lesson is Rossby waves in the atmosphere, but they can also exist in the ocean, too! Wild!

Rossby Waves are found all over the globe
Rossby waves on Earth

Cyclonic Activity

Rossby waves sound like a pretty neat phenomenon, don’t you think? But they actually influence weather patterns, including cyclones, which are winds that travel in a circular motion and have a low-pressure center.

It’s worth noting that the winds of a cyclone are counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones often cause rain or snow and generally poor weather.Let’s go back to Paul and his friends that are high in the atmosphere. Remember, Paul meanders up and down as he heads eastward in the jet stream. These ridges and troughs from the up and down movement can cause areas of high-pressure and areas of low-pressure. In fact, the ridges create high-pressure systems, and the troughs create low-pressure systems.

The low-pressure system becomes the center of the cyclone, whereas the high-pressure system becomes the center of an anticyclone. Here you can see areas of convergence and divergence take place on the wave.

Convergence and divergence can occur on Rossby waves
Example of a Rossby wave

You’re probably wondering what convergence, divergence, and anticyclone all mean, right? Let’s start with anticyclone. Like the name implies, it’s the opposite of a cyclone.

So, an anticyclone has a high pressure center surrounded by spinning air and, unlike a cyclone, is responsible for calm weather conditions. Anticyclones blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.Let’s quickly talk about convergence and divergence, which both are terms coined to describe airflow.

In convergence, more air is entering an area than is leaving an area. This can cause the ‘extra’ air to be forced upwards or downwards (depending on if this is occurring at the surface of the Earth or high in the atmosphere). Divergence, on the other hand, occurs when more air leaves an area than enters. This causes the air to spread out and take up more space. Convergence and divergence are responsible for creating pressure systems.

Lesson Summary

Following Paul around and learning about Rossby waves and cyclones was fun, but it was a lot of information, so let’s take a moment to review the main points. Paul and his air particle friends became Rossby Waves when they traveled in the jet stream and hit a mountain.

This caused them to get pushed northward. But, due to the rotating Earth, Paul was pushed southward, then northward, then southward. This up and down motion is the Rossby wave, or horizontal waves that are found in winds at high-altitudes.The ridges and troughs from the Rossby waves cause convergence and divergence of air. This helps create pressure systems. The high-pressure systems develop into anticyclones, and the low-pressure systems develop into cyclones.

Wow, for a teeny, tiny particle, Paul does get around!

Learning Outcomes

Learning about Rossby waves and cyclonic activity could prepare you to:

  • Detail the formation of Rossby waves in the atmosphere
  • Outline the pattern of Rossby waves
  • Discuss the role that Rossby waves play in the development of cyclones and anticyclones
  • Illustrate the formation of convergence and divergence points

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