A winter chill seems to be setting in for CBS’s “Big Brother.” The reality show, airing for the first time in the winter, stumbled in its second outing last night, falling from an already-low premiere.

“Brother” averaged a 2.1 in adults 18-49 in the 8 p.m. timeslot, according to Nielsen overnights, off 16 percent from a 2.5 final rating in its debut on Tuesday night.

True, last night “Brother” was facing Fox behemoth “American Idol” for the first time, and “Idol” pounded the competition in the timeslot, averaging a 9.7.

“Brother” finished third at 8 p.m., behind “Idol” and NBC’s “Deal or No Deal” and just ahead of Univision’s “Al Diablo con los Guapos.”

The franchise has never been a huge summer hit, but it’s been remarkably durable for the past seven years. CBS pulled it onto the regular-season schedule when the writers’ strike sent the networks scrambling for original content.

The racy show has long counted on sexy contestants in scanty clothes to pump ratings, and though it’s a bit colder outside, this year it’s no different. CBS has said it’s turning up the heat in the “Brother” house to encourage the usual clothes shedding.

Meanwhile, Fox led the night among viewers 18-49 with a 7.9 average overnight rating and a 21 share. NBC was second at 2.7/7, ABC third at 2.5/6, CBS fourth at 2.0/5, Univision fifth at 1.5/4 and CW sixth at 0.6/1.
 
Fox began the night in the lead with a 9.7 rating at 8 p.m. for “American Idol,” followed by NBC with a 2.7 for “Deal or No Deal.” CBS was third with a 2.1 for “Brother,” Univision fourth with a 2.0 for “Al Diablo con los Guapos,” ABC fifth with a 1.9 for “Wife Swap” and CW sixth with a 0.7 for an “America’s Next Top Model” clip show.
 
At 9 p.m. Fox was first again, this time with a 6.1 for “Moment of Truth,” while ABC moved to second with a 3.5 for “Supernanny.” NBC was third with a 2.5 for “Law & Order: Criminal Intent,” CBS fourth with a 1.8 for a repeat of “Criminal Minds,” Univision fifth with a 1.5 for “Pasion,” and CW sixth with a 0.5 for a repeat of “One Tree Hill.”
 
NBC took the lead at 10 p.m. with a 3.0 for “Law & Order,” with CBS second with a 2.0 for a repeat of “CSI: NY.” ABC was third with a 1.9 for “Cashmere Mafia” and Univision fourth with a 1.0 for “Don Francisco Presenta.”
 
Fox also finished first for the night among households, averaging an 11.5 overnight rating and an 18 share. NBC was second at 6.6/11, CBS third at 4.7/8, ABC fourth at 4.4/7, Univision fifth at 1.9/3 and CW sixth at 0.9/1.