Global Wind

As this warm air rises due to its lower density, it cools. Once it cools past the dewpoint, condensation occurs and clouds form. With continued rise and cooling, the air cannot hold the moisture and precipitation falls.

In response to that rising air, surface air must flow in to fill the vacated space. The rising air results in a low-pressure center. This is why when you hear about low pressure in the forecast, is typically associated with rising air masses and therefore with crummy weather. The air rushing in toward the equator defines the trade winds. These winds converge on the equator, but blow to the West because of Earth's rotation. This rotational effect is known as the Coriolis effect. We won't get into that in detail here, but if you are interested, check out the video below.

These flows drive convection cells, with dimensions that are controlled by the viscosity and density of air, and by the thickness of the atmosphere. The air that rose from the equator flows North and South at the top of the cell, and eventually descends at around 30 ° N or S latitude. As the cool, now dry air descends it warms. Sound familiar?

Just as occurs when air descends on the leeward side of mountain ranges and causes rain shadows, the amount of water that the descending and warming air could hold increases. But there is no additional moisture to be found, so the actual amount of water vapor in the air mass remains more-or-less fixed. These descending limbs of the Hadley cells form high pressure centers, and would be regions where persistent dry conditions should prevail – leading to the Earth's desert belts that include the Gobi, Sahara, Arabian, and the Australian Outback (not just a steakhouse!).

The equatorial convection cells are known as Hadley Cells. There are two more in each hemisphere, also driven by the uneven distribution of incoming solar radiation density; these are Farrell and Polar cells. Check out the diagram of this process below.

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Figure 19. Global Winds
Source: DWindrim - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons

• Global Wind Explained
• Formative Assessment 4: Hadley Circulation