How Air moves in the Atmosphere around the Globe

Figure 24.

HOW AIR MOVES IN THE ATMOSPHERE AROUND THE GLOBE

There are predictable ways that the atmosphere travels around the globe. Remember that the sun is stronger near the equator, so warm low pressure systems form. Near the top of the troposphere, some of the air moves northward while the other half moves southward. Of course, this air cools as it travels. Eventually sinks back down and gets sucked back toward the equator. You can see now that a circulatory pattern is set up. This circulatory pattern is a convection cell.

Remember also that the earth rotates. This complicates the convection cell, so that the cooled air lands further to the right than it should. This is only because the earth has rotated. This leads to what we call the Coriolis effect. Figure 25 shows how this works:

Figure 25.

In the northern hemisphere, the air coming from the equator meets the air coming back to the equator at about thirty degrees north latitude. Both of these batches of air descend, but for different reasons. Where this air mass descends, you have a high pressure area. You now know this as the subtropical high pressure zone. There is a tiny convection cell caused by this effect. It is called the Hadley cell, located between the equator and thirty degrees north latitude. Figure 26 shows all of these cells around the globe; it explains many of our typical wind and weather patterns:

Figure 26.

There are other convection cells in the Northern Hemisphere. It is the Ferrell cell between thirty and fifty degrees north latitude. Above that is the Polar cell between fifty degrees latitude and on up to the North Pole. In the southern hemisphere, the Coriolis effect makes objects such as air masses at the left, giving three mirror image circulation cells, for a total of six major convection cells on our globe.

Because of these convections cells and the earth's rotation, you now have global wind belts. These are stable wind patterns moving horizontally from high to low pressure. These are called the Northeast trade winds going in the southeast direction and linked to the Hadley cell, the westerlies in the higher latitudes going from west to northeast above 30 degrees north and linked to the Ferrell cell, and the polar easterlies from northeast to southwest. In the southern hemisphere, you now have the southeasterly trade winds below the equator heading in the northwest direction, the westerlies to the south going from northwest to southeast, and the polar easterlies.

The polar front is created by the boundary between the Ferrell cell and the Polar cell. Moist air from the equator mixes with high-pressure cold air. The end result is a potential for unsteady air in North America and much of Europe. The polar jet stream is very high and is where the cells meet. The temperature difference is large here so the wind is fast. You probably know that it's the jet stream that aircraft use to conserve fuel in some cases. This stream travels southward in the wintertime and northward during the summer months.

Between the Hadley cell and the Ferrell cell is the subtropical jet stream, which is less powerful than the polar jet stream, but it is based on the same principle. The air is warmer in both of these cells so the temperature variation is less.