Explain what causes the Sun angle and length of daylight to change throughout the year. Describe how these changes result in seasonal changes in temperature.

The amount of solar energy received at any location varies with latitude, time of day, and season of the year. Contrasting images of polar bears on ice floes with palm trees along a tropical beach serve to illustrate the extremes. The unequal heating of Earth’s surface creates winds and drives ocean currents, which in turn transport heat from the tropics toward the poles in an unending attempt to balance energy inequalities.

The consequences of these processes are the phenomena we call weather. If the Sun were “turned off,” global winds and ocean currents would quickly cease. Yet as long as the Sun shines, winds will blow and weather will persist. So, to understand how the dynamic weather machine works, we must understand why different latitudes receive different quantities of solar energy and why the amount of solar energy received changes during the course of a year to produce the seasons (Figure 2.1).

Earth’s Rotation and Orbit

Earth has two basic motions—its rotation (spin) on its axis and its annual orbit (or revolution) around the Sun. Rotation, the spinning of Earth on its axis, which is an imaginary line connecting the North Pole to the South Pole, takes 24 hours (1 day) and produces the cycle of day and night.

Earth also moves in a slightly elliptical orbit around the Sun that takes about 365¼ days (1 year). Because Earth’s orbit is not perfectly circular, the distance varies during the year (Figure 2.2). Each year, on about January 3, our planet is about 147 million kilometers (91.5 million miles) from the Sun, closer than at any other time—a position called perihelion. About 6 months later, on July 4, Earth is about 152 million kilometers (94.5 million miles) from the Sun, farther away than at any other time—a position called aphelion. The average distance between Earth and the Sun is about 150 million kilometers (93 million miles).

Although Earth is closest to the Sun and receives up to 7 percent more energy in January than in July, this difference plays only a minor role in producing seasonal temperature variations, as evidenced by the fact that Earth is closest to the Sun during the Northern Hemisphere winter.

What Causes the Seasons?

If variations in the distance between the Sun and Earth do not cause seasonal temperature changes, what does? You have undoubtedly noticed that the length of daylight changes gradually throughout the year. This is more noticeable the further you get from the equator. In fact, at the North Pole, daylight is continuous from March 21 through August 21. This accounts for some of the differences in the temperatures experienced in summer versus winter: Longer daylight hours result in warmer days.

In addition, the angle (altitude) of the Sun above the horizon affects the amount of solar energy that reaches Earth’s surface. When the Sun is directly overhead (at a 90° angle), the solar rays are most concentrated and thus most intense. At lower Sun angles, the rays become more spread out and less intense. This explains why tropical areas, which experience consistently higher Sun angles throughout the year, are much warmer than polar regions, where the Sun angles are lower (Figure 2.3). You have probably experienced this when using a flashlight. If the flashlight beam strikes a surface at a 90° angle, a small intense spot is produced. By contrast, if the beam strikes at any other angle, the area illuminated is larger—but noticeably dimmer.

The angle at which the Sun strikes a location varies seasonally. For example, for someone living in Chicago, Illinois, the Sun is highest in the sky at noon on June 21–22. (Comparisons of seasonal changes in Sun angles are based on noon solar time because that is when the Sun is highest in the sky.) But as summer gives way to autumn, the noon Sun gradually appears lower in the sky, and sunset occurs earlier each evening as daylight hours decrease. The lowest noon Sun angle and earliest sunset in Chicago occur on December 21–22.

Although Chicago and other midlatitude cities in the Northern Hemisphere experience their shortest day and lowest Sun angle in late December, their lowest average temperatures are usually experienced a few weeks later, in January. The reason for this temperature lag will be discussed in Chapter 3.

Sun angle also determines the path that solar rays take as they pass through the atmosphere (Figure 2.4). When the Sun is directly overhead, the rays strike the atmosphere at a 90° angle and travel the shortest possible route to Earth’s surface. Rays entering the atmosphere at a 30° angle must travel twice this distance before reaching the surface, whereas rays entering at a 5° angle travel a distance roughly equivalent to 11 atmospheres. The longer the path the rays must travel, the greater the chance that sunlight will be dispersed (scattered) or absorbed by Earth’s atmosphere, which reduces the intensity of sunlight reaching the surface. Changes in sun angle explain why noon is the brightest part of a clear day and why light dims as sunset approaches.

Daily Changes in Earth’s Orientation to the Sun

What causes fluctuations in Sun angle and length of daylight over the course of a year? Variations occur because Earth’s orientation to the Sun continually changes. Earth’s axis (the imaginary line through the poles around which Earth rotates) is not perpendicular to the plane of its orbit around the Sun—called the plane of the ecliptic. Instead, the axis is tilted 23½° from the plane of the ecliptic, called the inclination of the axis. If the axis were not inclined, Earth would lack seasons. Because the axis is always pointed in the same direction (toward the North Star), the orientation of Earth’s axis to the Sun’s rays is constantly changing (Figure 2.5).

For example, on one day in June each year, Earth’s position in orbit is such that the Northern Hemisphere is inclined or “leaning” 23½° toward the Sun (left in Figure 2.5). Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere “leans” 23½° away from the Sun (Figure 2.5, right). On days between these extremes, the “lean” of Earth’s axis is less than 23½° relative to the rays of the Sun. This change in orientation causes the spot where the Sun’s rays are vertical (striking the atmosphere at a 90° angle) to make an annual migration from 23½° north of the equator to 23½° south of the equator. In turn, this migration causes the angle of the noon Sun to vary 47° (23½° + 23½°) for all midlatitude locations during a year. New York City, for instance, has a maximum noon Sun angle of 73½° when the Sun’s vertical rays have reached their farthest northward location in June, and a minimum noon Sun angle of 26½° 6 months later—a difference of 47° (Figure 2.6). By contrast, a city on the equator will experience an annual migration of half that amount, 23½°. Box 2.1 explains how the angle of the noon Sun can be calculated for a given latitude.

Solstices and Equinoxes

Based on Earth’s position in orbit and the annual migration of the vertical rays of the Sun, 4 days each year are especially significant. On June 21 or 22, the vertical rays of the Sun strike 23½° north latitude (23½° north of the equator), a line of latitude known as the Tropic of Cancer (Figure 2.5). For people living in the Northern Hemisphere, June 21 or 22 is known as the summer solstice, the first “official” day of summer (Box 2.2).

Six months later, on December 21 or 22, Earth tilts (or leans) in the opposite direction, so the Sun’s vertical rays strike at 23½° south latitude. (Recall that Earth’s axis always points in the same direction; it is Earth’s changing position relative to the Sun that causes the apparent change in tilt.) This line of latitude is known as the Tropic of Capricorn. In the Northern Hemisphere, December 21 or 22 is the winter solstice, the first day of winter. However, on this same day, the Southern Hemisphere is experiencing its summer solstice.

The equinoxes occur midway between the solstices. September 22 or 23 is the date of the fall (autumnal) equinox in the Northern Hemisphere, and March 21 or 22 is the date of the spring (vernal) equinox. On these dates, the vertical rays of the Sun strike the equator (0° latitude) because Earth’s position is such that its axis is tilted neither toward nor away from the Sun.

The length of daylight versus darkness is also determined by the position of Earth relative to the Sun’s rays. The length of daylight on the summer solstice in the Northern Hemisphere, June 21, is greater than the length of night. This fact can be established by examining Figure 2.6, which illustrates the circle of illumination—that is, the boundary separating the dark half of Earth from the lighted half. The length of daylight is established by comparing the fraction of a line of latitude that is on the “day” side of the circle of illumination with the fraction on the “night” side. Notice that on June 21, all locations in the Northern Hemisphere experience longer periods of daylight than darkness (Table 2.1). By contrast, during the Northern Hemisphere winter solstice in December, the length of darkness exceeds the length of daylight at all locations in the hemisphere.

The seasonal changes in length of daylight and the Sun’s angle are the primary causes of the month-to-month variations in temperature observed at most locations. This is illustrated in Figure 2.7, which depicts the daily paths of the Sun over the seasons of the year for a location at 40° north latitude—New York City, for example. Notice that the length of daylight is longest during the summer solstice, whereas the opposite is true for the winter solstice. (Compare the length of the Sun’s path [yellow line] for each of these days.) New York City has about 15 hours of daylight on June 21, compared to 9 hours on December 21. In addition, the maximum Sun angle is 73½°on June 21, and only 26½°on December 21.

Also note from Table 2.1 that the farther north a location is from the equator on June 21, the longer the period of daylight it will experience. On this date, places located on or north of the Arctic Circle (66½° north latitude), experience the midnight Sun, a natural phenomenon in which the Sun is visible at midnight (Figure 2.8A). In fact, on this date the Sun does not set for a period that ranges from 1 day at the Arctic Circle, to about 4 months at 80° north latitude, and 6 months at the pole. Figure 2.8B shows the path of the Sun as seen by an observer located at 80° north latitude during the summer solstice. Notice that the Sun does not drop below the horizon at any time during the entire day. The Antarctic Circle, which is the corresponding latitude in the Southern Hemisphere, experiences the opposite situation—total darkness on June 21.

Figure 2.9 summarizes the characteristics of the solstices and equinoxes for a location in the Northern Hemisphere. When you examine Figure 2.9, you will see why a midlatitude location is warmest in the summer, when the days are longest and the angle of the Sun above the horizon is highest. Near the winter solstice, the reverse occurs: The days are shortest, and the Sun angle is lowest. During an equinox (meaning “equal night”), the length of daylight is 12 hours everywhere on Earth because the circle of illumination passes directly through the poles, thus dividing the lines of latitude in half.

All locations situated at the same latitude have identical Sun angles and lengths of daylight. If the Earth–Sun relationships were the only controls of temperature, we would expect these places to have identical temperatures as well. This is not the case. Other factors, such as a location’s elevation or its proximity to a large body of water, also influence local temperature and will be addressed in Chapter 3.

• Antarctic Circle: The parallel of latitude, 66½° south latitude, marking the northernmost location where the midnight Sun is visible on the southern summer solstice.

• aphelion: The point in the orbit of a planet that is farthest from the Sun.

• Arctic Circle: The parallel of latitude, 66½° north latitude, marking the southernmost location where the midnight sun is visible on the northern summer solstice.

• circle of illumination: The line (great circle) separating daylight from darkness on Earth.

• equinox: The point in time when the vertical rays of the Sun are striking the equator. In the Northern Hemisphere, March 20 or 21 is the vernal, or spring, equinox, and September 22 or 23 is the autumnal equinox. Lengths of daylight and darkness are equal at all latitudes at equinox.

• orbit: The elliptical path that Earth travels around the sun once each year.

• perihelion: The point in the orbit of a planet closest to the Sun.

• rotation: The spinning of a body, such as Earth, about its axis.

• solstice: The point in time when the vertical rays of the Sun are striking either the Tropic of Cancer (summer solstice in the Northern Hemisphere) or the Tropic of Capricorn (winter solstice in the Northern Hemisphere). Solstice represents the longest or shortest day (length of daylight) of the year.

• Tropic of Cancer: The parallel of latitude, 23½° north latitude, marking the northern limit of the Sun’s vertical rays.

• Tropic of Capricorn: The parallel of latitude, 23½° south latitude, marking the southern limit of the Sun’s vertical rays.

• The seasons are caused by changes in the angle at which the Sun’s rays strike Earth’s surface and the changes in the length of daylight at each latitude. These seasonal changes result from the tilt of Earth’s axis as it revolves around the Sun.

• When the Sun is directly overhead (at a 90° angle to Earth’s surface), the solar rays are most concentrated and thus most intense. At lower Sun angles, the rays become more spread out and less intense.