Global Atmospheric Circulation for Middle School Science

Feb 20

4 min read

Global atmospheric circulation is the way that the air molecules of the atmosphere move around the Earth due to the uneven heating of the planet by the sun. This topic connects weather, climate, and space. It is also a way to talk about environments and the plants, animals, and people who have adapted to them.

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Global Atmospheric Circulation

The air molecules that make up Earth's atmosphere are always moving. We call moving air molecules wind. The general wind patterns around Earth are called global atmospheric circulation.

Global atmospheric circulation is caused by uneven heating of the Earth by the Sun and by the Earth's rotation. The Earth is shaped like a ball. As a result, sunlight hits directly on the equator but at an angle everywhere else on Earth. Sunlight hits the poles at the sharpest angles, so these areas receive the least amount of light and heat from the Sun.

We use a three-cell circulation model to describe how air moves around Earth due to this uneven heating. There are actually six cells of air circulation in the three-cell circulation model: three cells above the equator and three cells below the equator.

The first cell is called a Hadley Cell. Warm air from the equator rises and moves toward the poles. At around 30° latitude, the air cools and drops back to the Earth. This cold air makes its way back to the equator, where it warms up and rises again.

Rising air causes an area of low pressure at the equator, and falling air creates high pressure at 30° latitude. Low-pressure areas have lots of precipitation because the rising air carries water vapor that condenses into clouds as it rises and cools. High-pressure areas have little rainfall and often form deserts.

If the Earth didn't rotate, air would move in a straight line from the equator to 30° latitude, but it doesn't. Again, the Earth is shaped like a ball. The middle of the ball is much wider than the top or bottom. That means the middle of the Earth is rotating faster than the poles. If the poles turned at the same speed as the equator, air would move in a straight line, but the change in speed means that air and water are deflected to the right in the northern hemisphere and the left in the southern hemisphere. Scientists call this phenomenon the Coriolis Effect.

The wind created by air moving from 30° latitude to the equator moves from northeast to southwest in the northern hemisphere and from southeast to northwest in the southern hemisphere. Since early sailors found these winds helpful for trade, they named them the trade winds.

At the poles, cold air sinks and moves toward the equator. The sinking air creates high air pressure at the poles. Antarctica is the largest, driest desert on Earth.

As a result of the Coriolis Effect, the winds caused by air moving from the poles to 60° latitude move from northeast to southwest in the northern hemisphere and southeast to northwest in the southern hemisphere. Meteorologists name winds based on where the winds start, so they call these winds the polar easterlies.

Around 60° latitude, the air from the poles warms and rises. The warm air circulates back to the pole where it cools and falls again. This movement of air makes up the Polar Cell of global atmospheric circulation.

At 60° latitude, the Polar Cell meets the Ferrel Cell that circulates air between 30° latitude and 60° latitude. Rising air creates a low-pressure zone with significant precipitation over North America and Europe.

At 30° latitude, the Ferrel Cell meets the Hadley Cell from the equator. Here, cold air from both cells sinks to Earth's surface. The air molecules then either flow toward the equator or the poles, whichever is closer to each air molecule.

The Coriolis Effect causes the wind from the air molecules traveling back up to 60° latitude within the Ferrel cell to move from southwest to northeast in the northern hemisphere and northwest to southeast in the southern hemisphere. As a result, meteorologists call these winds the westerlies.

There are two latitudes with very little wind: along the equator, where air rises, and at 30° latitude, where it falls. In the past, ships got stuck in these calm waters without a breeze to carry them out. They called the windless band around the equator the doldrums because it made them feel bored and depressed to wait for the wind. They named the area around 30° latitude the horse latitudes because sailors stuck there would sometimes throw horses on the ship overboard to conserve food and water.

The rising air at 60° latitude does not cause calmer winds because when the cold air of the poles meets the warmer air from the Ferrel Cell, the dramatic temperature difference causes unstable weather such as thunderstorms.

The global atmospheric circulation affects places on Earth in many ways. It creates deserts like the Sahara Desert in Africa and the Great Victoria Desert in Australia. It also contributes to our local weather and creates the jet streams that make flying from California to New York faster than flying from New York to California.

Most importantly, the global atmospheric circulation acts like an air conditioner. The equator collects more heat than it releases back into space, and the poles release more heat than they receive. By moving heat around through global atmospheric circulation, the Earth maintains a global temperature that can support life.