The treetops of Pittsburgh may be glistening tomorrow morning, but it's more likely to be the result of freezing rain than a snowy white Christmas.

Still, as the song says, you can dream.

People all over the country have been dreaming of white Christmases ever since 1942 when songwriter Irving Berlin and crooner Bing Crosby combined their talents to produce what remains the best-selling American song of all time.

But the nation has changed. It's no longer just like the one you used to know.

Population shifts toward the Sun Belt mean more and more Americans are steeped in nostalgia or simply hoping for a holiday miracle. Fifteen of the nation's top 25 population pockets are in regions that wouldn't have a snowflake's chance of a white Christmas.

Still, three of the most populated areas are in the north -- New York, Chicago and Philadelphia -- and the Midwest and New England states have their fair share.

The forecast for this Christmas in Pittsburgh calls for cold and rain, but probably not snow.

"It looks like the snow on the ground is all we're going to get," said meteorologist Carl Erickson, of AccuWeather in State College, Centre County.

This shouldn't come as a surprise. The National Climate Data Center in Asheville, N.C., puts the probability of Pittsburgh having a white Christmas -- meaning a new snow depth of greater than an inch on the morning of Dec. 25 of any year -- at 33 percent. But that is based on data collected since 1950.

From 1942, when the song "White Christmas" was released, through 1949, Pittsburgh saw only one snow-filled holiday (1945). Add those years into the equation, and the chances dip to almost 1-in-4.

The hope for a white Christmas predated Berlin's sentimental song, but no one this side of Currier & Ives tapped into it as successfully.

"Irving Berlin was a consummate songwriter," said Deane Root, a professor of music history and director of the Center for American Music at the University of Pittsburgh. "He grew up in New York City in the early part of the 20th century, where he was steeped in all the different musics of immigrants. He heard all this ethnic music, and he was able to imitate each of them. He had a good ear and he could hear the things that made each of these different styles distinctive."

Berlin's music mirrored America, whether he was writing ragtime dances, melancholy ballads or patriotic fanfare. Already established by the early 1940s, he was settled in the sunny southwestern United States when he penned the sentimental "White Christmas" as part of the score for Crosby's movie, "Holiday Inn."

"Thank God they had the film, because the film put the song into orbit," said Pittsburgh Pops conductor Marvin Hamlisch. "And it's the kind of melody that most composers want to be able to write, something that people can remember. But not something so simplistic that it becomes, 'Who cares?' Irving Berlin is probably the master of those kinds of melodies."

"The distinctive thing about this song was that it was as conservative as a song could get," said Dr. Root. "It's around one note, and the half-steps on either side of it. The listener is going to think he's in a safe and secure place. A very simple harmony, closed in, a tight little world. Even if the listener is not thinking about that aspect, they're going to hear it.

"It isn't a majestic song. It doesn't soar like 'O, say can you see,' which is rising over a whole octave. I think that's where a lot of its power is derived."

Singer B.E. Taylor annually presents a popular Christmas tour that begins in mid-October and stretches through the holiday with stops ranging from craft festivals to Heinz Hall. The shows include classic holiday hits, carols and a couple of Mr. Taylor's original tunes. He said he hasn't done "White Christmas," but he would like to perform it some year "with a lot of strings, almost a symphonic approach."

"There are a lot of great songs, but that one means more," said Mr. Taylor, an Aliquippa native. "Growing up, when we were kids, Christmas meant snow. My great aunt in Aliquippa would play the song on one of those old record players.

"The images it conjures up as it looks back to when we were younger. We see those snowflakes and we're all kids again."

Dr. Howard Friday, a clinical psychologist with Intercare Psychiatric Services in Bethel Park, said the image of snow represents peace and quiet to many people.

"It's the magic of Christmas," he said. "Santa Claus arrives on a sleigh, and that can only be accomplished if there's snow.

"But there's also the religious aspect, too. The whiteness associated with the light of Christ coming into the world at Christmastime, also lending itself to the good feeling."

For all its potential sadness, Dr. Friday pointed out, "White Christmas" also is a happy song.

"There's that part of the song where Bing Crosby even whistles," he said. "That's light-hearted, calm and enjoying life."

"It's one of those songs that is overly sentimental, but it doesn't matter," Mr. Hamlisch said. "People love it because it's Christmas. If the song was 'I'm Dreaming of a Good Sunday,' it probably would have been nothing. Christmas, you're allowed to be sentimental."

Any account of the song's success, however, has to address its timing. Coming out as it did in the early stage of World War II, it resonated with soldiers and families who were missing each other and the happy times together.

James Nolan, 84, of Mount Washington, was a corporal in the Marines stationed in the Pacific during World War II. He spent two Christmases -- 1944 and 1945 -- shuttling between Guam and Iwo Jima.

"The first Christmas, we didn't have much," he recalled. "We slept in tents on the ground, six men to a tent. We didn't have electricity or radios. That wasn't much of a Christmas because we were loading up for the invasion. Our Christmas dinner was sandwiches.

"The Christmas of 1945, our tents were on platforms. We had electric lights and radios and decent food. And that's when we heard 'White Christmas.'

"I was only a kid, but the older men were married with kids, and you'd see them crying. Imagine, you hear 'White Christmas' and you're over there. I don't give a damn how tough you are, those grown men cried."

Long after the war ended, the song retained its impact.

"It always will have a special meaning," Mr. Nolan said. "It's like our national anthem, I feel."

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