Global circulation is driven by pressure gradients in the atmosphere, the Coriolis force, and the friction of the atmosphere against the lithosphere. All of these may be summarized in a phenomenon we call ‘wind’. Wind is the horizontal movement of air in response to differences in pressure. Winds are the way that the atmosphere tries to balance the uneven distribution of pressure over the earth’s surface.
Pressure gradients and wind
We talked briefly at the beginning of this lecture about pressure gradients. Isolation differences are the key to the patterns of global circulation. The cold air around the poles of the earth sinks, causing regions of high pressure, while the warm, buoyant air around the equator tends to rise and cause regions of low pressure. We might then expect a gradual increase in pressure from the poles to the equator. However, the earth is composed of many dynamic ‘belts’ of high and low pressure which are far more complicated than the thermally induced low pressures at the equator and high pressure at the poles.
An English Scientist, Richard Hadley, gave this some thought several centuries ago, leading him to postulate a global circulation model where horizontal air movement was coupled with vertical air movement so that he envisioned the atmosphere as consisting of two huge convection cells with air rising at the equator, sinking at the poles and flowing from higher pressure to lower pressure, both at the earths surface and aloft. This has come to be known as the 'Hadley cell'. Here is how a Hadley cell works:
- Intense heating of air in the tropical areas, especially near the equator. As the density of air decreases, large-scale uplift of air between the equator and 5º N & S occurs, creating bands of low pressure
- Uplifted air is then pushed toward the poles.
- Air gradually cools, and sinks between 23.5º and 30º N & S, creating large bands of high pressure at the tropics.
Later work suggested that Hadley's simple model needed significant modification. By the late 1700's, a global circulation model based on a set of wind and pressure belts was in common use. This model presented a set of wind belts and pressure belts with a three cell global model where air flow, pressure belts, and vertical circulation cells all functioned together.
The Coriolis Effect and Wind
The Coriolis effect is the effect of the earth’s rotation on horizontally moving bodies such as the wind and ocean currents. Such bodies tend to be deflected to the right in the northern hemisphere (clockwise) and to the left in the southern hemisphere (counterclockwise). The amount by which the object is deflected depends on its speed and latitude. For a more in-depth explanation of this force and a very effective animated demonstration of its effect, look at the University of Iowa Coriolis page.
The Coriolis force has a different effect on high pressure systems than it does on low pressure systems. You might want to search for an illustration to examine the direction that winds will blow around a low pressure system (called a cyclone) and a high pressure system (called an anticyclone). Would a cyclone in the northern hemisphere blow in the same direction as a cyclone in the southern hemisphere? Why or why not?
The Friction of the Atmosphere Against the Lithosphere
Near the earth’s surface (up to about 1000 meters), frictional drag is very important because it reduces wind speed and causes ocean waves. One way to remember this is that on a very ‘calm’ day at the beach, the waves on the ocean are very flat. On a windy day, there are very large waves.