Module #7: Factors That Affect Earth's Weather
Uneven Thermal Energy Distribution
By far, the most complex factor that influences earth's weather is the uneven distribution of thermal energy that comes from the sun. As you have already learned, earth's axial tilt causes the sunlight to shine differently on different parts of the earth. The more directly the sun's light shines on the earth, the better it heats the planet. Well, the sun's light shines most directly on the equator, which is the imaginary line that runs directly between the Northern and Southern Hemispheres of the earth. That's why it is always warm at the equator. Above and below the equator, the sun's light shines on the earth at an angle, reducing its effectiveness at heating. The farther up or down from the equator, the more pronounced this effect is. At the North or South Pole, the sun's light is shining at such a large angle that its heating effect is very poor. That's why it is always cold at the poles.
If you think about this on a global scale, then, there is a tremendous temperature imbalance between the earth's equator and its poles. What happens as a result of this temperature imbalance? Wind! You see, when air gets hot, it expands. This causes a given mass of air to take up more volume. Because of this fact, a given volume of hot air is lighter than a given volume of cold air. As a result, hot air rises. As the hot air rises, there is a deficit of air left behind, creating a region of low pressure, referred to (reasonably enough) as a low. Conversely, cold air sinks. As the cold air sinks, it creates a buildup of air, resulting in a high pressure region called (you guessed it) a high.
Now what will air tend to do if there is a buildup at one location and a deficit at another? It will tend to move from the buildup to the deficit, in order to even out the amount of air everywhere. When air moves, we call it wind. Thus, temperature imbalances result in wind, and the wind blows near the surface of the earth from the cold region (the high) to the warm region (the low).
This situation is reversed at higher elevations. After all, as the hot air rises, it “piles up” at the higher elevations, causing an excess of air at higher elevations. In the same way, as the cold air sinks, it leaves behind a deficit of air at the higher elevations. Thus, at higher elevations, the wind will travel in the opposite direction (from the warm region to the cold region).
Remember why I started this whole discussion. The equator is much warmer than the poles. As a result, warm air rises at the equator, creating a low there. At the same time, cold air at the poles sinks, creating a high there. Thus, at the surface of the earth, winds tend to blow from the poles (the high) to the equator (the low). In the upper atmosphere, however, the situation is reversed, and winds blow from the equator to the poles.
Simple enough, right? Well, it would be if I stopped there. Unfortunately, there are three factors that complicate the situation immensely: changing air temperature, the Coriolis (kor ee oh' lus) effect, and local winds. The first of these effects is the easiest one to understand. Consider the cold air that is traveling near the surface of the earth from the North Pole towards the equator. As it makes its way there, it encounters warmer climate. This warms up the air, causing it to rise. When the air makes it to the 60° N latitude, it becomes so warm that it rises into the upper troposphere and begins moving back towards the pole! In the end, this sets up a loop of winds that travel continuously from the pole to a latitude of about 60° N and back again.
If we now turn our attention to the equator, we will see the exact opposite effect. As the warm air rises, it starts traveling towards the poles. At a latitude of about 30° N, however, it cools down enough to sink and begin traveling back towards the equator. Thus, from the equator, there is also a loop of winds that travel to a latitude of about 30° N and then turn around and come back again. In the middle of these two loops of wind there is a third loop that occurs as a reaction to these two loops. The result of all this mess is shown below.
Notice that I title the figure a “first approximation.” That's because I still haven't dealt with the next factor: the Coriolis effect.
Remember that the whole time air is moving, the planet is also spinning. This tends to distort the wind directions a bit. To understand why this happens, you first have to realize that each latitude north and south of the equator rotates with a different speed. Although this statement may surprise you at first, it should make sense after studying Figure 7.7.
So you see that because the circumference of the earth is different at different latitudes, the speed at which a point on that latitude of the earth rotates is different than that of points on other latitudes.
How does this relate to wind? Well, consider a mass of air sinking at the North Pole. Because it starts there, it is rotating around the earth at the speed of everything else that sits on the North Pole. As Figure 7.7 indicates, things at the poles rotate around the earth slowly. As the air starts to move towards the equator, however, the ground below starts moving faster, because things closer to the equator rotate around the earth faster than things closer to the pole. Thus, the ground beneath this air mass starts “outrunning” the air above it. This makes the wind bend away from the rotational motion of the earth. I am sure this is a bit confusing, so I will explain it again with a figure.
Animation
As the figure indicates, the apparent bending of the winds due to the rotation of the earth is called the Coriolis effect. This same effect alters currents in the sea as well.
Coriolis effect - The way in which the rotation of the earth bends the path of winds, sea currents, and objects that fly through different latitudes
When you add the Coriolis effect to the effect of changing air temperature discussed above, you get the following global wind patterns.
Please realize that these wind currents are only those found in the lower troposphere. The wind currents in the upper troposphere are even more complicated. Thankfully, I will not discuss them in this course!
Look at Figure 7.9 for a moment and try to understand why the air currents are bent in the way that they are bent. Begin by looking at the air currents near the North Pole. As Figure 7.7 indicates, things near the poles rotate rather slowly around the earth, because in the space of a day, they travel a shorter distance around than things closer to the equator. As a result, the winds that start out at the North Pole are rotating at a slow speed. As they travel south, they pass over land which is traveling faster. Figure 7.8 gives you the direction of earth's rotation, so the land will “outrun” the wind in that direction. This makes the wind look like it bends in the opposite direction. Thus, the winds near the poles bend opposite the direction of the earth's rotation. Between the latitudes of 30° N and 60° N, however, the winds travel from south to north. Thus, they start out rotating quickly and end up passing over land that is rotating more slowly. As a result, they “outrun” the land. This makes them look like they bend in the direction of the earth's rotation. In the end, then, winds traveling towards the equator get “outrun” by the land they are passing over and end up bending opposite the direction of the earth's motion. Winds that travel towards the poles “outrun” the earth they are passing over, so they bend in the direction of the earth's rotation.
Now please realize that Figure 7.9 shows the general wind patterns that exist on the planet. At any given time, however, there is no reason to believe that you can predict the way the wind is blowing by looking at Figure 7.9. Why? Well, remember that temperature imbalances are the driving forces for these winds, and that the temperature of a region is affected by several factors. For example, if the cloud cover over a region of the equator is high, then that portion of the earth will not receive as much insolation as another portion of the equator. This will make the cloudy region of the equator cooler, and the global winds in that area of the planet will be weakened. At the same time, another region near the equator might not be at all cloudy, causing temperature imbalance between different regions along the equator. This will set up a different wind pattern, interfering with the global wind patterns. In addition to all of this, local winds can dominate the global wind patterns completely, or they can just interfere a bit. In the end, then, Figure 7.9 shows us a general rule for global wind circulation, but the real picture is infinitely more complex!
Since local winds can affect the global wind patterns in Figure 7.9, it is important to know how local winds develop. Just like the global winds, they start as a result of temperature imbalance. Consider, for example, a region of the earth near a large lake. During the day, the sun's light warms the area. Water, however, does not warm up as quickly as soil and sediment. As a result, the land gets warmer than the sea when the sun's light shines on them both equally. This temperature imbalance causes a low to form over the land and a high to form over the lake. This results in a sea breeze, shown in Figure 7.10a. At night, the land cools faster than the water. This eventually causes the opposite temperature imbalance, and the breeze blows the other way, as a land breeze shown in Figure 7.10b.
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