Within "normal" temperature ranges (roughly -20 to 100+°F) the only difference as far as the engine is concerned (the rider may be disgruntled lol) is the fuel air mix as the colder it is outside, the denser the air is as others have mentioned (dense air = more of it per volume = leans fuel/air mix).
However, as it can be somewhat of a drastic difference in density with the average, noticeable performance change (with the HTs anyway) in an approximate 30° range (if tuned for 80° you'll note a pronounced difference at around 50° etc.) - as most HTs are running a touch rich - not only do you get a performance increase as you can get "more" air crammed into the combustion chamber per compression stroke (kinda like a weak supercharger), you'll get the added performance of a closer to correct fuel/air mix (NT & Speed carbs).
Needle clip position alone isn't really sensitive enough to compensate for this, nor is the difference in density usually enough to damage anything due to a lean condition (provided you're not running lean to begin with) but if you're noticing a pronounced improvement in performance on hot
days & running poorly on cold (test w/just a hair of choke for improvement) - you may be running too lean in general & should prolly check your plug's coloration & condition.
I ride year-round in conditions ranging from -40 to 110°F and although there's a somewhat drastic change in performance in that range, I don't bother re-tuning, needle clip position has a pronounced enough effect that it'll never be quite "perfect" anyway I figure - fortunately & purely coincidentally my winter beater loves the cold (that carby runnin' rich) and my summer cruiser the heat (& that 'un runnin' lean) anyway
Ofc as the "quality" control of these motors varies as greatly as the elevations they're used at (higher the elevation = lesser the air density), as I'm at sea level in the frozen northlands & we all favor different oil/fuel ratios (less oil = more fuel = richer air/fuel mix) my observations may vary greatly from yours heh