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WHY DOES WINTER HAPPEN? What causes frost, ice, snow . . . and pot-holes?

The answer to all of these, of course, is science - the laws and processes of nature.Winter storms in, sending us reaching for boots and overcoats, not because the sun moves farther away after autumn's leave-taking but because sunlight now falls upon the Earth at an angle, spreading its energy more thinly and coolly (the opposite occurs in the Southern Hemisphere, where the seasons are reversed).

In fact, the winter sun is several million miles closer to us than its summer counterpart.

Go outdoors on a wintry morning, and you can see your warm breath explode like white dragon's fire in the chill air: These frosty plumes arise when hot gas collides with cold, chilling your breath rapidly to the point where its moisture crystallizes around microscopic nuclei in the air to form myriad floating droplets.

Already you may have observed this condensation phenomenon in action twice earlier when your breath fogged the morning windowpane and your warm breakfast toast "exhaled" a glossy patina of droplets onto its plate.

And notice now on the porch the crown of ice on the dog's water, another signature of the season. Why does water freeze in cold weather, anyhow? And why at 32 degrees Fahrenheit?

Heat is motion and energy, you know - faster-moving molecules are hotter than slower ones. So as water cools in the frigid air, random pockets of more sluggish molecules transform into ice "islands" that in turn may melt again as they swirl into still-warm neighboring regions.

But soon the ice islands begin to grow in size and number until they overwhelm the warmer enclaves and start joining forces - chill island links up with chill island - in icy solidification.

Few of us winter watchers are aware that in fact very pure water won't freeze until it plunges close to minus 40 degrees Fahrenheit. This is because ice is formed of crystals that take shape around precipitating nuclei, or "seeds," so impurities in everyday water are instrumental in bolstering the size of those early ice islands to allow freezing at around the customary 32 degrees Fahrenheit.

Whether it's your dog's water or the nearby skating pond, water freezes from the surface down, of course. This happens not only because heat radiates away fastest at the surface but also because the warmer layers of water are heavier and sink, leaving the icier stuff to accumulate even faster at the top.

Because the earth at the bottom of a pond or river is usually warmer than the winter air, it can act as a "slow-burn" heater that delays or prevents freezing above. For this reason, transportation engineers will sometimes install bubbling systems at the bottoms of rivers to help float the deep warmer water toward the top in order to prevent freezing and thereby keep the waterways open for travel.

There's an old tale of freezing water that dates back to the time of Francis Bacon, the 16th-century English philosopher, who was astonished to notice one cold winter day that when he set out two pans of water, one hot and one only warm, the hot one froze first!

This phenomenon, widely observed in more arctic countries such as Canada and Iceland, still amazes people in warmer climes. Some years ago, a savvy high-school student in equatorial Tanzania reportedly dumbfounded his skeptical teacher by performing Bacon's water experiment before the class.

The story was picked up in scientific journals, creating a minor hubbub, and thus the phenomenon was rediscovered for thousands of winter watchers in more moderate zones.

The explanation for this, once you've stumbled onto it (as did Bacon), is simple enough: When hot water is placed outside, it evaporates much faster than warm water because its more energetic molecules tend to fly up out of the pan more readily and zip off into thin air, so that soon there's a lot less water in the once-hot pan, allowing freezing to occur faster.

Freezing is obviously a lot more troublesome when it strikes your water pipes: Here crystallization begins at the outer perimeter and moves inward until it eventually plugs things up. Since water expands by about 11 percent upon freezing, this can induce strain and eventual breakage at the pipes' weakest point.

One strategy for averting this, as many experienced winterers already know, is to allow outdoor taps to slow-drip continuously, thus keeping valves open and permitting a harmless expansion of any ice that forms inside.

WHAT'S WINTER without an occasional dose of that woolly white stuff (called by the ancient Greeks "woolly water" or "wet wool") floating down out of granite-gray skies?

A solitary snow crystal may fall around 40,000 feet, beginning as a simple microscopic six-sided "plate" in a high cirrus cloud, then becoming adorned with decorative branchings (these become lacy "lightning rods" that attract additional airborne water molecules) as it plunges through variable regions of temperature and moisture, often combining with myriad other growing crystals to form a complex flake, perhaps blown up and down, up and down for an hour or more before it settles gently on the Earth to whiten things up.

Yet don't be fooled by snow's virginal visual aspect, spreading at times as far as the eye can see: Snow's whiteness, like all color, is borrowed from elsewhere. The countless tiny crystalline facets of the flakes reflect the full spectrum of white sunlight without "selectively absorbing" any of the individual colors. (Elsewhere in the world, algae-tinted snows may appear pink, red, yellow, green or even blue.) Snow's brilliance stems from the fact that it is one of nature's best reflectors, with about 97 percent of the sunlight striking it bouncing back off.

Pile fluffy flakes high enough and, on a very cold day, you may hear them crunch squeakily underfoot as you walk, due to successive layers of flakes rubbing against each other noisily.

But as the temperature climbs toward freezing, the crunching disappears because now the pressure of your body's weight momentarily melts some of the snow underfoot and spreads a thin lubricating dampness that eliminates the squeaky sticking.

On a shivery afternoon walk to pick up the newspaper, you may remember a frigid time from childhood when a friend put his tongue to an outdoor railing at school and found it stuck there in the cold. In a panic, he jerked away and tore some skin. Nothing serious, though it could have been!

Something similar can occur when you handle or foolishly lick a frozen ice cube tray - the metal conducts a frozen bond that, if ruptured suddenly, can cause a painful tearing of flesh. To avoid injury, simply dip both tray and finger into a basin of hot water or pour liquid over the metal and stuck tongue (it's sloppy but painless).

Hanging from the eaves of an old house are icicles in a long row, each rather crooked and misshapen. With only constant gravity to tug on them, why aren't they straighter and more uniform? This forgets the other forces often at work, such as wind (which cools the hanging water unevenly) and an asymmetric water supply (grooves in the metal spouting, say, that direct water rivulets predominantly to one side.

As icicles form on tree branches or telephone wires, their growing weight often bends the branch or wire enough that the icicles start to grow new directions.

Driving to the store a while later in rather icy conditions, you feel the car bump and clatter on the washboard pavement, grown even worse in the cold. The repeated freezings and thawings of road materials are tough on surfaces, but often worse is the expansion of water into ice just beneath the pavement. Water often migrates up from the soil below and nearby, then collects there and finally freezes, wreaking pothole havoc on the road.

Ooops! On a sharp turn you suddenly encounter an icy patch that sends the car's rear end spinning out. Almost immediately, you turn the front wheels in the direction you intended to go, creating a sharp counter-torque that pivots the car straight again (this strategy may not work if you're driving too fast or if it's more a linear skid than a spinout).

After stopping for a light, you feel your car's wheels slipping somewhat when you try to accelerate. First you try a smoother, more gradual release of the clutch, but this doesn't help. So you slip the car into second gear, greatly reducing tire torque, and now off you go smoothly.

EVENING. HOME NOW at last around the hearth. It's time to stoke the fire and settle in before its cozy warmth.

From the steadiness of the flames, you know your chimney's in good working order: a smooth open airstream to the outdoors. Above the fire, the heated air rises up within the brick column as the colder, denser air of the room pushes against it, lifting it much as the waters of a swimming pool lift an old inner tube floating on the surface.

The taller the chimney, the greater the volume of light air in the column and the stronger the upward draft.

Every now and then, a blast of wintry wind from outside stuffs itself down the chimney, disrupting the smooth flow of upward air and causing huge cottony puffs of smoke to be released toward the starry sky. Soon everything stabilizes again, and the chimney resumes its uninterrupted stream once more.

Over on the mantel, the flame of a tall candle suddenly gulps and flickers, threatening to go out. Its sputtering is all part of a natural candle flame cycle that involves excess fuel being transported via capillary action up the wick and feeding the flame, which in turn burns off the fuel supply too rapidly, causing a choking or sputtering effect.

This can happen many times in the life of a candle. Usually, the longer the wick, the longer these cycles are, because longer wicks require more time for fuel to ascend in response to changing flame conditions.

Ah, the hour is growing late now, the candle is burning low, the old light-lean days of frosty winter seem really dug in. Yet tomorrow's another day, as promised sunlight brings new warmth and energy for more joyous romps and snow escapades.


Winter facts

Winter occurs because the sunlight falls upon the Earth at an angle. This spreads the energy more thinly and coolly.

Pure water won't freeze until it reaches -40 degrees Fahrenheit

About 97 percent of the sunlight striking snow bounces back off.

Warmer layers of water are heavier and sink, leaving the icier stuff to accumulate faster at the top.

NOTE: The winter sun is several million miles closer to us than the summer sun.

LIBRARIAN'S NOTE: See microfilm for graphic details.