Sometimes your string can stretch a little in the heat, causing your peep to be a little off which in turn, of course; makes you off.
I also have a secondary belief that depending on temperature, your arrow flight can be off, even if string etc is right on. Reason: Different air densities (pressure). Cold air = denser air, thus maybe more air resistance; Hot air = less dense air, thus less air resistance.
This is only a theory I have, but maybe someone that is a little more of a science geek than me can explain it better or straighten me out.
Of course, if this was true; altitude would also affect arrow flight due to the different air pressures.
~E Rock
