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  • 0:05 Earth's Rotation…
  • 1:23 The Equator, Doldrums,…
  • 2:38 Horse Lattitudes and…
  • 3:58 Polar Front and Polar Cell
  • 4:58 Lesson Summary
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The General Circulation of the Atmosphere

Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

The Sahara Desert, tropical rainforests, and cold polar winds are all due to Earth's rotation. In this video lesson, you will learn how this rotation has created global air circulation patterns on Earth and how it is responsible for such diverse climate patterns across the globe. Updated: 06/07/2019

Earth's Rotation Creates Wind Cells

Ever heard the phrase 'stuck in the doldrums?' It means you're stagnant, stuck, not really going anywhere. And, it comes from sailors who would get stuck in an area along the equator they called the Doldrums. If you're sailing through this area, you better have some oars handy because there's very little wind, and you won't get very far!

The Doldrums are also related to the rainforests along the tropics, the Sahara Desert in Africa and the Mojave Desert in the Southwest U.S., as well as the frigid winds of the polar regions. These all come about because Earth spins on its axis, which, as we learned in another lesson, deflects straight-moving winds and creates multiple cells of air circulation across the globe.

The Northern and Southern Hemispheres each have three air circulation cells. These cells help distribute heat and air across Earth. Each cell is like a wide belt around that part of Earth, and in these belts we get wind, which is horizontally moving air. Where each cell meets another is also like a belt, but these are opposite to our cells. They are thin, windless strips that girdle Earth. You can remember which is which because the windy cells are wide, which both begin with 'W.'

The Equator, Doldrums and Hadley Cell

Let's start along the equator. As you might expect, the air is very warm there, so it rises straight up. When warm air rises like this, it leaves behind an area of low pressure. You can remember this because the air is leaving, which creates an area of low pressure - 'leaving' and 'low' both begin with 'L.'

This particular area of low pressure along the equator is where we find our Doldrums - a long stretch of stagnant air and our first windless strip between cells. The warm air rising from the equator also brings a lot of moisture with it, and as the air rises, it cools. The water in the air is pushed out and comes back down as rain. It's certainly no accident that we find the world's tropical rainforests along the equator. This is where all that moisture from the hot equatorial air gets released!

The warm air rising from the equator eventually begins to spread horizontally, which creates those wide, windy circulation cells. On either side of the Doldrums are the Hadley cells - one for each hemisphere. You can remember the Hadley cell borders the equator because the equator is hot and 'hot' and 'Hadley' both begin with 'H.'

Horse Latitudes and Ferrel Cell

As the air spreads horizontally, we find that it begins to cool even more around 30° N and S latitudes. When the air is cool enough, it starts to fall back towards Earth's surface, but as it falls, it warms. So, if hot air leaving creates low pressure, hot air coming in creates the opposite - high pressure.

Because of this, the falling air creates our next thin, windless strip called the horse latitudes. They were named this because, like the Doldrums, there is little wind to push ships along the ocean. Legend has it that when this happened, horses on board the ships were either eaten as food supplies went low or thrown overboard to reduce the load.

The horse latitudes create areas of hot, dry surface air around Earth. You might not be surprised to learn that this is also where we find the world's great deserts, like the Sahara in Africa, the Mojave in the U.S. and the Great Victoria in Australia.

Some of the air heads back towards the equator as the warm, steady breezes of the trade winds, but much of it keeps heading toward the poles, creating our next cell, the Ferrel cell. The winds that blow towards the poles and create the Ferrel cells are the westerlies, which you can remember because they come from the west and blow to the east.

Polar Front and Polar Cell

When the warm, moist westerlies reach about 60° N and S (twice the latitude as our other thin, windless strip), they meet up with the cold, dry polar easterlies, which, as you have probably guessed, blow from the east. The polar easterlies make up our third and final circulation cell, the polar cell, and where the polar cell and the Ferrel cell meet is called the polar front. Here, just like at the equator, air rises vertically, and that leaving air creates an area of low pressure.

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