Earth as a System: Energy, Matter, and Life · Lesson 7 of 13
Planetary Winds
“The atmosphere has global travel routes, though it refuses to draw them as straight lines.”
• Locate the major global pressure belts. • Explain how large convection cells form. • Relate pressure belts to planetary winds. • Describe the deflection of moving air in each hemisphere. • Explain why planetary winds are not straight north-south flows. • Connect global winds with heat transport.
Introduction
A local breeze may cross one valley, but the unequal heating between equator and poles creates movements spanning oceans and continents. These broad, persistent patterns are shaped by rising and sinking air, pressure belts and planetary rotation.
Planetary winds are large-scale prevailing winds produced by global pressure belts and modified by the Earth’s rotation.
| Approximate latitude | Pressure tendency | Basic reason |
|---|---|---|
| Equator | Low pressure | Strong heating makes air rise |
| 30° north and south | High pressure | Air from higher levels cools and sinks |
| 60° north and south | Low pressure | Air masses converge and rise |
| Poles | High pressure | Very cold dense air sinks |
Air near the equator becomes very warm because the equator receives strong sunlight. Warm air is lighter, so it rises upward.
High above the ground, this air begins to move away from the equator toward the north and south. As it travels, it cools. Cooler air becomes heavier, so it sinks back toward the surface in regions away from the equator.
Some of this sinking air then moves along the surface back toward the equator. This creates a large circular movement of air called a circulation cell. Similar air-circulation patterns also occur in other parts of Earth. Together, these movements help carry heat from warmer regions near the equator toward cooler regions closer to the poles.
However, the air does not always travel in a perfectly straight line because Earth is rotating.
Imagine throwing a ball while standing on a spinning merry-go-round. The ball may appear to curve even though it is moving forward. Something similar happens to moving air on Earth.
In the Northern Hemisphere, moving air appears to bend toward the right.
In the Southern Hemisphere, moving air appears to bend toward the left.
This bending is caused by Earth’s rotation. It mainly changes the direction in which the wind travels. The original reason the air started moving is still the difference in air pressure between one region and another.
Problem
Predict the path of surface air moving from a subtropical high-pressure belt.
- 1.Pressure causes air to move toward the equatorial low-pressure belt.
- 2.In the Northern Hemisphere the moving air is deflected to its right.
- 3.In the Southern Hemisphere it is deflected to its left.
- 4.The resulting winds approach the equator from easterly directions.
- 5.The winds are curved because the surface beneath them rotates.
Problem
Why do mid-latitude winds commonly have a west-to-east component?
- 1.Surface air moves away from subtropical high pressure toward subpolar low pressure.
- 2.In the Northern Hemisphere it is deflected right; in the Southern Hemisphere it is deflected left.
- 3.In both hemispheres, this gives the flow a strong eastward component.
- 4.The direction reflects pressure-driven motion combined with rotation.
Continents, mountain ranges, seasonal heating and friction modify these ideal belts. The global pattern is therefore a framework rather than a collection of perfectly fixed lines. It is most useful for explaining broad prevailing directions and heat transport.
Use the direction in which the air is already moving. Deflection is to its right in the Northern Hemisphere and to its left in the Southern Hemisphere.
Quiz
Which description best matches Planetary Winds?
Which term matches this description: Planetary winds are large-scale prevailing winds produced by global pressure belts and modified by the Earth’s rotation.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- List the four kinds of pressure belts from equator to pole.
- Explain why air rises near the equator and sinks near the poles.
- Describe how rotation changes a pressure-driven wind.
- Why do real winds differ from an ideal global circulation diagram?
Key Takeaways
• Global heating differences create broad pressure belts. • Rising and sinking air form large circulation cells. • Planetary rotation deflects moving air. • Global winds redistribute heat and influence weather patterns.