Kansas City, Mo. ? Every year it happens, almost without fail.

First, the Midwestern air turns frosty and brisk, and seasonal music fills the airwaves. Excited children clamber onto the laps of department-store Santas.

And the Kansas City Chiefs become nearly unconquerable at home.

It doesn’t seem to matter if they’ve played poorly or well in September, October and November, or whether they’re barreling toward the postseason or playing out the string.

In December, at home, Kansas City does not lose.

The Chiefs are an astounding 20-1 in Arrowhead Stadium since 1995 in the calendar’s 12th month, riding an 18-game December home winning streak that’s the longest since the AFL-NFL merger.

Quarterback Trent Green has never lost a December home game. He’s 12-0 and out to make it 13-0 today against the Baltimore Ravens, who stand an outside chance of clinching a playoff berth if they can break their opponent’s inexplicable December home spell.

The players themselves are at a loss to explain their streak, beyond the obvious fact that Arrowhead is perpetually packed to capacity with nearly 80,000 noisy fans and always lends a distinct home-field edge.

“A lot of it is the crowd and the noise it generates,” Green said. “A lot of it also has to do with the weather, playing outside, playing on grass.”

The Ravens are not likely to be intimidated, however.

“They’re going to be coming in after a tough loss, trying to wrap up their division. I’m pretty sure they’re not going to get caught up in that,” Green added.

Guard Brian Waters thinks the elements are a factor.

“It’s the time of year when a lot of teams – I can’t say don’t want to play – but sometimes guys don’t do as well in this kind of weather,” he said. “I don’t know if it’s a psychological thing. But we just play better at home.”

The Ravens (9-3) lost to Cincinnati 10 days earlier, but still hold a two-game lead over the Bengals and will be a playoff team if they win today and Cincinnati, Denver and the New York Jets all lose.

It’s a possibility Brian Billick is not going to waste even a minute pondering.

“You know what, they’ll come tell us,” said the Ravens coach. “We’ll see.”

A more immediate concern is Larry Johnson. Kansas City’s quick, powerful running back has rushed for 1,312 yards, No. 2 in the league. Baltimore’s rush defense of 75.8 yards per game is also No. 2 in the league. The Ravens’ overall defense is No. 1.

“Larry Johnson is a great young running back,” said linebacker Ray Lewis. “They have a great offensive line, probably one of the best in football. They have arguably the best tight end in football over there, too. So they have a lot of people over there, but we have a lot of people as well, so here we go again. It’s one of those classic battles, and it’s going to be a good test for both sides.”

How Johnson plays when he doesn’t have the ball may also be key. Nobody on the Chiefs’ schedule is better than the Ravens at blitzing the quarterback from every conceivable angle.

It’s something the Chiefs have drilled on all week.

“It will be critical this week because they bring people from everywhere – linebackers, secondary guys,” said coach Herm Edwards. “This is what these guys do. They do a good job of bringing pressure.”