Days Until Fall

Fall (autumn) officially begins at the autumnal equinox — one of two days each year when day and night are approximately equal in length. In the Northern Hemisphere, the autumnal equinox falls around September 22–23 each year. After this point, nights grow longer than days, temperatures drop, and deciduous trees begin their spectacular display of changing foliage as they prepare for winter dormancy. The chemistry behind fall foliage is fascinating: the shorter days and cooler temperatures of autumn signal deciduous trees to stop producing chlorophyll — the green pigment used for photosynthesis. As chlorophyll breaks down, the underlying yellow and orange pigments (carotenoids) that were present all summer become visible. Red and purple pigments (anthocyanins) are actually produced fresh in autumn, created from sugars trapped in the leaves as the tree seals off its leaf connections. The most spectacular fall color occurs when warm, sunny days alternate with cool (but not freezing) nights — conditions that maximize anthocyanin production. Fall is the season of harvest festivals, pumpkins, apple picking, and the distinctive smell of fallen leaves. New England in the United States is world-famous for fall foliage tourism, drawing millions of 'leaf peepers' each October. Japan's autumn foliage (koyo) season is as celebrated as spring's cherry blossoms, with people traveling to mountains and temples to view the changing maple and ginkgo trees. Oktoberfest in Germany, Thanksgiving in North America, Diwali, Halloween, and Día de los Muertos all cluster in the autumn season.

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