Air density effect is the change in aerodynamic drag and lift a golf ball experiences as air gets thinner or thicker, from altitude, temperature, barometric pressure, and humidity. It has nothing to do with the golfer's swing: thinner air means less drag and less spin-generated lift, so shots fly lower and roll out more; a 250-yard sea-level drive typically carries roughly 265-275 yards at Denver's 5,280-foot altitude, an increase of about 1.1-1.2% per 1,000 feet of elevation. Counterintuitively, hot, humid air is actually less dense than cold, dry air, because water vapor molecules are lighter than the nitrogen and oxygen they displace, so a muggy evening plays slightly longer than a cold, dry morning. Launch monitors typically report a 'normalized' carry calculated at standard conditions (77°F, sea level, no wind) so a bay's number stays comparable across days and cities.