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The Chemistry Behind Autumn’s Awesome Hues

There are so, so, so many reasons to love autumn (milder weather, jackets, less crowded parks and trails, fewer mosquitos, less poison ivy, cozier camping, pumpkins, squash, gourds… we could go on forever), but the best—and certainly brightest—may be what happens to the leaves.

But what exactly does happen to them? Sure, they change from green to yellow and orange and red and even purple and magenta, but why? What’s going on within them that causes this celebrated transformation?

Wonder no more, because Compound Interest, an educational website run by UK-based chemistry teacher Andy Brunning, has created an eye-pleasing one-page graphic that plainly explains the chemicals responsible for autumn’s dazzling leaf show.

Click to enlarge:A graphic that explains the chemistry of the bright colors of autumn leaves and fall foliage.

Basically what’s going on is that flavonoids and carotenoids, the compounds responsible for fall foliage’s yellow and orange-red hues respectively, are present in leaves throughout the year, but they’re overwhelmed by high levels of vivid green chlorophyll during the summer months. Leaves can only produce chlorophyll—a necessary component of photosynthesis—in the presence of warmth and sunshine, which of course become increasingly scarce starting in late September. As chlorophyll production slows and then stops, yellow and orange flavonoids and carotenoids are allowed to shine through.

Beyond fall’s fiery hues, carotenoids are responsible for the salient shades of many foods including carrots, egg yolks, and tomatoes.

Autumn’s deepest reds, purples, and magentas are caused by anthocyanins, a type of flavonoid that unlike those discussed above is not present throughout the year. Its production is triggered only in the fall when sunlight interacts with seasonally higher concentrations of sugar in the leaves.

Head to Compound Interest to further increase the depth of your leaf-peeping knowledge.

The idea of resetting clocks forward an hour in the spring and back an hour in the fall was first suggested by Benjamin Franklin in his essay “An Economical Project for Diminishing the Cost of Light,” which was published in the Journal de Paris in April 1784.

Franklin’s suggestion was largely overlooked until it was brought up again in 1907 by Englishman William Willett, who penned a pamphlet called “The Waste of Daylight.” Although the British House of Commons rejected Willett’s proposal to advance the clock one hour in the spring and back again in autumn in 1908, British Summer Time was introduced by the Parliament in 1916.

Many other countries change their clocks when adjusting to summer time, but the United States only began doing so towards the end of World War I in an attempt to conserve energy. The House of Representatives voted 252 to 40 to pass a law “to save daylight,” with the official first daylight saving time taking place on March 15, 1918. This was initially met with much resistance, according Michael Downing, author of the book “Spring Forward: The Annual Madness of Daylight Saving Time.”

“When the Congress poked its finger into the face of every clock in the country, millions of Americans winced,” Downing wrote. “United by a determination to beat back the big hand of government,” daylight saving time opponents  “raised holy hell, vowing to return the nation to real time, normal time, farm time, sun time—the time they liked to think of as “God’s time.'”

Despite the public outcry, government officials enforced the time change until 1919, and allowed state and local governments to decide whether to continue the practice. It was reinstituted during World War II but, again, after the war the decision fell to the states.

In fact, even when Congress officially made the time change a law under the Uniform Time Act of 1966, it only stated that if the public decided to observe daylight saving time, it must do so uniformly. Hawaii and Arizona (with the exception of the Navajo Reservation), still choose not to partake in the convention, as do some U.S. territories, including American Samoa, Guam, Puerto Rico and the Virgin Islands.

Originally, clocks were sprung forward on the last Sunday in April and turned back on the last Sunday in October, but the Energy Policy Act of 2005 shifted the start of daylight saving time to the second Sunday in March and the end to the first Sunday in November.