16.7: Global Atmospheric Circulations

Global Atmospheric Pressure

Because more solar energy hits the equator, the air warms and forms a low pressure zone. At the top of the troposphere, half moves toward the North Pole and half toward the South Pole. As it moves along the top of the troposphere it cools. The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator. This describes the convection cells north and south of the equator.

If the Earth did not rotate, there would be one convection cell in the Northern Hemisphere and one in the Southern Hemisphere with the air rising at the equator and the air sinking at each pole. But because the planet does rotate, the situation is more complicated. The planet’s rotation means that the Coriolis effect must be taken into account. Due to the rotation of the Earth, an object moving in the Northern Hemisphere will be deflected to the right as a result of the Coriolis effect. Similarly, an object moving in the Southern Hemisphere will be deflected to the left. The Coriolis effect is greatest at the poles, and is nonexistent at the equator.

Let’s look at atmospheric circulation in the Northern Hemisphere as a result of the Coriolis effect. Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right. Remember that it just appears to deflect to the right because the ground beneath it moves. At about 30°N latitude, the air from the equator meets air flowing toward the equator from the higher latitudes. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone. Once on the ground, the air returns to the equator. This convection cell is called the Hadley cell and is found between 0° and 30°N.

There are two more convection cells in the Northern Hemisphere. The Ferrel cell is between 30°N and 50° to 60°N. This cell shares its southern, descending side with the Hadley cell to its south. Its northern rising limb is shared with the Polar cell located between 50°N to 60°N and the North Pole, where cold air descends.

There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis effect makes objects appear to deflect to the left. Ultimately, because there are three large-scale convection cells in the Northern Hemisphere and are repeated in the Southern Hemisphere, the model to understand these patterns is called the three-cell model.

Figure \(\PageIndex{1}\): The three-cell model of atmospheric circulation in the Northern Hemisphere. In the Hadley cell, warm air at the equator rises vertically, moves toward the pole in the troposphere, then sinks around 30°. In the Ferrel cell, surface air moves toward the pole, then rises around 60° and moves toward the equator at higher levels. In the Polar cell, air moves toward the pole, then descends at the pole.

Global Wind Patterns

Global winds blow in belts encircling the planet. The global wind belts are enormous and the winds are relatively steady. These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure. Technology today allows anyone to see global wind patterns in real-time, such as Earth Wind Map. Take a look at the Earth Wind Map and determine what patterns you can see occurring in the atmosphere in real-time. Are low pressure systems rotating counter-clockwise in the Northern Hemisphere? Are high pressure systems rotating clockwise in the Northern Hemisphere? Can you see the global wind patterns over the Atlantic and Pacific Oceans? Also notice how the winds flow faster over water than over continents because of land friction.

In the Hadley cell, air should move north to south, but it is deflected to the right by the Coriolis effect. So the air blows from northeast to the southwest. This belt is the trade winds, so called because at the time of sailing ships they were good for trade.

In the Ferrel cell air should move south to north, but the winds actually blow from the southwest. This belt is the westerly winds or westerlies. Why do you think a flight across the United States from San Francisco to New York City takes less time than the reverse trip?

Finally, in the Polar cell, the winds travel from the northeast and are called the polar easterlies. The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.

Figure \(\PageIndex{2}\): Global winds and atmospheric circulation model. The Ferrel cell is also known as the Mid-latitude cell.

The Polar Front and Jet Stream

The polar front is the junction between the Ferrel and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrel cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe.

The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the world’s most powerful. Jet streams move seasonally just as the angle of the Sun in the sky migrates north and south. The polar jet stream, known as the jet stream, moves south in the winter and north in the summer.

\(\PageIndex{3}\): The locations of the polar jet streams shown in blue and the subtropical jet streams shown in red. Jet streams form in the atmosphere where warm air masses meet cool air masses.

References

CC licensed content, Shared previously

• Dynamic Earth: Introduction to Physical Geography. Authored by: R. Adam Dastrup. Located at: http://www.opengeography.org/physical-geography.html. Project: Open Geography Education. License: CC BY-SA: Attribution-ShareAlike
• Jet stream cross section. Provided by: National Weather Service. Located at: https://commons.wikimedia.org/wiki/File:Jetcrosssection.svg. License: CC BY-SA 4.0: Attribution-ShareAlike

Public domain content

• Global winds. Provided by: NASA. License: Public Domain: No Known Copyright
• Jet streams. Provided by: NOAA/JPL-Caltech. Located at: https://scijinks.gov/jet-stream/. License: Public Domain: No Known Copyright