We got into a discussion about sighting in scopes and he made a comment that in warmer weather, the heat rising from the ground will cause the bullet to rise (maybe I should of asked him if a bullet sinks faster because of the cold air, just to get a laugh).
Well, a bullet DOES actually drop faster in cold air vs warm air. The reason is that cold air is denser, and therefore provides more resistance to the bullet's passage, thus slowing it down faster. (Think of it as lowering the bullet's B.C. a little.) This means that the trajectory curve will be a little steeper when fired in cold air vs warm air. This is one of the reasons artillery firing calculations include both air temperature and local atmospheric pressure in the variables.