# Why The Atmosphere Flows

We live at the bottom of a layer of gas that covers the Earth: the atmosphere. The Earth orbits the Sun about every 365 days and spins on an axis that is tilted about 23 degrees from the axis of its elliptic orbit (Figure 1.2). It is this tilt that is responsible for the change of seasons. When the northern hemisphere is tilted towards the Sun, it is northern hemisphere summer, and likewise for the southern hemisphere (Figure 1.2).

FIGURE 1.1: The Sun-Earth system. The Earth is spinning on its axis (in the counterclockwise sense when looking down from the north pole), which is tilted about 23 degrees. It also rotates about the Sun, making one complete orbit about every 365 days. The tilt of the Earth is responsible for the change of seasons as explained in the text.

FIGURE 1.1: The Sun-Earth system. The Earth is spinning on its axis (in the counterclockwise sense when looking down from the north pole), which is tilted about 23 degrees. It also rotates about the Sun, making one complete orbit about every 365 days. The tilt of the Earth is responsible for the change of seasons as explained in the text.

FIGURE 1.2: A schematic diagram of atmospheric motions. As the air in the Hadley cell moves toward the equator, the spin of the Earth about its axis moves the surface toward the right in the figure. An observer on the surface then feels a component of the wind coming from the east. Similarly, in the Ferrel cell in midlatitudes, the air tends to flow away from the equator and as the Earth's surface spins toward the right, there is a wind component from the west.

FIGURE 1.2: A schematic diagram of atmospheric motions. As the air in the Hadley cell moves toward the equator, the spin of the Earth about its axis moves the surface toward the right in the figure. An observer on the surface then feels a component of the wind coming from the east. Similarly, in the Ferrel cell in midlatitudes, the air tends to flow away from the equator and as the Earth's surface spins toward the right, there is a wind component from the west.

The Sun's rays impinge on the atmosphere nearly vertically in the tropics but at an increasing angle of incidence nearer the poles. This means that the regions closest to the equator receive more sunlight in a given area than regions closer to the poles, and as a result, they are warmer. Warm air is lighter than cooler air. It therefore tends to rise. As the warm, moist air near the equator rises, the water vapor in the air condenses into fine droplets, forming clouds. As the air nearest the equator rises, nearby air north and south of the equator must rush in to fill the void left behind. Thus, air from both sides of the equator converges toward the equatorial region, and meteorologists call this region the intertropical convergence zone. The rising air then turns poleward and descends, flowing downward toward the surface at around 30 degrees from the equator. This cellular motion is known as the Hadley cell1 and is illustrated in Figure 1.2. Because the descending air tends to be dried out, many of the Earth's deserts are located at latitudes near 30 degrees north or south of the equator.

Because the Earth is spinning on its axis, the air at the surface within the Hadley cell veers to the right toward the west. In other words, the Earth is spinning out from under the air as it turns on its axis, so that someone on the (moving) Earth experiences a wind from the east. The winds blowing toward the west are known as the trade winds. Poleward of the Hadley cell is the Ferrel cell, also illustrated in Figure 1.2. Air drawn down by the Hadley cell flows generally poleward near the surface, but because the Earth is rotating out from under these winds, we experience them veering toward the east. These midlatitude winds are known as the westerlies (from the west). Still further poleward, there is another weak cell: the polar cell. Again, the surface motion is generally toward the equator, but it veers to the right, so that the surface winds have an eastern component.

Near the equator, the surface winds are generally light, and for this reason, the region was named the doldrums (which means something like "low in spirit"). Similarly, in the region between the trades and the westerlies, the surface winds are light. In this region, in the days of sailing ships, vessels frequently became calmed for long periods, and it was named the horse latitudes (possibly because horses had to be eaten or thrown overboard when food and water shortages developed).

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