By
msnbc.com contributor

Explainer: The science of fall

  • Jacek Turczyk  /  EPA

    These are the sights of fall: Leaves piled high in the yard, pumpkins engorged on vines, flu shots, bears fattening up, shooting stars and the harvest moon. Hurricanes wind down and the Santa Ana winds begin to blow. Voters head to the polls and Oktoberfest beers flow. What's the science behind all these sights? Click the "Next" arrow above to find out.

  • Foliage is a great unmasking

    Nancy Palmieri  /  AP

    Scientists describe fall foliage like a great unmasking. Throughout the spring and summer growing season, trees produce chlorophyll — a green pigment that captures the energy in sunlight. As the days shorten in late summer and early fall, cells between the leaf and stem form a corky layer that blocks the flow of nutrients. Chlorophyll production ceases, the green fades away, and the underlying yellow and orange pigments are revealed. Reds and purples are produced by sugars trapped in the leaves. Eventually, these pigments, too, break down, the leaves turn brown and fall to the ground.

  • Harvest moon helps get the crops in

    Rene Johnston  /  ZUMA Press file

    Of all the named full moons — and every full moon is named — perhaps the best known is the harvest moon. This is the full moon that falls closest to the fall equinox and allows farmers extra hours of light to finish the chores of reaping what they have sown. Contrary to popular belief, the harvest moon does not linger in the sky any longer than other full moons, notes syndicated Space.com columnist Joe Rao. Rather, on the days surrounding the full moon, the moon seems to rise at nearly the same time each night instead of the normal average of 50 minutes later each day. Other full moons this time of year include the beaver, or hunter's moon, which gives extra light to hunters, and the cold moon, which comes as winter's grip takes hold.

  • Time to get flu shots

    AFP-Getty Images

    Fall is flu shot time. Every year, starting in January, the world health community scours the planet for strains of the influenza virus most likely to cause an outbreak of the flu. Three prime candidates are selected, combined with a strain that is safe for humans, and injected to a fertilized chicken egg. There, the mixture evolves into a single strain that will prompt the human immune system to produce antibodies to fight all the included strains. This seed virus is sent to vaccine manufacturers, who produce more of the virus in fertilized eggs. This vaccine is then harvested, treated and packaged for distribution. By early fall, flu shots are available.

  • Bears fatten up for hibernation

    National Park Service

    For bears, fall is the time to feast on the remaining summer berries, fish and other snacks in order to fatten up for the long winter snooze known as hibernation. As the days get shorter, the mammals will scout out and excavate a den, perhaps in a cave or amidst thick vegetation on a hillside. In Yellowstone National Park, biologists report, a grizzly might remove up to a ton of material getting its den ready. As the weather turns chilly, bears will hang out in front of their dens, stop eating, turn lethargic, and decide to head for cover once the first major snowstorm hits.

  • Breeding a better pumpkin

    Monsanto via PR Newswire

    Halloween is a favorite fall holiday for many people, and no Halloween is complete without a perfect pumpkin. Leave it to researchers at a subsidiary of the agricultural giant Monsanto to make sure no witches and ghouls get upset. The company has selectively bred more than 10 commercial varieties of pumpkin, each with desirable traits such as a firm, green stem that doesn't easily snap off and a deep orange color. Despite Monsanto's reputation for making so-called "Frankenfoods," the company has no immediate plans to genetically engineer the pumpkin, a researcher told the Associated Press.

  • Hurricane season winds down

    NOAA

    Though it may seem like the biggest and baddest hurricanes form during the dog days of summer, hurricane season officially extends to the first of November. In fact, the first weeks of fall can serve up some doozies. Take Wilma, for example, an October 2005 storm that peaked as a Category 5, the highest level possible, before it weakened slightly and tore across Mexico's Yucatan Peninsula. The storm then headed north and cut across Florida as a Category 3.

  • Santa Ana winds crank up, fan flames

    Kevork Djansezian  /  AP

    In California, the famed Santa Ana winds crank up and can flame ferocious wildfires that sweep down from the wilderness into the suburban sprawl that spreads out from Los Angeles and San Diego. The winds are triggered by high pressure that builds up over the desert areas of the Great Basin, including Utah and Nevada. As the winds race downslope, they heat up. Fires tend to erupt in October because the winter rains have yet to soak California, giving ample dry fuel for the flames, according to atmospheric and ocean scientist Robert Fovell at the University of California at Los Angeles.

  • Science seeks support at the polls

    Don Farrall  /  Getty Images stock

    There's a whole branch of science dedicated to the study of politics, and many science buffs are dedicated to raising the profile of the sciences on the political stage. As the election season ramps up each fall, talking heads from across this spectrum take to the airwaves offering up analyses and stumping for their positions. In the 2008 presidential election cycle, for example, backers of the ScienceDebate 2008 initiative persuaded the political camps of candidates Barack Obama and John McCain to spell out their stances on issues ranging from stem cells and global warming to funding for basic science research and space exploration.

  • Leonid meteors put on a sky show

    Stephen Shaver  /  AFP - Getty Images file

    Each November, Earth passes through streams of dusty debris shed by comet 55P/Tempel-Tuttle that result in the meteor shower known as the Leonids. The shower was a veritable storm with up to 3,000 so-called shooting stars per hour in the late 1990s and early 2000s. Astronomers are predicting a half-storm of about 500 meteors per hour on Nov. 17, 2009, as Earth orbits through a stream of debris shed in 1466. The 1998 shower is shown here as seen over the Great Wall of China.

  • Energy policy and Oktoberfest beer prices

    Wolfgang Rattay  /  Reuters file

    Each fall, millions of people flood into Munich, Germany, for Oktoberfest to eat hearty foods and drink copious amounts of beer. In recent years, the price of a mug has risen, German brewers say, because farmers are planting less barley — the staple ingredient of beer — in favor of subsidized crops such as corn and rapeseed that are used to brew biofuels. While biofuels are considered in some circles as more eco-friendly than fossil fuels like coal and oil, some beer drinkers are pushing for an end to the subsidies that appear to be messing with the price of their suds.

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