Nearly all the energy that drives Earth’s variable weather and climate comes from the Sun. Earth intercepts only a minute percentage of the energy given off by the Sun—less than 1 two-billionth, or 0.00000005%. This may seem to be an insignificant amount until we realize that it is more than ten thousand times the electrical-generating capacity of the world.

Solar energy is not distributed evenly over Earth’s land–sea surface. The amount of energy received varies with latitude, time of day, and season of the year. Contrasting images of polar bears on ice rafts and palm trees along a remote tropical beach serve to illustrate the extremes. It is the unequal heating of Earth that creates winds and drives the ocean’s currents. These movements of air and water, 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, the winds will blow and weather will persist. So, to understand how the atmosphere’s dynamic weather machine works, we must first know why different latitudes receive varying quantities of solar energy and why the amount of solar energy changes to produce the seasons. As you will see, the variations in solar heating are caused by the motions of Earth relative to the Sun and by variations in Earth’s land–sea surface.

Earth’s Motions

Earth has two principal motions—its rotation about its axis and its orbital motion around the Sun. The axis is an imaginary line running through the poles. Our planet rotates on its axis once every 24 hours, producing the daily cycle of daylight and darkness. At any moment, half of Earth is experiencing daylight and the other half darkness. The line separating the dark half of Earth from the lighted half is called the circle of illumination.

Each year, Earth makes one slightly elliptical orbit around the Sun. The distance between Earth and Sun averages about 150 million kilometers (93 million miles). Because Earth’s orbit is not perfectly circular, however, the distance varies during the course of a year. Each year, on about January 3, our planet is about 147.3 million kilometers (91.5 million miles) from the Sun, which is closer than at any other time—a position known as perihelion. About  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. 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 are not responsible for seasonal temperature changes, what is? The gradual but significant change in the length of daylight certainly accounts for some of the difference we notice between summer and winter. Furthermore, a gradual change in the angle (altitude) of the Sun above the horizon is also a contributing factor ▼. For example, someone living in Chicago, Illinois, experiences the noon Sun highest in the sky in late June. But as summer gives way to autumn, the noon Sun appears lower in the sky, and sunset occurs earlier each evening.

The seasonal variation in the angle of the Sun above the horizon affects the amount of energy received at Earth’s surface in two ways. First, when the Sun is directly overhead (at a 90-degree angle), the solar rays are most concentrated and thus most intense. The lower the angle, the more spread out and less intense is the solar radiation that reaches the surface ▼A. To illustrate this principle, hold a flashlight at a right angle to a surface and then change the angle (▼B).

Second, but of lesser importance, the angle of the Sun determines the path solar rays take as they pass through the atmosphere ▼. When the Sun is directly overhead, the rays strike the atmosphere at a 90-degree angle and travel the shortest possible route to the surface. This distance is referred to as 1 atmosphere. However, rays entering at a 30-degree angle travel through twice this distance before reaching the surface, while rays at a 5-degree angle travel through a distance roughly equal to the thickness of 11 atmospheres. The longer the path, the greater the chance that sunlight will be dispersed by the atmosphere, which reduces the intensity at the surface. These conditions account for the fact that we cannot look directly at the midday Sun, but we can enjoy gazing at a sunset.

It is important to remember that Earth’s shape is spherical. On any given day, the only places that will receive vertical (90-degree) rays from the Sun are located along one particular line of latitude. As we move either north or south of this location, the Sun’s rays strike at ever-decreasing angles. The nearer a place is to the latitude receiving vertical rays of the Sun, the higher will be its noon Sun and the more concentrated will be the radiation it receives ▲.

Earth’s Orientation

What causes fluctuations in Sun angle and length of daylight that occur during the course of a year? Variations occur because Earth’s orientation to the Sun continually changes as it travels along its orbit. 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. Instead, it is tilted 23.5 degrees from the perpendicular, as shown in the figure below ▼. This is called the inclination of the axis. If the axis were not inclined, Earth would lack seasons. Because the axis remains pointed in the same direction (toward the North Star), the orientation of Earth’s axis to the Sun’s rays is constantly changing ▼.

For example, on one day in June each year, the axis is such that the Northern Hemisphere is “leaning” 23.5 degrees toward the Sun (▲, left). Six months later, in December, when Earth has moved to the opposite side of its orbit, the Northern Hemisphere “leans” 23.5 degrees away from the Sun (refer to ▲, right). On days between these extremes, Earth’s axis is leaning at amounts less than 23.5 degrees to the rays of the Sun. This change in orientation causes the spot where the Sun’s rays are vertical to make an annual migration from 23.5 degrees north of the equator to 23.5 degrees south of the equator.

In turn, this migration causes the angle of the noon Sun to vary by up to 47 degrees (23.5 degrees + 23.5 degrees) for many locations during the year. For example, a midlatitude city like New York (about 40 degrees north latitude) has a maximum noon Sun angle of 73.5 degrees when the Sun’s vertical rays reach their farthest northward location in June and a minimum noon Sun angle of 26.5 degrees  months later.

Solstices and Equinoxes

Historically, four days each year have been given special significance, based on the annual migration of the direct rays of the Sun and its importance to the yearly weather cycle. On June 21 or 22, Earth is in a position such that the north end of its axis is tilted 23.5 toward the Sun (▼A). At this time, the vertical rays of the Sun strike 23.5 degrees north latitude (23.5 degrees north of the equator), a latitude known as the Tropic of Cancer. For people in the Northern Hemisphere, June 21 or 22 is known as the summer solstice, the first “official” day of summer.

Six months later, on about December 21 or 22, Earth is in the opposite position, with the Sun’s vertical rays striking at 23.5 degrees south latitude (Figure ▲B). This parallel is known as the Tropic of Capricorn. For those in the Northern Hemisphere, December 21 and 22 is the winter solstice. However, at the same time in the Southern Hemisphere, people are experiencing just the opposite—the summer solstice.

Midway between the solstices are the equinoxes. September 22 or 23 is the date of the fall equinox in the Northern Hemisphere, and March 21 or 22 is the date of the spring equinox. On these dates, the vertical rays of the Sun strike the equator (0° latitude) because Earth is in such a position in its orbit that the axis is tilted neither toward nor away from the Sun (▲C).

The length of daylight versus darkness is also determined by Earth’s position in orbit. The length of daylight on June 21 or 22, the summer solstice in the Northern Hemisphere, is greater than the length of night. This fact can be established from ▲A by comparing the fraction of a given latitude that is on the “day” side of the circle of illumination with the fraction on the “night” side. The opposite is true for the winter solstice, when the nights are longer than the days. Again, for comparison, let us consider New York City, which has about 15 hours of daylight on June 21 and only about 9 hours on December 21. (You can see this in ▲ and Table 1.) Also note from Table 1 that the farther north of the equator you are on June 21, the longer the period of daylight. When you reach the Arctic Circle (north latitude), the length of daylight is 24 hours.

This is the land of the “midnight Sun,” which does not set for about 6 months at the North Pole ▼.

The figure below ▼ summarizes the characteristics of the solstices and equinoxes for a location in the Northern Hemisphere. An examination of Figure 16.18 should make it apparent 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. The winter solstice facts are the reverse: 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. Obviously, such is not the case. Although the angle of the Sun above the horizon and the length of daylight are very important controls of temperature, other factors must be considered.

In summary, seasonal fluctuations in the amount of solar energy reaching various places on Earth’s surface are caused by the tilt of Earth’s axis relative to the Sun and the resulting variations in Sun angle and length of daylight.

• The two principal motions of Earth are (1) rotation about its axis, which produces the daily cycle of daylight and darkness, and (2) orbital motion around the Sun, which produces yearly variations.

• 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 are the result of the tilt of Earth’s axis as it orbits the Sun.

• circle of illumination: The boundary that separates daylight from darkness on Earth.

• fall equinox: One of two times of the year when the sun is exactly over the equator and day and night are of equal length. Occurs on about September 22 or 23 in the Northern Hemisphere and March 21 or 22 in the Southern Hemisphere. Also called the autumnal equinox.

• inclination of the axis: The tilt of Earth’s axis from the perpendicular to the plane of Earth’s orbit.

• revolution: The motion of one body about another, such as Earth’s orbit around the Sun.

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

• spring equinox: One of two times of the year when the sun is exactly over the equator and day and night are of equal length. Occurs on about March 21 or 22 in the Northern Hemisphere and  September 22 or 23 in the Southern Hemisphere.

• summer solstice: One of two times of the year when the sun is directly over the Tropic of Cancer or the Tropic of Capricorn, marked by the shortest or longest daylight hours of the year. Occurs on about June 21 or 22 in the Northern Hemisphere and December 21 or 22 in the Southern Hemisphere.

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

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

• winter solstice: One of two times of the year when the sun is directly over the Tropic of Cancer or the Tropic of Capricorn, marked by the shortest or longest daylight hours of the year. Occurs on about December 21 or 22 in the Northern Hemisphere and  June 21 or 22 in the Southern Hemisphere.