Hygrometer
The definition
Absolute humidity is simply how much water is in the air, per unit volume. Take a cubic metre of air, extract every molecule of water vapour from it, weigh that water: that mass in grams is the absolute humidity.
It is an amount, not a comparison. That single property is what makes it useful, because two amounts can be compared and two percentages cannot.
A sense of scale
A cubic metre is roughly the volume of a large fridge. The water in it, on an ordinary day, is a couple of teaspoons.
| Temperature | Capacity | Roughly |
|---|---|---|
| −10°C / 14°F | 2.4 g/m³ | half a teaspoon |
| 0°C / 32°F | 4.8 g/m³ | a teaspoon |
| 10°C / 50°F | 9.4 g/m³ | two teaspoons |
| 20°C / 68°F | 17.2 g/m³ | a tablespoon |
| 30°C / 86°F | 30.3 g/m³ | two tablespoons |
| 40°C / 104°F | 51.0 g/m³ | a small espresso |
Note how the capacity roughly doubles every 10°C. That geometric climb is the reason humid heat is so much more oppressive than dry heat, and the reason cold air is always dry in absolute terms however high its percentage reads.
Scaled to a room rather than a fridge, the quantities stop being trivial — the calculator puts it in litres.
How it is calculated
Nothing measures absolute humidity directly. It is derived from a relative humidity reading and a temperature, via the vapour pressure:
This is just the ideal gas law applied to the water vapour on its own, treating it as a gas that happens to share space with the nitrogen and oxygen. Water vapour behaves nearly ideally at atmospheric conditions, so the approximation is a good one.
The one thing it does not survive
Absolute humidity is a density, and densities depend on volume. Warm a sealed parcel of air and it expands; the same water now occupies more cubic metres, so the g/m³ figure falls even though not a molecule has left.
The effect is modest — around 5–6% across a 17°C warming — but it means g/m³ is not a conserved property of an air mass. For that you want the mixing ratio in grams per kilogram of dry air, which is unaffected by heating, cooling, compression or lifting.
For comparing two conditions at ground level, g/m³ is perfectly good and considerably more intuitive. For tracking one body of air across a weather map, it is the wrong tool.
When to reach for it
- Comparing seasons or climates. The only honest way to say one is more humid than another.
- Working out whether something will dry. The gap between the current figure and the capacity is the room the air has left.
- Understanding why winter feels dry indoors. Cold air brought inside carries its few grams with it, and warming it adds none.
And when not to: anything about a rate or a surface — drying speed, mould, static — is properly a relative humidity question.