Atmospheric sink, and then return to the Equator.

Atmospheric circulation is responsible for the weather you experience. This lesson will explain how atmospheric circulation works and then it will explore how it impacts the weather in a couple of different locations.

Atmospheric Circulation

When you hear the word ‘circulation’, a beating heart, arteries, and veins probably comes to mind. But what about atmospheric circulation? Does the atmosphere have a giant heart with blood vessels that pumps air through the atmosphere? Of course not. But you know what? Atmospheric circulation is kind of similar to your beating heart and blood vessels with a few differences.Instead of blood, there’s air and instead of a heart that pumps blood, unequal heating of the Earth and the Earth’s rotation drives atmospheric circulation. So, in a nutshell, atmospheric circulation means air traveling around the Earth due to unequal heating and the Earth’s rotation.

Let’s take a moment to delve a little deeper into that definition, starting with unequal heating.As the Sun’s energy reaches the Earth, more heat energy reaches the Equator compared to the poles, thus creating unequal heating. The warm, equatorial air rises and heads towards the poles. As it heads north and south, however, it starts to cool, sink, and then return to the Equator. This creates a cell, or a cycle of air.And because the Earth is rotating, the air gets deflected in what is known as the Coriolis Effect.

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In the Northern Hemisphere, winds are deflected to the right as they blow from high- to low-pressure systems and the opposite is true in the Southern Hemisphere where winds are deflected to the left as they blow from high- to low-pressure systems.Now that you have a general idea of how atmospheric circulation works, let’s scale down a bit so you can get a clearer picture.

Scale and Observation

The atmospheric circulation we just went over is a little more complicated than equatorial air heading to the poles and polar air heading to the Equator. There are actually several cells that occur on the Earth. For example, there is the Polar Cell that occurs near the poles, the Hadley Cell that occurs near the Equator, and the Mid-latitude Cell that occurs (you guessed it) at mid-latitudes. And in reality, it’s even more complicated than that!But let’s scale down even further and see what weather someone would observe if she were living on the east coast of the United States.

We’ll focus on the Polar Cell in the Northern Hemisphere, but there’s one in the Southern Hemisphere too!Remember, due to unequal heating by the Sun, the North Pole has cold, dense air whereas the Equator has warm, less-dense air. The cold, dense air flows from the high-pressure system in the North Pole toward the Equator in what is known as Polar Easterly winds. And remember, the Earth is rotating, so these winds get deflected to the west. They get their name because of where they originate, at the poles and from the east.Let’s scale down a little more and take a look at these winds. They tend to be dry, cold and weak and blow from the poles to about 60 degrees latitude.The Polar Easterlies meet up with warm, wet wind that is blowing toward the poles (or the Prevailing Westerlies) at about 60 degrees North and South latitude.

The warm, wet wind from the prevailing Westerlies and the dry, cold wind from the Polar Easterlies create the polar front. A front, in case you’re wondering, is just a transition between two different types of air; in this case the cold, dry air and the wet, warm air.Now let’s scale down even further to see how these winds and the polar front impact weather. On the east coast of the United States, the polar front is responsible for mid-latitude cyclones, which create blizzards and storms. In fact, in 1993, a storm occurred that was so severe it’s been called the ‘Storm of the Century’. This storm was one of the most expensive natural disasters during the 20th century in the United States with snow piling up from Maine to Alabama, and Florida experiencing flooding, all followed by extreme temperatures.Let’s take a moment to zoom in on another cell and see how it impacts the weather.

This time, let’s head closer to the Equator to look at the Hadley Cell.Here, the warm equatorial air rises and heads towards the poles. At about 30 degrees north and south latitude, the air cools, sinks, and returns to the Equator. Let’s scale down further and check out the winds that are created by the Hadley Cell, or the trade winds.Like the Polar Easterlies, the trade winds are deflected due to the Coriolis Effect so they blow from the northeast in the Northern Hemisphere. Let’s scale down a little more to see how someone living in Hawaii would observe the trade winds.

The trade winds come into contact with Hawaii as they blow towards the Equator and they tend to bring cooler temperatures and precipitation to the windward side of the islands. When the trade winds die down, many find it feels muggy and humid. In addition to bringing increased precipitation to the windward side of the Hawaiian Islands, they help steer storms that form over bodies of water including the Pacific Ocean, Atlantic Ocean, and parts of the Indian Ocean.

These storms eventually reach North America and parts of Asia and Africa.

Lesson Summary

Atmospheric circulation is similar to the circulation that occurs in your body. But here, we aren’t dealing with a heart and blood vessels. Instead, it’s the movement of air in the atmosphere due to unequal heating of the Earth and the Earth’s rotation.The Sun heats the Equator more than the poles, so warm equatorial air rises, heads to the poles where it sinks and heads back to the Equator. This causes a cell, or cycling of air.

Of course, this is large scale and is actually more complicated.We zoomed in on two cells: the Polar Cell and the Hadley Cell. The Polar Cell creates the Polar Easterly winds which are caused by the cold, dense polar air heading towards the Equator. The Polar Cell occurs from about 90 degrees latitude to 60 degrees latitude. These winds create a polar front when they meet with the Prevailing Westerlies.

And, if you were to scale down even further, you might observe the weather patterns that are created by the polar front such as mid-latitude cyclones which can bring increased storms to the United States, especially the East Coast.Next we zoomed in on the Hadley Cell, which is caused when warm, equatorial air heads towards the poles, but cools and sinks at about 30 degrees latitude. The Hadley Cell causes the trade winds near the Equator throughout the world, but we zoomed in on the Hawaiian Islands where we saw trade winds bringing precipitation to the windward side of the islands.So, just like your heart and blood vessels circulate blood through your body, these cells and winds circulate air and weather throughout the atmosphere!

Learning Outcomes

Strengthen your knowledge of atmospheric circulation, and then test your recollection by achieving these goals:

  • Explain how atmospheric circulation works
  • Describe the Polar Cell and the Hadley cell
  • Sum up their effects on weather in other areas of the world

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