MIKE MOSS SAYS: Jay, From a meteorological point of view, you can break it down reasonably simply as long as you hold some factors constant and imagine one variable at a time changing. Basically, with a golf ball in the air (and assuming no changes in the way it is struck and that there are no raindrops, fog droplets, etc to exert additional drag on the ball, only gases and the inevitable trace particulates) we're interested in the frictional resistance to its flight imparted by the air. That resistance increases and decreases along with density so that the ball theoretically should fly farther, all elese being the same, when the air is less dense and vice versa.
As given by the Equation of State, density of dry air is proportional to pressure divided by temperature. Likewise, the density of air that contains some water vapor is proportional to the pressure divided by something called "virtual temperature." The concept of virtual temperature allows us to use the equation of state in such a way as to incorporate humidity conditions, and it works out that virtual temperature and the actual temperature are the same for perfectly dry air, with virtual temperature increasing as air becomes more humid. In the equation above, this tells you that all else held the same, the more humid the air becomes (and therefore the larger virtual temperature becomes), the lower the density will be. The reson for this is that water vapor molecules (H2O) have a lower molecular weight than the (mainly) nitrogen and oxygen (N2 and O2) molecules they displace.
With that as background, we would expect the air to be less dense when
1) The pressure is lower
2) The temperature is higher
3) The humidity is higher
Any of these, if it is the only factor changing, should lead to a longer flight, and vice versa. Real life may not be so simple, though, due to combined changes that may offset each other, behavior of winds that may correlate with certain pressure, temperature and humidity variations (and the discussion above assumes wind is calm or at least always the same - but wind can easily outweigh all of the density considerations above). Also, there may be characteristics of the ball itself, the ground and vegetation beneath the ball, the grip, or the golfer (!) that may vary under different weather conditions and work together with or against the air density effects.