You’re standing over a shot that measures exactly 175 yards. For most of us, that’s an easy club to pull. An average male amateur might select a 5-iron. A PGA Tour pro would probably hit a 7- or 8-iron. An average female golfer might go for a 3-wood. The choice obviously depends upon how far you hit the ball. But it also depends on how far that 175 yards really is.
Compared to what? If you’re playing golf in Denver, the shot will play shorter than if you’re playing in Death Valley, Calif. That’s because air is denser at 300 feet below sea level than it is at 5,000 feet above, and thus offers more resistance at the lower altitude. So, in effect, there’s more air to penetrate during the duration of the ball’s flight when you’re closer to sea level – or under it.
What’s the formula for calculating effective distance? Don’t ask an engineer. They’ll just give you an engineer’s answer. “It depends on launch angle and spin rate,” said Frank Thomas, former USGA technical director and now proprietor of his own firm, Frankly Consulting, in Chester, N.J. As Thomas makes clear, effective distance is a function of a series of interactive factors, including altitude, the vertical climb (or fall) of the shot, humidity, wind and temperature. Each of these elements exercises a different effect upon the golf ball’s coefficient of lift and drag.
Don’t have time for calculating differential equations in the middle of a round? Here’s a quick and dirty guide to figuring out effective distance.
These data are my own nonlinear extrapolations from USGA Research and Test Center studies of driver distances and represent ballpark estimates at best. As with everything else in golf, more research is needed.
For altitude, a good rule of thumb is 2 percent per 1,000 feet of elevation. Actually, the figure is about 1.7 percent for drivers, probably a little less for mid-irons given their higher spin rates and launch angles. But ballpark estimates work well enough, so for each 1,000 feet of elevation on that 175-yard shot, figure a 3-yard bonus. That means at 5,000 feet above sea level, the shot will play like 160 yards.
Hot air is thinner than cold air, thereby offering less resistance and allowing greater distance. On drives, the bonus approaches 1 yard for each 5-degree temperature increment, less than that for a mid-iron. If that shot plays 175 yards at 75 degrees Fahrenheit, it will play about 170 at 115 degrees Fahrenheit and 180 at 35 degrees Fahrenheit.
Humidity has a much smaller effect on distance than either altitude or temperature, but it does count, even if it’s barely discernible. Contrary to conventional wisdom, humid air is thinner and drier air is denser.
This one goes out to certain television commentators. The ball flies farther in humid conditions than in arid ones. If folks think the ball travels better when they play in the desert, that’s a function of altitude, not humidity.
Wind is the obvious wild card in any golf game. Interestingly enough, on long shots (with a lower trajectory), its effect on roll actually exceeds its effect on carry. For that 175-yard shot, the effect is about the same, but a fair number for calculation is plus/minus 1 yard for each mile per hour of head/tail wind. Into the teeth of a 15 mph breeze, the shot plays 190 yards; straight downwind it plays 160.
Let’s not forget the vertical climb or fall of the shot. We all know downhill shots travel farther and uphill shots travel less. But how much?
This one is very tough to estimate because it varies so much with the shot trajectory. Shots with a higher angle of launch (and descent) will be affected less. Confused? Figure the difference at about 1.5 percent per 10 feet of elevation change
It helps to be able to eyeball elevation shifts. Let’s consider a 175-yard par 3. Uphill to a green 30 feet above the tee, the shot plays 183 yards; downhill to a green 30 feet below the tee, it plays 167.
It’s all pretty simple to calculate. Who says golf isn’t rocket science?