For leaf peepers Fall is the time for the best leaves. They turn from bright green to yellow, then orange and if the conditions are right red and purple too. But what makes fall colors so spectacular?

Science can explain.

First just about everybody knows that chlorophyll gives leaves their green color. But chlorophyll is more than just a molecule that gives plants a green pigment. Chlorophyll absorbs sunlight and uses its energy to create carbohydrates from carbon dioxide and water. Trees drink in water through their roots and they breathe in carbon dioxide from the air.

That process we all know is called photosynthesis. It’s what all land-based and many sea-based plants rely on for food. That’s why we call leaves nature’s food factories.

Like all things in nature plants and trees are built for efficiency. As the hours of sunlight decrease during the second half of the year deciduous trees begin to conserve energy so they don’t freeze during the winter. They accomplish this by shedding their leaves. But each tree doesn’t consciously decide to drop the excess baggage. That’s also part of the process which leads to fall foliage being so brilliant.

Before dropping their leaves, trees slow down production in their food factories by producing less chlorophyll. That allows other colors to become visible. First, the yellow pigment which was there all along begins to emerge. That is created by molecules called carotenoids. The two common leaf carotenoids are carotene and xanthophyll.

They are the pigments that gives carrots and sweet potatoes their color. Carotenoids are masked by chlorophyll until fall approaches. When the chlorophyll production slows down carotenoids become visible. They are responsible for yellow and orange leaves.

Ever present but hidden by other colors is another pigment called anthocyanin. Anthocyanin is part of a third pigment family called flavonoids. Lycopene is a flavonoid that gives tomatoes their red color. But anthocyanin acts as the natural sunscreen for plants, blocking harmful ultraviolet rays from the sun. As chlorophyll production shuts down for the year, anthocyanin production ramps up. These give tree leaves their deep red, magenta and purple colors. That increase in anthocyanin is in response to surging sugar concentrations, building up in the leaves.

That glucose glut tends to concentrate where leaf meets stem, making the extra weight a target point for wind or rain to knock the leaves from the trees. After all, that’s the trees’ goal. Get rid of the leaves so they can survive the sunless winter without dying of starvation or freezing to death.

While trees go through this incredibly complicated process to get ready for winter, we marvel at their transformation from vibrant green blowing in the mid-summer sun to yellows, oranges, reds and purples buffeted by fall storms. Perhaps we watch in awe as the slow and steady march of time goes on before our very eyes.

Key Leaf Chemistry

The color of leaf we get is highly dependent on the types and amounts of pigments that are present. Internal plant chemical interactions, especially from variable soil acidity or pH level can have a dramatic impact on leaf color.

With the steadily decreasing length of day and cooler temperatures as fall approaches trees biologically trigger the formation of a corky layer of cells called the abcission layer across the base of the leaf where it attaches to the tree. This formation gradually decreases the supply of water and minerals to the leaf, reduces the manufacture of chlorophyll and traps sugars in the leaf.

In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight during the day and the cool nights of autumn cause the leaves turn this glucose into a red color. With enough sunny days, those anthocyanins turn leaves bright reds and purples.

The muted brown color of oak leaves is made from wastes left in those leaves.

Why is so hard to predict when the leaves will be the best and brightest? In short, it depends on the weather.

Warm wet springs, summers that don’t scorch and bright sunny fall days with cool nights all lend to the brilliance of fall color. The degree of visual splendor and leaf vibrancy requires a lot of variables lining up just right. And when they do, the beauty is unparalleled.

Phenology of Fall Foliage

Phenology is the study of the timing of nature. This ranges from the dates that tulips bloom in the spring to the fall migration of Canadian geese. And it definitely includes watching the leaves turn color in the fall.

But scientists have been struggling to find a direct link between later leaf drops and climate change in the U.S. Europe and Asia have been conducting studies that seem to point to later fall foliage. But it’s been a difficult case to prove in the U.S. Part of that reason could be the wildly fluctuating weather from year to year.

A wet spring, followed by the deluge of Hurricane Irene, a mind-summer drought and normal-ish fall have trees in New England all confused. Some tree leaves are bypassing fall colors, turning brown and dropping early. Some plant biologists believe that’s due to a fungus that relished the wetness and took hold.

Those factors make identifying long term trends difficult at best.

Several organizations are enlisting the help of the public to collect data in hopes of finding the signal for such change in the noise or fluctuating weather.

According to the Wall Street Journal:

“There are signs everywhere that things are changing — how is the question. Some species are being affected while others are not,” said Esperanza Stancioff of the University of Maine cooperative extension and Maine Sea Grant, who has trained 195 citizen scientists to enter data online in her “Signs of the Season” phenology project.

To assist both backyard observers and researchers alike, the National Phenology Network has spent the last four years coming up with standards to be used by observers in reporting foliage color changes. Final tweaks on the uniform reporting standards should be completed in a few weeks, Weltzin said.

Another part of the effort to study climate change through the lens of fall foliage is being conducted from space by the U.S. Geological Survey utilizing satellites from NASA and the National Oceanic and Atmospheric Administration.

Right now, the effort is focused on Shenandoah National Park in Virginia, where scientists are attempting to understand the factors that go into the metrics to ensure proper analysis of the photos taken from above, said John W. Jones, a research geographer with the USGS outside of Washington, D.C.

2011 has been the deadliest tornado year in over 50 years. Already 1,151 twisters have formed and cut a wide swath of destruction across many states. That is more than double the average. And 481 people have died as a result of severe weather this year. That is more than six times the average and makes it the deadliest season since 1953.

Joplin, Missouri is the latest urban center to be demolished, following a very rare April tornado outbreak that leveled Tuscaloosa, Alabama.

Stan Gedzelman, a meteorologist at the City College of New York doesn’t see anything unusual in this year’s weather pattern that would indicate an especially severe weather spring. He also believes the warning system is performing as well as it could, giving residents up to 20 minutes to prepare for an incoming tornado.

But he says the instability in the atmosphere that allows these super storms to form is just about as large as he’s ever seen.

Tornadic activity occurs when cool dry air from the north collides with warm moist air from the Gulf of Mexico. That convergences creates convection as cold air sinks and warm air rises. Spinning storm cells can spit out tornadoes.

The Joplin tornado was an F-4 tornado with winds approaching 200 miles per hour that cut a six-mile long debris track three-quarters-of-a-mile wide through a suburban and commercial area of the city of 47,000 in the southwest corner Missouri.

The Associated Press science reporter Randy Schmid explains why the Joplin tornado was so destructive.

Early results from the late April burst of tornadoes that ripped across the southeast, killing over 300 show that 2011 is already the third worst tornado season on record. And it isn’t over yet. The month of April broke the record for most tornadoes, as a swath of destruction tore from Oklahoma to Virginia, hitting Alabama especially hard.

Meteorologists have been analyzing the data associated with the tornado outbreak and have counted over 300 separate tornadoes during April 26-28, including two F-5s, the strongest that can be measured with winds over 200 mph.

They also explain the perfect storm that set up to create the tornado outbreak

San Francisco Bay area teachers are using the 2010 Winter Olympics to teach kids about math and science.

The Silicon Valley Education Foundation teamed up with NBC Learn — the educational arm of NBC News — and the National Science Foundation to provide free lesson plans and video clips.

For more information, visit Lessonopoly.org.

The western half of North America has been baking in a heat wave that shattered all-time records while the east coast is shivering in one of the coldest summers on record.

A giant ridge of high pressure and a giant low pressure trough that span the continent set up and became locked, creating a formidable weather pattern that has everyone talking.

And many are wondering whether the record-breaking 103 temperature is the first evidence of global warming in Seattle.

Listen here.

As Congress tackles one of the most important and expensive revamps of the American health care system, some are attacking the science used to measure how effective treatments are.

This research–known as comparative effectiveness research–does not attempt to ration health care. And, could even be the key to reducing overall health care costs.

President Obama has challenged Congress to resolve health care reform bills currently wending their way through the House and Senate by the fall. In his national news conference on Wednesday, the President endorsed evidence-based medicine as a means to streamline health care.

The new Star Trek movie is fueling a science fiction discussion about what technologies could eventually become science fact. The ability to travel at warp speed–the speed of light is at the root of this discussion. Is it possible and if so when?

Well, yes and no. Some scientists believe that the science involved does not violate the laws of physics but there still is a long way to go before intergalactic travel becomes a reality.

Two scientists at Washington State University–dubbed the positron pair–are working to contain antimatter so they can control the immense energy created when electrons and the their antimatter counterparts positrons cancel each other out in the process of annihilation.

That sounds like science fiction but that science fact is already improving medical devices, computers and may power space ships to send to the center of the Milky Way…someday.

Listen here.

Running a marathon can be a grueling effort, even for well-trained athletes. Runners can suffer electrolyte imbalances and even kidney problems. An important factor in all this is hydration. But how much and what should runners drink? Those are questions doctors at Mayo Clinic want to answer. They’re studying the effects running has on the human body in hopes of being able to tell runners and other endurance athletes how best to prepare for an event.

And with marathon season nearly upon us, take care and stay hydrated during your run.

Dr. Helen Fisher, the author of Why Him, Why Her talks about the power of love–from a chemical point of view. The chemicals dopamine, serotonin, estrogen, and testosterone and are powerful forces in attracting a mate.

Does the opposites-attract idea hold up under scientific scrutiny? Or is like drawn to like? The answer? It depends who you are. Find out your chemical profile, based on the research of this anthropologist and scientific advisor for dating site Chemistry.com.

Flickr photo courtesy of squidminion

According to official numbers, 2008 is the coldest year on record since 2000 (and it is still .05 degrees Celsius warmer than seasonal averages) because of the influence of La Nina cooling pattern in the Pacific Ocean.

And winter has been brutal so far, with temperatures dropping to record low levels. The cold in the upper Midwest and East coast this week is just the latest in a string of frigid storms coast to coast. (And all this is happening during a solar miniumum.)

Just in: From University of Alabama, Huntsville, December 2008 temperatures (preliminary)

Global composite temp.: +0.18 C (about 0.32 degrees Fahrenheit) above
20-year average for December.

Northern Hemisphere: +0.41 C (about 0.74 degrees Fahrenheit) above 20-year
average for December.

Southern Hemisphere: -0.05 C (about 0.09 degrees Fahrenheit) below 20-year
average for December.

November temperatures (revised):
Global Composite: +0.25 C above 20-year average
Northern Hemisphere: +0.34 C above 20-year average
Southern Hemisphere: +0.16 C above 20-year average

(All temperature variations are based on a 20-year average (1979-1998) for
the month reported.)

Notes on data released Jan. 12, 2009:

The eleventh warmest global December in 31 years was also the fifth warmest
in the Northern Hemisphere, according to Dr. John Christy, a professor of
atmospheric science and director of the Earth System Science Center (ESSC)
at The University of Alabama in Huntsville. At the same time, the Southern
Hemisphere saw its tenth coolest of the past 31 Decembers.

WARMEST DECEMBERS, NH
2003 +0.62 C
2006 +0.54 C
1987 +0.52 C
1998 +0.42 C
2008 +0.41 C
2005 +0.40 C

Since November 1978, the Northern Hemisphere atmosphere has warmed more than
three times as fast as the Southern Hemisphere atmosphere (+0.19 C to +0.06
C per decade).

With a global average temperature that was 0.05 C warmer than seasonal
norms, 2008 goes into the books as the coolest year since 2000. Global
temperatures during 2008 were influenced by a La Nina Pacific Ocean cooling
event.

Another La Nina appears to be forming in the Pacific, which could chill
temperatures through 2009.

Color maps of temperature anomalies available on-line at:
http://climate.uah.edu/

If you are out in real cold–say when it’s 11 or 12 below zero–here are few experiments you can do to stay warm.

Bryan Wunar visits from the Museum of Science and Industry in Chicago to share some great experiments in the cold.

The United States is in a recession. The signs are all around. But how is an official recession calculated or determined? The National Bureau of Economic Research announced that a recession began in December 2007. It took a year of housing foreclosures, bank failures and massive layoffs to slow economic growth after a long 15-year run.

The United States is in a recession. The signs are all around. But how is an official recession calculated or determined? The National Bureau of Economic Research announced that a recession began in December 2007.

It took a year of housing foreclosures, bank failures and massive layoffs to slow economic growth after a long 15-year run.

The United States is in a recession. The signs are all around. But how is an official recession calculated or determined? The National Bureau of Economic Research announced that a recession began in December 2007. It took a year of housing foreclosures, bank failures and massive layoffs to slow economic growth after a long 15-year run.

science_of_a_recession_120208.mp3

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At last, a science-based reason to show the famous Diet Coke-Mentos geyser video from YouTube! A group of college Sophomores in North Carolina decided to study the physics of this pop culture experiment.

And, their work is published in the June issue of American Journal of Physics.

After all the anticipation about the landing, the real work begins now that the Phoenix Lander is safely planted on Mars.

The three-month mission gets underway this week and will search for clues to the red planet’s watery past. A robotic arm on the lander will dig up soil and ice samples and analyze them right there on the surface.

Prices are rising, food seems scarce. Wars are raging in far corners of the world. But is our present time one of more or less violence?

A group of Maryland researchers have been following the numbers. It turns out we are living in a very peaceful time. Why doesn’t it feel like it?

Dr. Michio Kaku examines the link between science fiction and real science. From the technological advances of teleportation in Star Trek and a death star in Star Wars, Kaku rates the impossibility of such realities in the future in his new book, Physics of the Impossible.

This year, February 29 appears on the calendar. That’s because 2008 is a leap year. It’s just an easy way to keep our calendars clear and consistent.

Why do we get an extra day every four years? The answer is in the way the Earth moves around the Sun.

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