Hmmm......tough one.
First off, if it's hot, it doesn't automatically mean that there's less water in the air. Trust me, I have walked in the equatorial rainforests, and if two things apply there, it's: A) It's hot! And B) The humidity in the air is insanely high!
Also, the coldest days of the year are always really dry days....(moisture in the air actually traps warmth, when it's cloudy at night, it doesn't cool off as much as during a very clear night.....).
But back to the question at hand. I have no real idea about the influence of the weather on the trajectory of a paintball. When it's warm, most things will expand, including barrels and paintballs. I get the feeling that the paint is more susceptable to this phenomenon that the barrel is, but this can ofcourse be offset by using a larger bore size, so no big chances in that respect.
All I can see happening is just indirect influences. If it's cold, the shells will get brittle, which can cause a lot of barrelbursting, resulting in no trajectory at all. When paintballs are exposed to warmth for too long, the shell will get a bit softish, and consequently it might start to dimple, which will give you that unpredictable trajectory.
Well boys and girls, all people complain about the weather, but us paintballers are the only ones to have good reason to do so....
I'm sorry, I don't think that answered your question much, but I tried.
Peace.
