Hygrometer
Two measurements of the same thing
There is a fixed quantity of water in any given parcel of air. Both figures describe it; they just divide by different things.
- Absolute humidity — the mass of that water per cubic metre. A quantity. 12 g/m³ means twelve grams, wherever and whenever you measure it.
- Relative humidity — that same water as a percentage of the maximum the air could hold at its current temperature. A ratio, and the denominator moves.
The denominator is the entire story. It roughly doubles for every 10°C of warming, which means a percentage is only meaningful alongside the temperature it was measured at.
The claim, with the arithmetic
| Winter | Summer | |
|---|---|---|
| Air temperature | 0°C / 32°F | 25°C / 77°F |
| Relative humidity | 80% | 40% |
| Capacity at that temperature | 4.8 g/m³ | 23.0 g/m³ |
| Water actually present | 3.9 g/m³ | 9.2 g/m³ |
| Dew point | −3.0°C / 27°F | 10.5°C / 51°F |
Twice the percentage, less than half the water. The winter air is not "80% humid" in any sense that would make it feel damp — it is nearly empty air that happens to be close to its own very low ceiling.
Why this explains winter indoors
This single mechanism explains something almost everyone notices and few can account for: why houses feel so dry in cold weather, even when the outdoor forecast says the humidity is high.
Take that 0°C air at 80%, carrying 3.9 g/m³. Bring it inside and warm it to 21°C. No water has been added — it is still 3.9 g/m³. But air at 21°C can hold about 18.3 g/m³, so the relative humidity is now about 21%.
That is desert-dry, and it is where the static shocks, nosebleeds and cracked lips come from. Nothing dried the air out; the air was always that dry, and heating it merely revealed the fact.
The one wrinkle: absolute humidity is a density
Grams per cubic metre is a quantity, but it is a quantity per volume — and air expands when it warms. So a sealed parcel of air, heated, shows a slightly falling absolute humidity even though not a molecule of water has left.
The effect is small (13.2 → 12.5 g/m³ across a 30°F warming) but it is real, and it is why meteorologists prefer the mixing ratio in g/kg, which is per kilogram of dry air and is genuinely unchanged by heating, cooling or lifting.
For everyday comparison, g/m³ is fine. For tracking one air mass across a map, it is not.
When each is the right number
| If you want to know… | Use |
|---|---|
| Is today more humid than last month? | Absolute humidity or dew point |
| Will it feel muggy? | Dew point |
| Will this surface fog up? | Dew point |
| How fast will the washing dry? | Relative humidity |
| Can mould grow on this? | Relative humidity |
| Why so much static? | Relative humidity |
Converting between them
The absolute humidity calculator goes both ways, and how much water is in the air turns the density into litres for a volume you choose — which is the version that makes it stick. If you want the working rather than the answer, the conversion is here.