Sandwiched between blazing summer and chilly winter, autumn is the "cooling off" season. Nighttime arrives earlier, temperatures begin to drop and most vegetative growth decreases. Animals begin to prepare for the dearth of food that generally comes during the winter, gathering supplies or traveling to warmer climates.
The cycle of seasons is caused by Earth's tilt on its axis and the planet's orbit around the sun. When the axis points toward the sun, that hemisphere experiences summer. The hemisphere tilted away from the sun experiences winter. [INFOGRAPHIC: Earth's Solstices & Equinoxes Explained]
After Earth travels a quarter of the way around the sun, the axis is pointed along the planet's path, parallel to the star. At the equator, the sun is directly overhead at noon. These moments in time are called equinoxes, when the length of daylight and nighttime are approximately the same.
"Astronomical autumn" begins with the autumnal equinox and ends with the winter solstice. In the Northern Hemisphere, that is from around September 22 to about December 22, though it varies from year to year. In the Southern Hemisphere, the season runs from about March 20 to June 21. Some cultures, however, consider the equinox to be the midpoint of the season.
Weather forecasters and some countries define the season as "meteorological autumn" by the three months in which the weather changes. In the Northern Hemisphere, this is September, October, and November. In the Southern Hemisphere, meteorological autumn occurs in March, April, and May.
Autumn brings a nip in the air. How much of a change is felt depends on the location of the area on Earth. Regions near the equator experience fairly constant temperatures throughout the year, while those farther north or south experience greater variations.
For the continental United States, autumn temperatures average 53.9 degrees F (12.2 C) — an 18-degree (10 degrees C) drop from average summer temperatures. Average autumn temperatures range from a high of 72.7 F (22.6 C) in Florida to a low of 26.7 F (minus 2.9 C) in Alaska.
Why do leaves change color?
Autumn is also known as "fall," particularly in the United States, because during autumn, leaves change color and fall from the trees.
In response to chilly temperatures and fewer daylight hours, leaves stop producing green-tinted chlorophyll, which allows them to capture sunlight and make energy. Because chlorophyll is sensitive to the cold, certain weather conditions like early frosts will turn off production more quickly.
Meanwhile, orange and yellow pigments called carotenoids — also found in orange carrots — shine through the leaves' washed out green. The red color in some leaves comes from anthocyanins, which unlike carotenoids, are only produced in the fall. They also give color to strawberries, red apples, and plums. [Image Gallery: A Rainbow of Fall Leaves]
But red leaves are also signal of distress. If you see leaves of a tree turning red early, in late August, the tree is most likely suffering from a fungus or perhaps a ding from a reckless driver. [Turning Leaves: The Rich Colors of Fall Foliage]
A time of celebration
Autumn is generally regarded as the end of the growing season. Also known as the harvest season, autumn ushers in a time of celebration for many farming cultures when they gathered in their crops.
Thanksgiving is one such holiday, celebrated in the United States and Canada. Native American tribes engaged in a variety of festivals.
In China, Taiwan, and Vietnam, the Mid-Autumn, or Moon, Festival celebrates the end of the fall harvest, and occurs near the fall equinox.
The people of Britain celebrate a harvest festival around the first full moon near the fall equinox.
Sukkot, or the Feast of Tabernacles, celebrated by those of the Jewish faith, lasts seven days and takes its roots from agriculture.
The season is often regarded as a melancholy time and has inspired many writers and poets. Here are some quotes about autumn:
"Autumn is a second spring when every leaf is a flower." — Albert Camus
"Delicious autumn! My very soul is wedded to it, and if I were a bird I would fly about the earth seeking successive autumns." — George Eliot
"Autumn … the year's last, loveliest smile." — William Cullen Bryant
"Every leaf speaks bliss to me, fluttering from the autumn tree." — Emily Bronte
"Now Autumn's fire burns slowly along the woods and day by day the dead leaves fall and melt." — William Allingham
"Autumn in New York, why does it seem so inviting?" — Vernon Duke
"Summer ends, and autumn comes, and he who would have it otherwise would have high tide always and a full moon every night." — Hal Borland
"I saw old Autumn in the misty morn stand shadowless like silence, listening to silence." — Thomas Hood
"Autumn is the mellower season, and what we lose in flowers we more than gain in fruits." — Samuel Butler
"Autumn is as joyful and sweet as an untimely end." — Remy de Gourmont
— Nola Taylor Redd, LiveScience Contributor