When Is Autum

This definition stays consistent every year, making it easier for scientists to compare weather patterns over decades. 3. The Phenological Start: Nature’s Cues

After the equinox, the days continue to get shorter and the nights longer until the Winter Solstice in December.

Autumn (also known as fall) generally lasts from September to December in the Northern Hemisphere, and from March to June in the Southern Hemisphere.

As the Earth tilts away from the sun, the intensity of sunlight decreases, leading to the signature "sweater weather."