Yellow and orange were hidden all summer
Carotenoid pigments are present year-round but masked by green chlorophyll until autumn.
Nature & Biology
Why Do Leaves Change Color in Autumn?
Leaves do not suddenly invent yellow and orange in autumn. Those colors were mostly there already, hidden until chlorophyll breaks down.
The short answer
Leaves are green in summer because they are packed with chlorophyll, the molecule that absorbs sunlight for photosynthesis. Chlorophyll reflects green light, which is what you see. Yellow and orange pigments were in many leaves the whole time, hidden by the stronger green. In autumn, shorter days and cooler temperatures signal trees to shut down their leaves. Without fresh chlorophyll being produced, existing chlorophyll breaks down and the green fades. The yellow and orange carotenoids are more stable and become visible. Red is different. Red anthocyanin pigment is produced fresh in autumn when sugars become trapped in the leaf. Warm sunny days and cool nights create the brightest reds because they promote sugar accumulation and anthocyanin production.
Red is made fresh in autumn
Anthocyanins that cause red and purple color form from trapped sugars in cool, bright conditions.
Myth: cold temperatures cause the color change
Cold affects intensity, but shorter day length is the main seasonal trigger.
Why some years are more colorful
Warm sunny days and cool but not freezing nights produce the strongest autumn color.
Related Articles
Related why questions
Nature & Physics
Why Is Snow White?
Snow is white because millions of tiny ice crystals scatter all wavelengths of light equally in every direction. Your eyes receive the full spectrum at once, which looks white.
snowlight
Weather & Physics
Why Does Lightning Happen?
Lightning happens when electrical charges separate inside storm clouds and the voltage difference becomes large enough to discharge as a massive spark.
lightningthunder
Earth & Geology
Why Do Volcanoes Erupt?
Volcanoes erupt because molten rock from inside Earth finds a path to the surface, driven by buoyancy, pressure, and expanding gases.
volcanoesmagma
Body & Immune System
Why Do Mosquito Bites Itch?
The itch is not from the bite itself. It comes from your own immune system reacting to mosquito saliva. Here is what is actually happening under your skin.
mosquitoitch
Body & Senses
Why Do Ears Pop on Airplanes?
Your ears pop because the air pressure outside changes faster than the pressure inside your middle ear can equalize. Here is what is happening and how to fix it.
earspressure
Body & Nervous System
Why Do We Blush?
Blushing is an involuntary nervous system response that floods your face with blood. You cannot stop it on command, and that is by design.
blushingembarrassment
Keep Exploring
More ways to keep going
Jump back to this shelf, browse generated topics, or let TinyThat choose the next question.