7: General Circulation
- Page ID
- 46824
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Have you ever wondered why it rains at some times of year and not others? Or why some regions don't get much rain and are desert-like, while others get constant rain, resulting in lush rainforests (Figure \(\PageIndex{1}\))? While Chapter 5 showed how precipitation forms in general, we haven't yet covered its distribution worldwide. The primary reason for where (and when) precipitation falls is where storms travel, and that is dependent on wind circulations. The reason we have global wind patterns is ultimately due to a differentially heated, rotating Earth. The differential heating of Earth continually creates an imbalance in air pressure and temperature around the world, which, in turn, drives a continuous general circulation of winds that attempt to restore balance.
While actual winds in a given place and time may differ from the average general circulation, the average can explain how and why the winds prevail from a particular direction in a certain place. For example, the prevailing surface winds tend to be westerly in the continental United States, but northeasterly in Hawai’i. The general circulation also serves as a model for the transport of heat and momentum from the equator to the poles.
The focus of this chapter is on the typical wind circulations found on Earth as a result of the forces affecting wind in the atmosphere, as introduced in Chapter 6. The average global winds are called the general circulation of the atmosphere. To determine typical wind circulation, one needs to average wind speed and duration over a long period. Averaging over time removes short-duration fluctuations, allowing the primary sense of movement to be visualized.
By the end of this chapter, you should be able to:
- Describe the differential heating Earth experiences, and how heat is redistributed
- Diagram vertical atmospheric circulations (Hadley cell, Ferrel cell, Polar cell)
- Diagram surface wind directions (trade winds, belt of westerlies, etc.)
- Discuss the distribution of heat over Earth’s surface and how it drives global circulation, including its connection to the Coriolis force
- 7.1: Differential Heating
- This page explains how Earth's curvature impacts solar energy distribution, causing increased intensity at the equator and cooler temperatures at the poles. This results in warmer tropics and cooler polar regions, which affect global atmospheric circulation. Despite ongoing energy loss through infrared radiation, the tropics have a net surplus while poles cool down.
- 7.2: Single-Cell Model
- This page discusses the single-cell model of global circulation, which assumes a uniform, water-covered Earth without seasons or Coriolis effects, leading to one large Hadley cell per hemisphere. It describes the process of warm air rising at the equator and cold air sinking at the poles, establishing pressure differences.
- 7.3: Three-Cell Model
- This page explains the three-cell model of atmospheric circulation on a rotating Earth, detailing the Hadley, Ferrel, and Polar cells. It describes how warm air rises at the equator, resulting in low pressure and trade winds, alongside high-pressure systems formed at 30° latitude due to descending air. Additionally, it discusses polar easterlies, the polar front, and how air masses interact to create weather.
- 7.4: Jet streams
- This page explains jet streams, focusing on the subtropical and polar jets in the Northern Hemisphere. The polar jet, found at 50° to 60° latitude, is influenced by temperature differences between polar and subtropical air, strengthening in winter. The subtropical jet is located around 30° latitude.
- 7.5: Exercises for Chapter 7
- Some exercises to review the topics covered in this chapter.
Thumbnail: Differential Heating of the Earth by NASA is in the Public Domain