Cool autumn weather reveals nature’s true hues

The science behind why leaves change color

A favorite American pastime in fall is to pack a picnic basket and set off with loved ones on a Sunday drive along one of the nation’s many scenic byways. It’s a time of year when people enjoy crisp cool weather and marvel at the transforming landscape as tree leaves turn from lush green to gorgeous shades of yellow, orange, red, purple and brown.

Red leaves on a fall day.

Red leaves on a fall day. (Image credit: NOAA)

While we relish the opportunity to frolic in a big pile of freshly raked leaves, we don’t often think about the science behind why leaves change color and eventually fall from their branches. The answer may surprise you!

Recipe for fabulous foliage? Cool nights and sunny days

Fall foliage.

Weather factors such as temperature, sunlight, precipitation and soil moisture influence fall color arrival, duration and vibrancy. According to the United States National Arboretum, a wet growing season followed by a dry autumn filled with sunny days and cool, frostless nights results in the brightest palette of fall colors. Changes in weather can speed up, slow down or change the arrival time of fall’s colorful foliage. For example:

  • Drought conditions during late summer and early fall can trigger an early “shutdown” of trees as they prepare for winter. This causes leaves to fall early from trees without reaching their full color potential.
  • Freezing temperatures and hard frosts can kill the processes within a leaf and lead to poor fall color and an early separation from a tree.

How can you find the best place and time to see fall foliage in your area? NOAA’s National Weather Service regional forecast offices can provide you with a seven-day forecast of weather conditions — including the arrival times of different air masses — to help you determine the best location in your area for vibrant fall color.

True colors come from inside

Trees actually begin to show their true colors in autumn, and here’s why.

Fall leaves.

Trees actually begin to show their true colors in autumn, and here’s why.

The four primary pigments that produce color within a leaf are: chlorophyll (green); xanthophylls (yellow); carotenoids (orange); and anthocyanins (reds and purples). During the warmer growing seasons, leaves produce chlorophyll to help plants create energy from light. The green pigment becomes dominant and masks the other pigments. 

Trees must replenish the chlorophyll because sunlight causes it to fade over time. As days get shorter and nights become longer, trees prepare for winter and the next growing season by blocking off flow to and from a leaf’s stem. This process stops green chlorophyll from being replenished and causes the leaf’s green color to fade.

The fading green allows a leaf’s true colors to emerge, producing the dazzling array of orange, yellow, red and purple pigments we refer to as fall foliage.

Following the feast of fall colors

Fall’s color “parade” varies from region to region and year to year, depending on weather conditions. For areas under calm and dry high pressure, cool nights and sunny days can lengthen fall color displays. Cold or warm fronts can produce strong winds and heavy rain that cause leaves to fall off trees more rapidly.

If you’re planning a trip to see fall foliage, first check out the current and near-term weather conditions in your area from NOAA’s National Weather Service.

Other resources:

The United States National Arboretum is a great source of information about the pigmentation processes that occur within leaves.