May I also suggest you don't clean it every time after you shoot. A fowling shot or two is usually needed on the hummer. It will throw the next few hundred perfect after that. If your cold bore is off of a clean barrel every time. Your second or third shot will shift POI. which may be enough for a miss out a ways. Have a cold bore fouled barrel and it will duplicate over and over. BTW velosity difference will not be enough from summer to winter to make a noticeable change to impact. So sight it in when ever you are comfortable with the temperature. Don't worry about the barrel.
Blessings