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Climate and the Seasons Study Guide (page 2)

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Updated on Sep 25, 2011

Earth's Spin and Tilt

People long ago did not know that Earth spun around once a day. They thought that the stars turned around, as a great sphere far above the atmosphere, and that the sun, moon, and planets traveled around the earth on their own sphere or path, with different periods. It was not known that Earth was also a planet. Dawn came each day when the sun literally rose; the earth, obviously because it was so huge, stood still, or so people thought.

With the discoveries of Copernicus and Galileo, the true nature of the solar system was recognized. The stars move at night and the sun rises and sets because Earth spins. We say it spins once a day. But don't for get that the day is in fact defined by the earth's spin.

If the earth spun only half as fast, then the day would be 48 hours. Or would it still be 24 hours, with each hour being 120 minutes long? I'll leave you to ponder these issues as we move on to the direction of the earth's spin.

Which way does the earth spin? Imagine yourself looking at a globe, or in space looking down at the real Earth of clouds and continents, the way astronauts do. Suppose you face the United States the way it is normally shown on maps with Central and South America below it, in the direction of your feet. Which way is the earth spinning? Left to right or right to left? Think of the answer before reading on.

The spin is from left to right. How did you figure this out? One way is to realize that dawn comes to the east coast before the west coast (that is, to New York before Los Angeles). Sunset is the same. The only way this can happen is if the spin is from left to right. Now what if you were turned upside down, so your head is toward South America and your feet are toward Canada?

Earth spins around an axis that stays constant. The spin axis is an imaginary line that goes through the diameter of the earth, from what we call the South Pole to the center of the earth and then up through the North Pole. Furthermore, and crucially for our lives, the spin axis is not perpendicular to the plane of Earth's orbit around the sun. Earth's spin axis is tilted at 23.5° away from that perpendicular direction to the plane of Earth's orbit.

The other important fact about Earth's spin axis, which will prove consequential for understanding the seasons and climate in the next section of this lesson, is that it keeps pointing to the same direction in space. Today, the axis always points to the North Star (which is really, really far away).

The facts, (1) that the spin axis is tilted and (2) that it always points to the same direction in space, together create the same changes that those who live in the high latitudes see in the length of the daylight hours. (Note that we speak of the daylight hours as the day, but day also means the total 24 hours for the earth to spin once around. Yes, it's a bit confusing, but that's our English language.) In Figure 3.3, you can see how day length changes with time of year. Please study it closely before moving on to the practice questions.

Figure 3.3

Seasons and Climate

We now have the pieces we need to understand the seasons and climate. These pieces include: latitude and longitude, how the amount of sun energy varies with latitude (because the earth is nearly a sphere, an oblate spheroid), and how the earth's spin axis is tilted and remains pointed in the same direction in space during the course of the year.

Figure 3.4 The Sun and the Seasons

The easiest way to grasp the essentials of the situation is to ask: At what latitude is the sun overhead at noon on June 21 and December 21? Recall that these dates are the two dates with extreme points in the earth's tilt relative to the plane of the earth's orbit. On June 21, the summer solstice, the sun is overhead at noon for an observer at latitude 23.5° N. Study this fact in Figure 3.4, and then we'll discuss the situation at the time of the winter solstice.

On June 21, the summer solstice, the sun is overhead at noon at the latitude 23.5° N. At the same time, you can see that for a place in the northern hemisphere, say New York City at 41° N, the sun is very high in the sky and the ground receives a large amount of solar energy per square meter (if necessary, review Figure 3.2). This time of year, of course, is summer in the northern hemisphere. On December 21, the winter solstice, the situation is quite different. Now the southern hemi sphere is maximally tilted toward the sun and the latitude of 23.5° S has been "lifted" to the place where the sun at noon is directly overhead. During the winter solstice, the northern hemisphere is tilted away from the sun; in fact, all sites above 67.5° N are in complete darkness all that day. If we consider an observer in New York City during the winter solstice, you can see from the figure that the observer sees the sun very low in the sky. That site receives relatively little solar energy per square meter because the sun's rays are spread out along the ground (if necessary, review Figure 3.2).

Figure 3.2

On the winter solstice, December 21, it is latitude the 23.5° S that sees the sun overhead at noon, as explained in the previous paragraph. The southern hemisphere is in summer and the northern hemi sphere is in winter. The fact that the tilt of the earth remains pointing to the same position in space (always at the north star) makes the noon sun's angle change everywhere on Earth during the course of the year. At the latitudes of sites in the United States, for example, the sun at noon is high during summer and low during winter. This changes the amount of solar energy received per square meter of ground, which causes the seasons' changing amount of solar energy per square meter over the course of the year is the cause of the seasons.

On the equinoxes, September 22 and March 20, the sun is overhead at noon for places exactly on the equator. In fact, at the equator, the sun is almost always overhead, varying from 23.5° from vertical in one direction at one solstice, to exactly vertical at the equinox, then to 23.5° from vertical in the other direction at the other solstice, to vertical at the other equinox, and then back to the first solstice. The large amount of solar energy per square meter received by the equator and places near the equator is the reason that the temperature is warm all year round. In fact, the temperature is tropical. We call these low latitudes the tropics. Basically, this includes all the latitudes between 23.5° N and 23.5° S. These 23.5° latitude lines are known, respectively, as the Tropic of Cancer and the Tropic of Capricorn.

The way the sun changes during the course of the year provides Earth with three main climate bands. The climate actually varies continuously with latitude, but humans tend to classify nature into regions. The first we've already seen—the tropics. The second is called the midlatitudes, which includes the United States, Europe, and China in the northern hemisphere.

Climate in this belt is also called temperate. The midlatitudes have a high amount of seasonality, with definite summers and winters. The annual average temperatures in the midlatitudes are lower than the annual average temperatures of the tropics. Of course, the specific average temperature and how much the seasons vary depend on the latitude within the mid latitudes. Contrast Florida, for example, with Vermont, both of which are in the midlatitudes.

The third major climate belt is in the high latitudes, which includes the types of climates called sub arctic and polar. Where the high latitudes begin is arbitrary. Some would say around 50° N and 50° S, then stretching to the poles in both hemispheres. Others would say at the Arctic and Antarctic Circles (67.5° N and 67.5° S, respectively), the places that have at least one full day each year in which the sun doesn't rise and one full day each year in which the sun doesn't set. In the high latitudes, the seasons are extreme, and the annual temperatures are lower than those of the midlatitudes.

The changes in daylight hours in the high latitudes are amazing. The most dramatic examples come from the poles themselves. From March 20 to September 22, the sun is continuously above the horizon at the north pole and continuously below the horizon at the south pole. From September 22 to March 20, the sun is continuously above the horizon at the south pole and continuously below the horizon at the north pole. The south pole and north pole only have one sunrise and one sunset per year. In fact, sunsets and sunrises last almost a week at the poles. That's a nice, leisurely time to enjoy the colors in the sky, if you happen to be there and are dressed warmly enough.

We can now see how so much about our lives is determined by the simple geometric relationships among the spinning sphere of the earth, its orbit around the sun, and the rays of energy from the sun that warm our planet.

Practice problems of this concept can be found at: Climate and the Seasons Practice Questions

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