12.2: What Causes Winds?

Winds—movements of air—arise from an imbalance of pressure. They are nature’s way of bringing balance to an unbalanced atmosphere. To achieve that balance, winds blow from regions of high pressure to regions of low pressure. To understand winds, we need to understand something about the forces generated by differences in air pressure between different locations (following Ahrens and Henson 2018).

Hydrostatic Equilibrium, a Balance of Forces

As we learned above, air pressure decreases with altitude. It’s highest at Earth’s surface and lowest at the top of the atmosphere. So if winds blow from high to low pressure, why aren’t winds blowing upward from Earth’s surface? The answer is gravity. The force of gravity counteracts the force generated by the upward difference in pressure. When these two forces are equal, and no net vertical motion of the atmosphere occurs, a hydrostatic equilibrium is maintained. The upward forces are balanced by the downward forces, and the fluid is at rest. The hydrostatic equilibrium concept (i.e., a balance of forces) applies to vertical motions in the ocean as well (as we learned in Chapter 13). We’ll visit this topic again in our chapter on ocean circulation.

Of course, some vertical motions in the atmosphere (and ocean) do occur. Sinking air generally accompanies high-pressure systems, while rising air characterizes low-pressure systems. Thermals—rising columns of warmed air (sought out by birds, hang glider enthusiasts, and presumably robotrosses)—also represent vertical motions of air. Violent vertical air motions occur during thunderstorms and tornadoes. Nevertheless, the strongest motions of air lie with pressure differences across horizontal scales—that is, the force that causes winds.

The Pressure Gradient Force

To express the strength of the pressure differences that cause winds, meteorologists and oceanographers define the pressure gradient force (PGF), which is generated by differences in air pressure between two locations. Mathematically, we represent the PGF between two locations as:

PGF = Pressure difference / Distance

(Eq. 15.1)

The strength of the PGF determines the strength of the winds. This equation predicts that when the pressure difference between two locations is high, the PGF will be strong. When the pressure difference between those two locations is low, it will be weak. What really matters is how pressure changes over distance—the gradient of pressure changes. In an isobaric map, steep changes appear as closely spaced isobars, while gradual changes in pressure are represented by isobars that are farther apart. Steep changes in pressure over short distances bring about very high winds; weak winds will be present where isobars are widely spaced.

A Description of the Coriolis Force

One other force plays a role in the motions of air (and water) on our planet. Formulated by French mathematician Gaspard Gustave de Coriolis (1792–1843), the Coriolis force refers to the apparent deflection of moving objects across Earth’s surface from the standpoint of an observer on Earth. This deflection occurs because the object is in motion on a rotating frame of reference—the spinning Earth. Though not correct in physical terms, the Coriolis force has been compared to what happens to a ball thrown to the other side of a playground merry-go-round. By the time the ball reaches the other side, the target has moved. From the standpoint of a person on the merry-go-round, it appears as if the path of the ball curves, though, of course, it doesn’t.

If this sounds complicated, it is, and a proper understanding would take many pages and some advanced mathematics. Many valiant attempts have been made in textbooks to explain the Coriolis effect, but they generally confuse and muddy the topic more than they help students understand it (e.g., Kearns 1998; Perrson 1998; Shakur 2014). Simply knowing what happens as a result of the Coriolis force suffices for our purposes here. The Coriolis force underlies the clockwise and counterclockwise motions of winds and currents around centers of high and low pressure, helps to explain the formation and circulation of hurricanes, and a lot more.

As a result of the Coriolis force, winds and currents appear to be pulled sideways, or perpendicular to the direction of their motion. In the Northern Hemisphere, the Coriolis force causes moving objects to deflect toward the right (clockwise). In the Southern Hemisphere, the Coriolis force causes moving objects to deflect toward the left (counterclockwise). In other words, the Coriolis force influences the direction of winds and currents. If that’s all you remember about the Coriolis force, you’ll be fine.

For the sake of completeness, you should be aware that the strength of the Coriolis force—the degree to which it influences direction—increases with latitude and the speed of the object. Horizontal motions along the equator experience no Coriolis force. As well, the Coriolis force acts largely over long distances, on the order of tens of kilometers—scales that are important for the circulation of the atmosphere and the ocean. Despite popular notions, the Coriolis force does not cause water in sinks and bathtubs to swirl clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Compared to the forces within the sink or bathtub (such as the shape of the basin and whether it’s level), the Coriolis force is too small to alter the rotation of water down the drain.

To visualize the effect of the Coriolis force on the direction of winds and currents, I recommend that you employ the three-legged handperson model. You’ll need a right hand (or a reasonable facsimile) and a map or piece of paper representing a map. Let your three middle fingers be the legs of your handperson (yes, this handperson has three legs). Let your thumb represent the left hand of your handperson and your pinky finger represent the right hand. Now walk (using your three legs) across a map to the north of the equator. As the handperson moves toward the North Pole, move it in the direction of your pinky, or toward the right. Turning your handperson to the right in the Northern Hemisphere represents the change in motion that occurs as a mass of air or water moves in the Northern Hemisphere. The direction you travel makes no difference; when you are in the Northern Hemisphere, a moving object will be deflected toward the right. Alternatively, when you walk your handperson into the Southern Hemisphere, you will turn in the direction of your thumb—toward the left. Remember that your knuckles are the eyes of the handperson so you’re always facing in the correct direction as you walk across the map. In this way you can visualize and better understand the consequences of the Coriolis force as it acts on winds or currents.