Behind the Forecast: How weather affects Fall colors

Listen to Science Behind the Forecast with Meteorologist Tawana Andrew every Friday on 89.3 WFPL at 7:45 a.m.
Updated: Sep. 18, 2020 at 8:11 AM EDT
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LOUISVILLE, Ky. (WAVE) - Fall is known for the changing colors of leaves. The cooler temperatures in our forecast can significantly impact those changes.

Leaves change color in response to the decrease in sunlight. The Earth’s daylight hours start decreasing on the summer solstice (June 20/21/22); this lasts 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.

Plants produce chlorophyll through photosynthesis throughout the year in order to turn sunlight into the glucose they need to survive. Chlorophyll saturates the leaves, causing them to appear green. A lack of photosynthesis leads to a reduction in the amount of chlorophyll. Less chlorophyll means the leaves' true 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, guarantee that the sugars produced during the day remain trapped in the leaf. Large amounts of sugar and light result in the creation of anthocyanin {an·thow·sai·uh·nin} pigments that produce red, crimson, and purple colors. Trees produce more anthocyanin during the fall, helping to protect their leaves and extending their time on trees.

Carotenoid pigments are always present in leaves so yellow and gold leaf colors are less influenced by the weather. One of the most common carotenoids is beta-carotene. It strongly absorbs blue and green light, reflecting red and yellow light which gives leaves their orange color.

Flavonols, part of the family of flavanoid’s proteins, always exists in leaves but aren’t seen because of the chlorophyll. Flavonols produce the yellow color in leaves.

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:

The Fall Foliage Prediction Map may help you plan to watch the colors change this autumn. According to the map, we could see a peak in fall color during the last week of October.

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