Since I was a boy, autumn has always been my favorite season.  When I would tell my buddies that I liked fall better than summer, they would eye me with a strange look and say, “But you have to go to school in the fall!  Why autumn?”  Maybe it was because I could sense the changes in the air, and the changes felt mysterious and exciting.

Even now I still get excited when I see the shorter days and feel the cooler nights, and as I watch leaves turn from plain green to vivid colors of gold and red.  Now, as an on-camera meteorologist for The Weather Channel, I look on the changing season with even more wonder, knowing that nature is bringing many events together at one time to cause autumn’s colorful changes.

The Sun Starts It All

The changes start with the sun and the earth’s orbit around it.  Often people think that our temperatures get cooler in autumn because the earth moves farther from the sun.  But this is not true. What actually changes is not the distance of the earth to the sun, but the angle of the sun’s rays on the earth.   In our summer, the sun shines more directly on the top half of the earth, or northern hemisphere.  That’s when we see long warm days.  In our winter, the sun shines more directly on the bottom half of the earth, or southern hemisphere.  With less and less direct sunlight on the top half, the days here get shorter and temperatures get colder as winter nears.  Those shorter days and cooler temperatures act as a signal to the trees and plants around us to get ready for winter.

Chlorophyll: The Magic Chemical

 All summer long, a tree’s leaves have been making food for the tree so it can grow. An amazing chemical in the leaves, called chlorophyll, uses sunlight to turn carbon dioxide and water into the tree’s breakfast, lunch and dinner.   Chlorophyll gives the leaves their green color.  Also in the leaves are other chemicals with yellow and orange colors—the same chemicals that give color to some flowers, carrots, and even bananas.  You don’t see those colors in summer because there is so much green chlorophyll in the leaves hiding the other colors.

In the winter, the short days don’t provide enough sunlight for the trees to make their food, so the trees live off the food they stored during summer.  Before winter arrives, the shorter days make the trees slow down their food making, and the green chlorophyll disappears from the leaves.  That’s when the yellow and orange colors take over.  At the same time, other chemical changes may happen in the leaves, forming other colors such as red or purple.  Like an artist mixes paint for his canvas, the chemicals mix to form different colors in different trees.  Eventually some trees, but not all, lose their leaves.

How Bright Can It Get?

You may have noticed that in some years, the leaves seem more brightly colored than in others.  Again, weather is the reason.  The leaves are brightest when the late summer is warm and dry and autumn has a lot of sunny days and cool nights with temperatures in the upper thirties or low forties.  The sunny days will help the leaves make some food, but the cool nights will keep the food from moving out of the leaves.  Under those weather conditions, the trapped food will form brilliant  purple and red chemicals in the leaves. On the other hand, if the autumn days are cloudy and the nights are warm, the leaves won’t produce as much trapped food and so they will not be as colorful.  If temperatures go below freezing at night, the frost will dull the leaves’ colors. 

Sometimes leaves on the same tree may have slightly different colors from one another.  That’s because different leaves receive different amounts of sunlight, so some leaves produce more red and purple chemicals than others.  On some trees, leaves that receive a lot of sunlight may turn red, while the leaves in the shade may be yellow.  Again, it is the sun that is responsible for nature’s wide palette of colors.

Look Around

As you are playing outside or riding in your car this fall, take a closer look at the trees around you.  Take a moment to think about the colors you see and how they got there.  And as you observe the changes in the season and the changes in the weather, you just might be able to forecast how bright this year’s autumn colors will be!

And maybe autumn will become your favorite season too. 

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©Copyright 2004 Nick Walker/Small Gate Media