Baby, it's cold outside. And isn't it wonderful?

Not everyone would agree, but that's the way retailers feel as the mercury plummets 30 degrees from the balmy, unseasonable temperatures of the past three weeks. As hats, coats and scarves pile up in stores and executives prepare to cut prices so people will buy them, in comes Mother Nature with a lifeline.

It's a cold, hard fact that holiday sales are affected by the weather. Sure, retailers complain all the time about the temperature, the rain, the wind and the snow, but truly, this is the one time of year when it actually does make a difference.

"In July, when it's hot or really hot, it isn't going to matter as much as November when it feels like August," said Amy Brackin, director of marketing for Planalytics Inc. of Wayne, Pa., which advises consumer companies about the weather and its business impact. Among its clients are Gap Inc. and Ross Stores Inc., but also Campbell Soup Co. and Perrier.

The good news for department stores and clothing chains is that it's not just cold weather that helps spur sales, it's the rapid change, according to Planalytics. Consumers tend to overreact to the sudden drop and rush out to buy warm and fuzzy outerwear. And once they're shopping, they start buying holiday gifts.

"Consumers are trained to buy things when we need them," said Paul Walsh, senior vice president of Planalytics. "When the weather turns and the temperature drops 40 degrees, it really compels people to go buy the coats they haven't bought yet. That translates into traffic in malls, the malls are decked out for the holidays, and it creates a more positive environment overall."

At Hecht's, the mufflers and cashmere sweaters are just waiting for buyers, and company spokeswoman Nancy Chistolini said executives are "anticipating a very big weekend."

During the warm lunchtime hour downtown yesterday, workers enjoying the weather in McPherson Square suggested there is some pent-up demand among consumers.

"It's hard to get in the spirit until it turns cold," said Elizabeth Olson of Silver Spring, eating her lunch on a bench. She hasn't bought anything for Christmas and has been waiting to beef up her fall wardrobe. "The last time I went shopping was the last time it was cold, like a month ago," she said. "And I haven't been able to wear any of it."

The weather not only affects how much people buy, it affects what people buy, retailers say. When it's warm, people buy electronics and non-seasonal merchandise. When it's cold, they buy apparel. Yesterday, Filene's Basement tried to gin up some weather worry with an ad in the Washington Post that warned, "Baby, It's Gonna Get Cold Outside."

They seem to be right.

This burst of cold "is going to last, and just get colder," according to WRC-TV meteorologist Veronica Johnson, who predicts no more stretches of 60-degree weather. That's a marked change from last year, when November was warm right through the end of the month, delaying serious holiday sales traffic in stores until well after Thanksgiving.

Of course, there's always a reason for retailers to worry. "When the weather turns like this, there is an underlying risk for snow," Walsh said.

After enduring 70-degree days, that's exactly what retailers don't want to hear. Other than hurricanes, snow is the worst-case scenario.

"We're eager for the cold, but we're not eager for the snow," said Chistolini of Hecht's. "You see, we're very, very particular."