It is not the water vapour that costs you distance over a pond
- Distance
- Ball flight
- Physics
"A shot over water gets swallowed by the pond because vapour hangs over the water, and that heavy air kills the carry."
You hear it often, and it sounds reasonable. Water is heavier than air, air carrying water vapour is heavier, and a ball does not fly as far through heavy air.
The first step is wrong. Humid air is lighter than dry air. Here is what the same ball-flight model behind the Carry Range Finder says about the size of it.
Water vapour is lighter than air
Air is roughly 78% nitrogen and 21% oxygen, with molecular weights of 28.0 and 32.0; dry air averages 28.96 g/mol.
A water molecule, H₂O, comes to 18.0 g/mol — one oxygen atom (16) with two light hydrogens attached, so it is lighter than a nitrogen molecule.
What matters next is that at a given pressure, a given volume holds a given number of molecules. Raising the humidity means that within that fixed count, heavy nitrogen and oxygen are replaced by light water molecules. That is why humid air is lighter.
The feeling that humid air is "heavy" is about sweat not evaporating. It is not about density.
How much lighter
Air density at sea level, 1013 hPa:
| Temperature | 0% RH (dry) |
50% RH | 100% RH (saturated) |
0% → 100% |
|---|---|---|---|---|
| 10°C | 1.2466 | 1.2438 | 1.2409 | −0.46% |
| 20°C | 1.2041 | 1.1988 | 1.1936 | −0.87% |
| 30°C | 1.1644 | 1.1552 | 1.1459 | −1.58% |
| 35°C | 1.1455 | 1.1335 | 1.1214 | −2.10% |
In kg/m³. The gap widens with temperature because warmer air can hold more vapour. Even saturated at 30°C, water vapour accounts for only 4.2% of the total pressure. The effect of humidity is small because there is only ever that much of it to add.
Through the ball-flight model
Carrying a pond is a summer problem, so the calculation runs at 30°C: saturated air over the water, against the 50% humidity of an ordinary fairway.
| Club | Base carry | 50% RH | 100% RH | Difference |
|---|---|---|---|---|
| Driver | 200 m | 203.46 m | 203.94 m | +0.48 m (+0.52 yd) |
| 7-iron | 128 m | 130.31 m | 130.63 m | +0.32 m (+0.35 yd) |
| PW | 96 m | 97.37 m | 97.56 m | +0.19 m (+0.21 yd) |
The sign is the other way round. Over the humid air above the pond the ball carries slightly further.
And the size of it is tiny. 32 cm with a 7-iron — 0.25% of a 130 m shot. That is not what leaves you short of the far bank.
Across temperatures the difference never reaches half a metre.
| Temperature | 7-iron at 50% RH | 100% RH | Difference |
|---|---|---|---|
| 10°C | 127.20 m | 127.31 m | +0.10 m |
| 20°C | 128.78 m | 128.96 m | +0.18 m |
| 25°C | 129.55 m | 129.79 m | +0.24 m |
| 30°C | 130.31 m | 130.63 m | +0.32 m |
| 35°C | 131.06 m | 131.48 m | +0.42 m |
Pushing it to the extremes changes nothing. Bone-dry air (0% RH) against saturated air over the pond (100% RH), both at 30°C, differ by 64 cm.
Put another way: going from 50% to 100% humidity is the same as raising the temperature from 30°C to 32.1°C. Two degrees of warmth. That is all the water vapour amounts to.
So why does the ball go in?
If it is not the vapour, what is it? Here are the factors that are genuinely present on a shot over water, through the same model. Everything is measured against 30°C, 100% RH, 7-iron (base 130.63 m).
Headwind: 17 times the humidity
| Headwind | Carry | Difference |
|---|---|---|
| 1 m/s | 129.00 m | −1.63 m |
| 2 m/s | 127.20 m | −3.43 m |
| 3 m/s | 125.22 m | −5.41 m |
| 5 m/s | 120.70 m | −9.93 m |
A breeze of 3 m/s costs 5.4 m — 17 times the humidity effect (+0.32 m), in the opposite direction.
There are no trees or buildings over a pond. The wind the ball meets can easily be stronger than the wind you feel on the tee, and whatever you discount as "not much wind" comes straight off the distance.
Air temperature over the water: larger than humidity, and the other way round
Water warms and cools slowly, so on a summer afternoon the air over it can be cooler than the air around it. Cold air is denser, and that does shorten the shot.
| Temperature | Carry | vs 30°C |
|---|---|---|
| 29°C | 130.46 m | −0.17 m |
| 28°C | 130.29 m | −0.34 m |
| 27°C | 130.12 m | −0.50 m |
| 25°C | 129.79 m | −0.84 m |
Three degrees cooler costs 50 cm — enough to cancel the humidity effect (+32 cm) with a little to spare.
So if the sense that "it does not carry over water" has any physical basis, it is water temperature, not water vapour. Even then it is a matter of tens of centimetres, and the sign flips with the season: from autumn into winter the water is the warmer of the two, so it works the other way.
Dispersion: an order of magnitude apart
Everything so far has been tens of centimetres to a few metres. Set that against how much the same club varies from swing to swing.
On a 130 m shot, dispersion of ±3% is ±3.9 m, and ±5% is ±6.5 m. The 32 cm from humidity is more than an order of magnitude smaller.
The pond is in front of you
And this may be the real answer.
Carrying water is a layout where the punishment depends entirely on which way you miss. Come up 4 m short with your usual dispersion and, without a pond, you are chipping from the apron. With a pond, it is a penalty stroke. The same miss; only the punishment changed.
You remember the ball you put in the water. You do not remember the ball that finished exactly as short on a hole with no water. The impression that "it does not carry over water" may well be built out of that.
What about mist?
Water vapour is a gas: there is nothing in the ball's path. Fog, with liquid droplets suspended in it, is a different question.
Run the numbers and raising the air density by 1% would take 11.5 g of liquid water per cubic metre — like atomising a litre of water into a one-metre box and keeping it there. In fog that thick you would not see the ball, or the far bank.
Flying through real fog, the contribution to density is smaller still than the effect of the vapour.
In short
- Humid air is lighter than dry air, because a water molecule (18) is lighter than nitrogen (28) or oxygen (32)
- At 30°C, going from 50% to 100% humidity makes a 7-iron carry 32 cm further — the equivalent of two degrees of warmth
- What actually matters over water is the headwind (5.4 m at 3 m/s) and your own dispersion (±3.9 m at ±3%). Different orders of magnitude
- Cooler air over the water does shorten the shot, but that is temperature, not vapour, and it stays under a metre
- A pond only punishes the short miss. What makes it memorable is not lost distance but the cost of the penalty
Taking an extra club over water is not justified by the reason usually given, but it is not a bad decision. It is just that what you are allowing for is the wind, your dispersion and the asymmetry of the punishment — not the water vapour.
This calculation treats humidity only as a change in air density. The change in viscosity is negligible at these speeds. The wind is taken as constant with height, launch angle and spin use representative values per club, and roll-out is not included.
To check it against your own clubs and today's conditions, put the temperature, humidity and wind into the Carry Range Finder. The model is described on the about page.