I've read a lot about the effects of cold and warm weather on the effects of golf ball distance, as well as elevation. Really elevation can correlate to air pressure since the higher the elevation, typically the lower the air pressure.
Yesterday I was playing a round of golf in the PNW (Vancouver specifically). Temperature was around 46F (8C) and of course humidity was around 90%. I did not know what the air pressure was at the time.
On the second hole, I'm lying 126 to the pin and I've got around 7 yards from pin to back of green. I hit a PW, which would normally travel 120 yards at 46F (summer distance is near 130). I walk to the green to discover I flew the green. My ball mark was on the very back; that's about 132 yards carry. I'm shocked, so I jump on the iPhone to look at the air pressure; it's 99.4Pa (994mb for you Americans). This is super low air pressure for the PNW and especially at sea level. Standard air pressure is 101.5Pa or 1015mb. The air pressure in the PNW is typically around 102Pa or 1020, slightly high air pressure. We all know we have to adjust for distance at sea level, unless you are playing in Florida, the temp offsets the air pressure I believe.
Hole 9, I have 136 to the pin and around 10 yards to the back. I take out a 9 iron and choke down 1/2 an inch and swing 3/4. I airmail the green. Drives were going around 270 (which is respectable in summer at sea level); the tour average at last years RBC Canadian Open (2011), was around 270. I was driving this distance in soft conditions and at 46F. Had it been summer, I would have been driving 300 all day, which I've never done at sea level.
I've read quite a bit that temperature has greater effect on ball distance than elevation. I'm not sure this is the case. I think whichever measure is more extreme, might be the more significant factor. If they are both at the extreme end, it could mean a 2 club difference.
My golf buddy's think its funny that I check the air pressure before a round. Most of the time it's irrelevant, but for those times when the air pressure is extremely abnormal, I know it's made a big difference; 5 yards could make the difference between birdie and boggie.