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 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, 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 September 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 longterm precession of seasonal patterns.

Although the most dramatic changes in illumination occur 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.

See also Atmospheric composition and structure; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Revolution and rotation; Solar energy; Year, length of

Seasons

Seasons, which generally coincide with annual changes in weather patterns, are most pronounced in temperate zones. These zones extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. Within these latitudes, nature generally exhibits four seasons; spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature , amounts of precipitation , and the length of daylight.

Seasonal observations have been noted in the earliest known written records of history. In fact, seasonal changes have affected the course of history in the outcomes of battles or movements of peoples in search of longer growing seasons has often been greatly influenced by seasonal changes. Spring comes from an Old English word meaning to rise; summer originated as a Sanskrit word meaning half year or season. Autumn comes originally from an Etruscan word for maturing. Winter comes from an Old English word meaning wet or water . The equatorial regions or torrid zones have no noticeable seasonal changes and one generally finds only a wet season and a dry season in these zones. In the polar regions the seasons are closely related to the amount of sunlight received, resulting in a light season and a dark season.

Seasons are tied to the apparent movements of the Sun and stars across the celestial sphere. In the Northern Hemisphere, spring begins at the vernal equinox (around March 21) when sunlight is directly incident on the equator with equal distribution of light to the Northern and Southern Hemispheres. Summer begins at the summer solstice (approximately June 21) when the Sun is at its apparent maximum declination. Autumn begins at the autumnal equinox around September 23, and winter at the winter solstice (minimum declination in the Northern Hemisphere) that occurs approximately December 21. Because every fourth year is a leap year and February then has 29 days, the dates of these seasonal starting points change slightly. In the Southern Hemisphere, the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in June. Seasons in the Southern Hemisphere are generally milder due to the moderating presence of larger amounts of ocean surface as compared to the Northern Hemisphere.

Changes in the seasons are caused by Earth's movement around the Sun. Because Earth orbits the Sun at varying distances, many people assume that the seasons result from the changes in the Earth-Sun distance. This belief is incorrect. In fact, Earth is actually closer to the Sun in January compared to June by approximately three million miles.

Earth makes one complete revolution about the Sun each year. The major reason that the seasons occur is that the axis of Earth's rotation is tilted with respect to the plane of its orbit. This tilt, called the obliquity of Earth's axis, is 23.5 degrees from a line drawn perpendicular to the plane of Earth's orbit. As Earth orbits the Sun, there are times of the year when the North Pole is alternately tilted toward the Sun (during northern hemispheric summer) or tilted away from the Sun (during northern hemispheric winter). At other times, the axis is generally perpendicular to the incoming Sun's rays. During summer, two effects contribute to produce warmer weather. First, the Sun's rays fall more directly on Earth's surface and this results in a stronger heating effect. The second reason for the seasonal temperature differences results from the differences in the amount of daylight hours versus nighttime hours. The Sun's rays warm Earth during daylight hours; Earth cools at night by re-radiating heat back into space . This is the major reason for the warmer days of summer and cooler days of winter. The orientation of Earth's axis during summer results in longer periods of daylight and shorter periods of darkness at this time of year. At the mid-northerly latitudes, summer days have about 16 hours of warming daylight and only eight hours of cooling nights. During mid-winter the pattern is reversed, resulting in longer nights and shorter days. To demonstrate that it is the daylight versus darkness ratio that produces climates that make growing seasons possible, one should note that even in regions only 30° from the poles one finds plants such as wheat, corn, and potatoes growing. In these regions the Sun is never very high in the sky but because of the orientation of Earth's axis, the Sun remains above the horizon for periods for over 20 hours a day from late spring to late summer.

Astronomers have assigned names to the dates at which the official seasons begin. When the axis of Earth is perpendicular to the incoming Sun's rays in spring the Sun stands directly over the equator at noon. As a result, daylight hours equal nighttime hours everywhere on Earth. This gives rise to the name given to this date, the vernal equinox. Vernal refers to spring and the word equinox means equal night. On the first day of fall, the autumnal equinox also produces 12 hours of daylight and 12 hours of darkness everywhere on Earth.

The name given for the first day of summer results from the observation that as the days get longer during the spring, the Sun's height over its noon horizon increases until it reaches June 21. Then on successive days, it dips lower in the sky as Earth moves toward the autumn and winter seasons. This gives rise to the name for that date, the summer solstice, because it is as though the Sun "stands still" in its noon height above the horizon. The winter solstice is likewise named because on December 21 the Sun reaches the lowest noon time height and appears to "stand still" on that date as well.

In the past, early humans celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient Britain, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Christ, Christians in the fourth centurya.d.chose to celebrate Christmas near the winter solstice.

See also Atmospheric circulation; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Latitude and longitude; Seasonal winds; Solar illumination: Seasonal and diurnal patterns

Daylight ★★ 1996 (PG-13)

After a massive explosion seals both ends of New York's Holland Tunnel (The Tunneling In-ferno?), a small band of stock disaster flick survivors are trapped under the waters of the Hudson River. Fortunately, a cab driver with a really square jaw who, conveniently, is an ex-emergency rescue worker AND who knows the entire layout of the tunnel happens to be driving toward the tunnel at the exact moment of the disaster. Coincidence? Nope, just Hollywood. Kit Latura (Stallone) leads the survivors through cave-ins, floods, fire, rats, and poison gas (The Poison-Hiding Adventure!) only to discover to their dismay that they have emerged in New Jersey. Long on special effects but short on character development, this nod to the catastrophe movies of the '70s is still enjoyable for those who like to watch things go boom. 109m/C VHS, DVD, HD DVD . Sylvester Stallone, Viggo Mortensen, Amy Brenneman, Stan Shaw, Claire Bloom, Renoly Santiago, Sage Stallone, Dan Hedaya, Jay O. Sanders, Karen Young, Vanessa Bell Calloway, Colin Fox, Danielle Harris, Jo Anderson, Mark Rolston, Rosemary Forsyth, Barry Newman; D: Rob Cohen; W: Leslie Bohem; C: David Eggby; M: Randy Edelman.

day·light / ˈdāˌlīt/ • n. 1. the natural light of the day: [as adj.] daylight hours. ∎  the first appearance of light in the morning; dawn. ∎ fig. distance between one person or thing and another: the growing daylight between himself and the leading jockey.2. (daylights) used to emphasize the severity or thoroughness of an action: he beat the living daylights out of them.PHRASES: see daylight begin to understand what was previously unclear.

daylight daylight robbery blatant and unfair overcharging (the implication is that it is the equivalent of theft that would be carried out under cover of darkness).
daylight saving the achieving of longer evening daylight, especially in summer, by setting the clocks an hour ahead of the standard time; the originator of the system was the English builder William Willett (1865–1915). The first Daylight Saving Bill was introduced into the House of Commons in the following March, but did not become law until (as a wartime measure) 1916.