The Single-Cell Model of General Circulation

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  • 0:01 What's a Model?
  • 0:35 Single-Cell Model
  • 2:58 Problems with…
  • 4:44 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Have you ever wondered how air circulates in the atmosphere? This lesson will explain an early model of air circulation, the single-cell model. It will also look at some of the problems with this early model of air circulation.

What's a Model?

Welcome to the fashion runway. Fancy shoes, bright lights, celebrities, and beautiful models. Wait, what? You mean another type of model? Oh, you mean something developed by scientists that helps explain how things occur in nature. Okay, sure. I can work with that, and I can even think of some scientific models I've heard of, like the atomic model or a model of the solar system. And I know that models change as scientists learn more; for example, models of the solar system and of the atom have changed substantially over time!

Single-Cell Model

So, just like the solar system and the atom, there are models about how air circulates through the atmosphere. And these models have changed over time, too! Let's talk a little about one of these models, the single-cell model, and, no, this isn't a single-celled organism that's a model.

In fact, here, 'cell' refers to the cycling of air. Back in 1735, a physicist and meteorologist from England named George Hadley developed the single-cell model in order to explain trade winds. This model says cold air sinks at the poles and warm air rises at the equator, resulting in a cell. Sometimes this cell is referred to as a Hadley cell. Wow! Lucky George Hadley! To have a cell named after him? Wouldn't that be great!

Let's look a little closer at Hadley's model. As the sun shines on the earth, more heat energy hits the equator versus the poles, so the air at the equator is warmer than the air at the poles. Makes sense, right?

So, George, or Mr. Hadley, was definitely onto something! The warm air is less dense, so it rises until it reaches the tropopause, which is the area in the atmosphere between the troposphere and stratosphere, both of which are layers in the atmosphere. A really quick note about air before we go on. The molecules in warmer air are further apart compared to colder air, so this is why warmer air is less dense.

From the tropopause, some of the warm air heads towards the North Pole and some heads towards the South Pole. Now, as it heads to the North and South Poles, the warm air cools, thus becoming more dense and sinking. Once it sinks, it cycles back to the equator where it gets warmed again, rises to the tropopause and heads back to the poles. This creates one giant cell above each hemisphere: the warm air that went to the North Pole would be over the Northern Hemisphere and the warm air that went to the South Pole would be over the Southern Hemisphere. And, yeah, I realize that's a little confusing since the name is single-cell model, and there are actually two cells. So, now, you can see why the single-cell model has nothing to do with a single-celled organism or a fashion model; instead it's how air circulates through the atmosphere!

Problems with the Single-Cell Model

George Hadley's single-cell model helps explain how air moves; however, he failed to consider several factors. Does that mean the model is garbage? Of course not. Remember, models are always being revised and updated, so the single-cell model explains some aspects of air circulation and was used to develop more complex models.

Let's check out what Hadley neglected to take into account. First, in the single-cell model, the sun must be directly over the equator. Now, we know the equator gets a lot of sun, but the sun isn't constantly over the equator, which results in seasonal winds that aren't represented by the model.

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