At lower temps the fuel burns more efficient. Greater efficiency yields more power. Keep in mind that going to cold, say 140 or 150 deg. t-stat can actually hurt power. Ever notice that on cool days the car runs better and pulls harder? That's why.
Maybe someone like Terry can give a more technical answer?
Mike

I have a dream, and that is a 69 Boss 302 with Jules Asner in it.
Current - 2002 Chevy Silverado
Former - 98 SVT E0 T-Red 4/4/97 #173 of 6535(so long girl, you'll be missed)