Hygrometer
The chart
Each cell is the grams of water per cubic metre that combination actually contains. Read across a row to see how much a fixed percentage varies with temperature — that variation is the whole point of the table.
| RH | 0°F | 10°F | 20°F | 30°F | 40°F | 50°F | 60°F | 70°F | 80°F | 90°F | 100°F |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 20% | 0.3 | 0.4 | 0.6 | 0.9 | 1.3 | 1.9 | 2.6 | 3.7 | 5 | 6.8 | 9.1 |
| 40% | 0.5 | 0.8 | 1.2 | 1.8 | 2.6 | 3.8 | 5.3 | 7.4 | 10.1 | 13.6 | 18.2 |
| 60% | 0.8 | 1.2 | 1.8 | 2.7 | 3.9 | 5.6 | 7.9 | 11 | 15.1 | 20.5 | 27.3 |
| 80% | 1 | 1.6 | 2.4 | 3.6 | 5.2 | 7.5 | 10.6 | 14.7 | 20.2 | 27.3 | 36.4 |
| 100% | 1.3 | 2 | 3 | 4.5 | 6.5 | 9.4 | 13.2 | 18.4 | 25.2 | 34.1 | 45.6 |
The bottom row is saturation — the most the air can hold at that temperature. Every other row is a fraction of it.
What the rows tell you
Follow the 60% row from left to right. At 0°F it is under 1 g/m³, air so dry that static is inevitable and nothing outdoors stays damp. At 100°F the same 60% is over 27 g/m³, air so full that sweat barely evaporates.
Same number. Same "60% full". Thirty-five times the water.
This is why a forecast humidity figure quoted without its temperature is close to useless, and why the dew point — which encodes the quantity directly — is the better single number to carry around.
The capacity curve
| Temperature | Capacity | Water in 40 m³ |
|---|---|---|
| 0°F | 1.29 g/m³ | 0.05 L |
| 20°F | 3.02 g/m³ | 0.12 L |
| 40°F | 6.55 g/m³ | 0.26 L |
| 60°F | 13.24 g/m³ | 0.53 L |
| 80°F | 25.21 g/m³ | 1.01 L |
| 100°F | 45.55 g/m³ | 1.82 L |
The doubling every ten degrees or so is not a coincidence or an approximation — it falls out of the Clausius-Clapeyron relation, which describes how the saturation vapour pressure of any liquid changes with temperature. It is a property of water itself.
Using it in practice
- Comparing two days — find both cells. The larger number is the more humid air, whatever the percentages say.
- Working out whether something will dry — compare the cell with the saturation figure in the same column. The gap is the room the air has left.
- Understanding indoor winter dryness — find the outdoor winter cell, then look at what that same quantity would be as a percentage at 68°F. It is usually startling.
That last one is worked through in relative vs absolute humidity, and the converter will do it for any pair of values you like.