Autumn, also known as fall in North American English, is one of the four temperate seasons. Outside the tropics, autumn marks the transition from summer to winter, in September (Northern Hemisphere) or March (Southern Hemisphere), when the duration of daylight becomes noticeably shorter and the temperature cools considerably. Day length decreases and night length increases as the season progresses, until the Winter Solstice in December (Northern Hemisphere) and June (Southern Hemisphere). One of its main features in temperate climates is the shedding of leaves from deciduous trees.
Some cultures regard the autumnal equinox as “mid-autumn”, while others with a longer temperature lag treat the equinox as the start of autumn. In the English-speaking world, Autumn traditionally began with Lammas Day and ended around Hallowe’en, the approximate mid-points between Midsummer, the autumnal equinox, and Midwinter. Meteorologists (and most of the temperate countries in the southern hemisphere) use a definition based on Gregorian calendar months, with autumn being September, October, and November in the northern hemisphere,and March, April, and May in the southern hemisphere.