As a very general rule, I think you can say most of the time patterns are better with warmer temperatures, but not always. I have shot enough in very cold weather to see better patterns in the 20's and low 30's than in the 40's or low 50's with certain shells and guns. This is totally dependent on the individual shell, the muzzle velocity, and gun being used. The size of the bore and length of the forcing cone are certain factors in this. When it gets cold enough with some shells, the powder burn rate can be slowed enough, and the velocity lowered to where patterns IMPROVE under these conditions. Again, I believe this to be more common with two things 1, an overbored shotgun and 2, a longer forcing cone.
My suspicion is that once you reach a certain temp, patterns will improve again regardless of gun model/shell as the lack or air resistance will override the gains that may be seen from a reduced muzzle velocity in colder temps,
