Bench Notes › Altitude
Correcting Carry for a Mile of Missing Air
My job in a fitting session is partly to write things down and partly to make sure that what gets written down means something a month later. The largest single threat to that, in this city, is a carry figure with no altitude attached to it.
Elevation is the headline, density is the number
This bench sits at 5,312 feet. That is the figure everybody quotes and it is only half of the story, because air thins with heat as well as with height. On a hundred-degree afternoon in June, the air over the strip out back behaves like air at somewhere near eight thousand feet. On a cold January morning it behaves like air at around four thousand.
Density altitude is the single number that folds elevation, temperature and barometric pressure into one figure. It is what pilots use to work out whether a runway is long enough, and it is the correct input for anything to do with how far a golf ball flies. Elevation alone is a starting point that is wrong most of the year.
So we log pressure and temperature at the start of a session and again at the end. If the two readings disagree meaningfully, which they do on spring afternoons, the correction is applied in two halves rather than one.
What thin air does, in the order it does it
Drag falls. Fewer air molecules in the way means less resistance, so the ball retains speed longer. On its own this lengthens every shot in the bag.
Lift falls too. This is the part that gets forgotten. Backspin generates lift by pushing air around the ball, and thinner air gives it less to work with. The ball climbs less and holds its height less well at the end of the flight.
The two do not cancel. For most clubs drag reduction wins comfortably and the shot flies further. But the balance shifts by club. A high-spinning, high-launching wedge loses a meaningful share of the lift that was holding it up, and gains less from reduced drag because it was never travelling fast enough for drag to dominate. A driver gains almost entirely.
The rule of thumb we do not use. Add ten percent to everything. It is roughly right for a driver on a mild day and wrong by several yards in both directions elsewhere in the bag. Applied to a whole set it produces a gapping chart that looks tidy and plays badly.
Roughly what the corrections look like
These are indicative figures for a mid-handicap male player at our bench on a mild day, comparing what a sea-level chart would predict against what we measure and correct to. They are here to show the shape of the effect, not to be used as your own numbers.
| Club | Approximate gain | Why it lands there |
|---|---|---|
| Driver | 8 to 11 percent | Highest ball speed, drag reduction dominates |
| Fairway wood | 8 to 10 percent | Same effect, slightly more spin working against it |
| Long iron or hybrid | 7 to 9 percent | Drag still dominant, apex lower |
| Mid iron | 6 to 8 percent | Lift loss beginning to bite into the gain |
| Short iron | 5 to 7 percent | High spin, high launch, more lift to lose |
| Wedge, full swing | 4 to 6 percent | Lift loss largest here relative to drag gain |
The consequence nobody expects: the bag compresses
Look at that table as a set rather than as individual clubs. The top of the bag gains around ten percent and the bottom gains around five. Which means the distance between your driver and your three-wood grows, and the distance between your pitching wedge and your nine-iron grows, but the distance between the long clubs and the short clubs grows unevenly and the whole ladder stops being evenly spaced.
In practice the most common finding is at the top. A player arrives from a lower altitude with a three-wood and a five-wood that gapped perfectly at home, and here they land eight yards apart because both gained ten percent of a large number and the difference between them was small to begin with. One of those clubs is now redundant and the hole it leaves is further down.
The second most common finding is below the pitching wedge, where strengthened set lofts have already created a hole and altitude has widened it. That one usually gets solved with a loft change on a club already in the bag rather than a purchase.
What goes on the card
Two columns, always. The left one is what the monitor said in the conditions of the day, unedited. The right one is the same shot corrected to a standard reference for this city, which is what you should be planning your yardages against.
Underneath both, the conditions themselves: temperature, pressure, the density altitude that came out of them, the ball model, and whether the block was hit indoors or on the strip. A number without those attached is not data. It is decoration, and it will mislead whoever reads it next, including you.
Travelling with corrected numbers
The card is deliberately built so it travels. If you play a week in San Diego, the raw column is not much use and the corrected column is not either, but the conditions line lets any competent fitter or caddie work out what your numbers should be at sea level in about a minute.
The reverse trip catches people out more. Players who learn their yardages here and then compete at lower elevation frequently come up short for two days before they adjust, because everything they know about their own distances was learned in air that does not exist where they are standing.
Where this gets used in a session
The correction happens in the fourth block, after testing and before the card is written. It is not applied during testing, because during testing we are comparing one spec against another in identical conditions and the raw numbers are the honest comparison. Correcting mid-session would add arithmetic without adding information.
It matters most in iron sessions, where gapping across the set is the entire point, and in the full bag ledger, where fourteen corrected carries laid out as a strip is usually the moment a player sees the problem for themselves.
Related: the session walkthrough and how the correction block fits into the two hours.