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Wednesday, August 27, 2014
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Published: Wednesday, 10/24/2007

The science of fall color

When the car's trunk is full of pumpkins, you also know the trees are full of colorful leaves. Usually, the third week in October brings the peak of color in southeast Michigan and northwest Ohio. But, raise your hand if you want to know why the leaves change.

OK, hands down. The leaves are actually dying, and they go out in a blaze of color. When the tree starts to slow down and doesn't send sugar water through its system as fast, the reds and yellows will show up. These showy colors are always there, but when the tree is growing strong in the spring and summer, the leaves are flooded with green chlorophyll.

Leaves start to turn colors because they get less sunlight. The best weather for leaf-turning is to have bright days and cold nights. If the high temperature is in the mid-60s or low 70s and the lows are between 35 and 50, the trees will start turning.

The lower sun angle and shorter days cause a chemical change in plants.

Sept. 23 was the official fall equinox, where we get 12 hours of daylight and night because the earth is perpendicular to the sun. As the calendar moves closer to the winter solstice, the shadows get longer and the days get shorter. Dec. 22 brings the winter solstice and the shortest day of the year, with only nine hours of sunlight.

Beech, hickory, and birch trees turn yellow, orange, and brown. They turn these colors because their chemical leaf structure contains carotenoid pigments. Trees that have a lot of sugar in them, like sweetgum, dogwood, maple, and oak, will turn red and purple. They have anthocyanin pigments in their leaves and will turn darker colors.

Yellows come from xanthophyll pigments, and the brown colors usually signal the end of the road. These are the tannins in the plant and are considered its waste product.

Fall also is a good time to see some spectacular sunsets. Raise your hand if you want to know what causes the vibrant sunrise and sunset color. OK, hands down. The National Weather Service credits the color to the angle of the sun. We usually see blue skies during the day. Ordinary sunlight goes the whole color spectrum, from red to violet. Our eyes see the light scatter off the air molecules and it bounces back to our eyes. The color is really in the violet spectrum, but our eyes are more sensitive to blue instead, so we see blue during the day.

The color changes as the sun goes down. Since the sun isn't overhead, it has to go through more of the atmosphere. And a longer sun angle means that little beam of light will have a lot more particles to go through.

A lot of the violet and blue part of the light spectrum is lost along the way, so the only thing that is left will be red, yellow, and orange. There's a scientific name for all of this. Its called Rayleigh scattering, named after a British physicist and math wizard who lived back in the 1800s.

Dust also changes the color. The light bumps into tiny bits of dust and pollution and scatters the light into pretty reds and orange colors. Scientist say you can see the most intense blues around 10 a.m. and 3 p.m. on cloudless days. Reds and oranges are intensified in the fall when the sun is closer to the earth, but at a sharp angle.



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