Autumn

Autumn is one of the four seasons of the year in regions with seasonal climate variation. It falls between the end of summer and the beginning of winter. Its defining characteristics are shortening days, lengthening nights, and a gradual drop in temperature.

The seasons arise from the tilt of Earth's axis (approximately 23.4 degrees) relative to the plane of Earth's orbit around the sun. For this reason, when the Northern Hemisphere is in autumn, the Southern Hemisphere is in spring. Consequently the seasons are opposite between the two hemispheres.

Astronomical dating

Astronomically, autumn begins at the autumnal equinox, the day when day and night are nearly equal in length, and ends at the winter solstice. In the Northern Hemisphere this falls between mid-September and mid-December. Meteorologically, the season is typically defined as three full months: September, October, and November.

Characteristics and life

In temperate regions, tree foliage changes color and falls as plants stop producing chlorophyll. Many animals store food or prepare for winter hibernation during this season; many birds migrate to warmer regions. In many parts of the world, autumn is the harvest season for agriculture.

Use of the Somali term

In Somali, the word dayr typically refers to one of the four seasons in the Somali climate calendar: gu', xagaa, dayr, and jiilaal. The Somali dayr is a season of moderate rainfall that follows xagaa (dry season) and is used for pasture and agriculture. For this reason, the local meaning and the astronomical meaning of 'autumn' are not equivalent, although they fall near the same time of year.