Meteorological Fall, Plus Why Leaves Change

Color spotter, Samantha Lindberg recently used the phrase “meteorological fall” and it peaked my interest. After doing a little research it seems the term refers to a season created by scientists (meteorologists). It begins September 1 and ends November 30, according to wane.com, because this is the time period that the general public associates with “fall.”

While the date of the actual autumn equinox changes every year, meteorological fall stays the same. The equinox arrives on September 22 in 2023 in California, but with cooler temperatures in the state this Labor Day Weekend, perhaps the scientists behind meteorological fall are on to something.

Either way, the reason leaves change color throughout the season stays the same. Blake Engelhardt with the US Forest Service explains.

“During the summer growing season the leaves are constantly producing chlorophyll, which gives them their green color,” she says. “In the fall, longer nights trigger the formation of the abscission layer.”

According to Engelhardt, this is basically a thickened layer of cells at the base of the leaf where it attaches to the stem that blocks the transport of water and sugar into the leaf.

“Thin, deciduous leaves won’t survive the freezing temperatures of winter, so there’s no reason to keep feeding them,” she adds. “Once chlorophyll synthesis stops, the green quickly disappears and other more persistent pigments become visible yellows, reds, purples.”

Yellow comes from Xanthophylls (compounds) and Flavonols (proteins) that reflect yellow light.

Orange is found in leaves with lots of beta-carotene, a compound that absorbs blue and green light and reflects yellow and red light, giving the leaves their orange color.

Red comes from the Anthocyanin compound.

The best fall color occurs when days are warm and nights are clear and cold. California’s cloudless skies and extreme range of elevations (sea level to 14,000′) provide ideal conditions for the development of consistently vivid fall color, as seen in these reports. 

Peak fall color will begin appearing in the Eastern Sierra above 9,000 feet (you can drive right to it) during the last two weeks of September.