Solar Illumination: Seasonal and Diurnal Patterns

Earth rotates about its polar axis as it revolves around the Sun. Earth's polar axis is tilted 23.5° to the orbital plane (ecliptic plane). Combinations of rotation, revolution, and tilt of the polar axis result in differential illumination and changing illumination patterns on Earth. These changing patterns of illumination result in differential heating of Earth's surface that, in turn, creates seasonal climatic and weather patterns.

Earth's rotation results in cycles of daylight and darkness. One daylight and night cycle constitutes a diurnal cycle. Daylight and darkness are separated by a terminator—a shadowy zone of twilight. Earth's rate of rotation—approximately 24 hours—fixes the time of the overall cycle (i.e., the length of a day). However, the number of hours of daylight and darkness within each day varies depending upon latitude and season (i.e., Earth's location in its elliptical orbital path about the Sun).

On Earth's surface, a circle of illumination describes a latitude that defines an extreme boundary of perpetual daylight or perpetual darkness. Tropics are latitudes that mark the farthest northward and farthest Shadow obsuring majority of Earth, only small crescent of Earth is visible. Photograph by M. Agliolo. Reproduced by permission.
southward line of latitude where the solar zenith (the highest angle the Sun reaches in the sky during the day) corresponds to the local zenith (the point directly above the observer). At zenith, the Sun provides the most direct (most intense) illumination. Patterns of illumination and the apparent motion of the Sun on the hypothetical celestial sphere establish several key latitudes. The North Pole is located at 90° North latitude; the Arctic Circle defines an area from 66.5° N to the North Pole; the Tropic of Cancer defines an area from the Equator to 23.5° N; the Tropic of Capricorn defines an area from the equator to 23.5° S; the Antarctic Circle defines an area from 66.5° S to the South Pole.

There are seasonal differences in the amount and directness of daylight (e.g., the first day of summer always has the longest period of daylight, and the first day of winter the least amount of daylight). With regard to the Northern Hemisphere, at winter solstice (approximately December 21), Earth's North Pole is pointed away from the Sun, and sunlight falls more directly on the Southern Hemisphere. At the summer solstice (approximately June 21), Earth's North Pole is tilted toward the Sun, and sunlight falls more directly on the Northern Hemisphere At the intervening vernal and autumnal equinoxes, the both the North and South Pole are oriented so that they have the same angular relationship to the Sun and, therefore, receive equal illumination. In the Southern Hemisphere, the winter and summer solstices are exchanged so that the solstice that marks the first day of winter in the Northern Hemisphere marks the first day of summer in the Southern Hemisphere.

At autumnal equinox (approximately September 21), there is uniform illumination of Earth's surface (i.e., 12 hrs of daylight everywhere except exactly at the poles which are both illuminated). At winter solstice (approximately December 21), there is perpetual sunlight within the Antarctic Circle (i.e., the Antarctic Circle is fully illuminated). At vernal equinox (approximately March 21), the illumination patterns return to the state of the autumnal equinox. At vernal equinox, there is uniform illumination of Earth's surface (i.e., 12 hrs of daylight everywhere except exactly at the poles which are both illuminated). At summer solstice (approximately June 21), there is perpetual sunlight within the Arctic Circle (i.e., the Arctic Circle is fully illuminated).

The illumination patterns in the polar regions—within the Artic Circle and Antarctic Circle—are dynamic and inverse. As the extent of perpetual illumination (perpetual daylight) increases—to the maximum extent specified by the latitude of each circle—the extent of perpetual darkness increases within the other polar circle. For example, at winter solstice, there is no illumination within the Artic circle (i.e., perpetual night within the area 66.5° N to the North Pole). Conversely, the Antarctic Circle experiences complete daylight (i.e., perpetual daylight within the area 66.5° S to the North Pole). As Earth's axial tilt and revolution about the Sun continue to produce changes in polar axial orientation that result in a progression to the vernal equinox, the circle of perpetual darkness decreases in extent round the North Pole as the circle of perpetual daylight decreases around the South pole. At equinox, both polar regions receive the same illumination.

At the Equator, the Sun is directly overhead at local noon at both the vernal and autumnal equinox. The Tropic of Cancer and the Tropic of Capricorn denote latitudes where the Sun is directly overhead at local noon at a solstice. Along the Tropic of Cancer, the Sun is directly overhead at local noon at the June 21 solstice (the Northern Hemisphere's summer solstice and the Southern Hemisphere's winter solstice). Along the Tropic of Capricorn, the Sun is directly overhead at local noon at the December 21 solstice.

Precession of Earth's polar axis also results in a long-term precession of seasonal patterns.

Although the most dramatic changes in illumination occurs within the polar regions, the differences in daylight hours—affecting the amount of solar energy or solar insolation received—cause the greatest climatic variations in the middle latitude temperate regions. The polar and equatorial regions exhibit seasonal patterns, but these are much more uniform (i.e., either consistently cold in the polar regions or consistently hot in the near equatorial tropical regions) than the wild temperature swings found in temperate climates.

Differences in illumination are a more powerful factor in determining climatic seasonal variations than Earth's distance from the Sun. Because Earth's orbit is only slightly elliptical, the variation from the closest approach at perihelion (approximately January 3) to the farthest Earth orbital position at aphelion six months later (varies less than 3%). Because the majority of tropospheric heating occurs via conduction of heat from the surface, differing amounts of sunlight (differential levels of solar insolation) result in differential temperatures in Earth's troposphere that then drive convective currents and establish low and high pressure areas of convergence and divergence.