2 things to keep in mind-
1.In regards to humidy, keep in mind that colder air holds much less watever vapor than warmer air. Therefore "80 percent' humidity in 100 degrees is WAY different than 80 percent in 40 degrees. The effect of humidity is almost nothing once you are below 40 degrees as the air cannot hold a lot of moisture so.
Point is that colder temps are a "double benefit" in that you get the colder denser O2 and you get much less water vapor mixed in for any given humidity.
The small tradeoff is some states change their gas formulation for "winter formula" contains more butane in order to modify the relative vapor pressure of the of the fuel between winter and summer. Butane, similar to enthanol makes our fuel have less energy content and basically winter fuel is a bit less potent than summer fuel. Still a net beneft in winter thought
2.The most accepted rule of thumb is that 10 degrees F change= 1 percent hp change.
So about 4hp for every 10 degrees for our cars. so from summer 100 degrees to 30 degrees is almost a 30hp relative increase. this does not take into account the lower humidy effect in winter so with that its probably more along the lines of a 30-45 hp difference between winter and summer. That shows you how much a true 30 hp difference is, VERY noticable.