Behind the Forecast: How weather affects Fall colors

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Science Behind the Forecast: How weather affects Fall colors

LOUISVILLE, Ky. (WAVE) - While temperatures may still say summer, fall is right around the corner. Fall is known for the changing colors of leaves. But how does weather impact those changes?

Leaves change color in response to the decrease in sunlight. The Earth's daylight hours decrease starting on the summer solstice (June 20/21/22) and lasting through the winter solstice (December 21/22) when the day is shortest and night is longest. Photosynthesis slows down as the days get shorter in the fall months.

A lack of photosynthesis leads to a reduction in the amount of chlorophyll, the pigment that gives leaves their green hue. Less chlorophyll means other colors, like red, yellow and orange, get to shine through. The amount of red and the time that the leaf’s color is displayed is directly dependent on moisture and temperatures present before and during the time that chlorophyll amounts begin to decrease.

The best fall foliage displays come after warm, sunny days and cool (but not freezing) nights. Sunshine promotes the creation of sugars in leaves, according to the Michigan State University (MSU) Extension. Cooler nights along with narrowing leaf veins during the fall guarantees that the sugars produced during the day remain trapped in the leaf. Large amounts of sugar and light result in the creation of anthocyanin pigments that produce red, crimson, and purple colors. Carotenoid pigments are always present in leaves so yellow and gold leaf colors are less influenced by the weather.

Soil moisture content must remain adequate throughout the year for beautiful foliage displays to occur. A late spring or summer drought can delay fall color, according to the MSU Extension. Warm fall temperatures can lower the intensity of fall colors and even trigger leaf drops before the colors have the opportunity to change.

For brilliant fall colors to occur a warm, wet spring followed by calm summer weather and warm, sunny fall days with cooler nights are needed.

Here’s a breakdown of fall colors based on species from the U.S. Forest Service:

  • Oaks: red, brown, or russet
  • Hickories: golden bronze
  • Aspen and yellow-poplar: golden yellow
  • Dogwood: purplish red
  • Beech: light tan
  • Sourwood and black tupelo: crimson
  • Maple:
    • Red maple: brilliant scarlet
    • Sugar maple: orange-red
    • Black maple: glowing yellow
    • Striped maple: almost colorless

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