What's in a snowflake?

Making a snowflake is dirty business. High above the ground, water vapor condenses onto dust particles and freezes into ice crystals. As more water vapor condenses on the surface, the crystal forms into a small hexagon -- a tiny, six-sided piece of ice. Gradually, the sides of the crystal grow and form icy arms.

As the newly formed flake tumbles through changing temperatures and humidity, it is reformed into shapes that range from stars to needles. These microscopic changes lend credence to the old saying that "no two snowflakes are alike." However, tiny crystals that fall without fully developing into flakes might be very much alike.

If you find two identical snowflakes out there, just keep it to yourself.

Really heavy snow

The amount of snow that falls each year weighs about a million billion kilograms, according to physicist Jon Nelson at Ritsumeikan University in Kyoto, Japan. (You'll have to do the conversion to tons yourself.)

A typical snow crystal weighs roughly one millionth of a gram, Nelson said. A cubic foot of snow can contain roughly one billion crystals. A rough estimate of the number of snow crystals that fall per year is "about 1 followed by 24 zeros," Nelson told LiveScience in 2007. "If another scientist says that I'm off by one or two zeros, then I won't quibble."

The milk-and-bread phenomenon

One mystery about snow that science has yet to explain is why everybody in Roanoke runs out of milk and bread the very night the weatherman calls for a snowstorm.

Eric Williams, store manager for the Kroger at Towers Shopping Center, is aware of this phenomenon. Whenever the forecast calls for snow, even if it's just for an inch or two, shoppers slam the grocery store like it's a Walmart door-buster sale. Shoppers stock up on essentials that will sustain them for the, oh, two or three hours they'll be cooped up indoors.

That's when grocery stores go to DEFCON 4. In the case of a bona fide heavy snowstorm, like last weekend's, Williams said his suppliers will add extra deliveries to keep his store well-stocked. For example, Kroger works with a Lynchburg dairy that adds trucks in case of snow.

Even a little snow is great for business.

"The best thing for us is when they call for [wintry] weather, then you get a little shot of it that's not enough to keep people inside," Williams said.

When that happens, Williams said that customers accuse him of being in cahoots with the TV weather people.

"Customers say, 'We know y'all pay the weatherman to say it's going to snow,' " he said.

Movies for a snowy day

Christmas season

• "Holiday Inn" (1942) This Bing Crosby/Fred Astaire classic introduced the song "White Christmas."

• "Scrooge" (1951) Alastair Sim's portrayal of Ebenezer Scrooge is considered the best of all the Scrooges -- which means he's the worst of all the Scrooges.

• "Die Hard" (1988) Nothing says "Joy to the World" like Bruce Willis killing international terrorists during a Christmas Eve office party.

All winter long

• "The Lord of the Rings" trilogy, director's-cut DVDs. That's almost 12 hours worth of hobbits and wizards -- and lots of talking about hobbits and wizards. Especially good for a blizzard. A blizzard of wizards!

• "Casablanca" (1942) Of all the movies in all the DVD players in all the world, this one has some of the greatest movie lines in history. Spend a snowy afternoon working on your Humphrey Bogart impersonation.

• "Doctor Zhivago" (1965) If you've always wanted to watch this 3-hour, 17-minute epic but never had the time, save it for a snowy day. Has lots of snow scenes, too.

• "Blue Hawaii" (1961) Elvis, Hawaii and "Rock-A-Hula-Baby" will melt those wintertime blues.

Best places for public sledding

Ghent Park, Roanoke

The long, steep hill near Memorial Bridge is one of the most popular sledding spots in town.

Salem Municipal Golf Course

Hop on your ride near the first tee.

Longwood Park, Salem

The hill is so long and smooth, they ought to rename it long-sled park.

Fishburn Park, Roanoke

The hill behind the tennis courts used to feature a tow rope for skiers.

Blacksburg Municipal Golf Course

For years, the snow-covered hills near the fairways for holes 2 and 5 have attracted families, snowboarders and Virginia Tech students.

Snowfall totals

Winter 2008-09

• Roanoke: 4.4 inches

• Blacksburg: 19.1 inches

Dec. 18-19

• Roanoke: 17.8 inches

• Blacksburg: 14.2 inches

1959-60

• 62.7 inches: Roanoke's snowiest winter

Roanoke's snowfall average

Over the past 60 years, Roanoke has averaged about 21 inches of snow per year. However, annual totals have varied from 2 inches to nearly 63 inches.

Snow days

Any student loves to stay home for a "snow day." But you've got to make the days up.

According to the Code of Virginia (leg1.state.va.us), if schools are closed due to weather for five days or fewer, the schools must make up all five days, either by adding days to the schedule or hours to the school day. (Many school divisions include a few makeup days in their academic calendars.)

If schools are closed six days or more, they must make up the first five days, plus one day for every two days missed after that. If they use up their built-in makeup days, that's when they start adding extra days to the school year.

So, enjoy those snow days, but don't whine when you're still in school on Flag Day in June.

Schools must provide 180 teaching days and/or 990 teaching hours per school year.

Makeup days by school system

These are the makeup days specifically set aside for days missed due to inclement weather. School divisions also use other holidays and breaks for makeup days.

• Roanoke -- 0*

• Roanoke County -- 7

• Salem -- 2

• Franklin County -- 5

• Montgomery County -- 6

• Radford -- 1

• Botetourt County -- 5

• Bedford County -- 10

• Floyd County -- 8

*Roanoke's extended school days allow the system to accumulate more than enough instructional hours to compensate for missed days. They hope.

Sources: SnowCrystals.com, LiveScience.com, Kevin Myatt's Weather Blog, National Weather Service