Autumn

Autumn, also known as fall in North American English, is one of the four temperate seasons on Earth. Outside the tropics, autumn marks the transition from summer to winter, beginning in September (Northern Hemisphere) or March (Southern Hemisphere). Autumn is the season when the duration of daytime 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 striking change in colour of the leaves of deciduous trees as they prepare to shed.

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 of high latitude countries, autumn traditionally began with Lammas Day and ended around Halloween, the approximate mid-points between midsummer, the autumnal equinox, and midwinter. Meteorologists (and Australia 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.

In the higher latitude countries in the Northern Hemisphere, autumn traditionally starts with the September equinox (21 to 24 September) and ends with the winter solstice (21 or 22 December). In the United States, Labor Day (the first Monday in September) is often informally treated as the end of summer and start of autumn; certain summer traditions, such as wearing white, may be frowned upon after that date. As daytime and nighttime temperatures decrease, trees change colour and then shed their leaves.