Hygrometer
The literal answer
Relative humidity, as a percentage. That is the measurement. Everything else on the display is arithmetic performed on it.
The reason it is relative humidity specifically, rather than an amount, is that the sensing element physically responds to how close the air is to saturation. A polymer film takes up water in equilibrium with the surrounding air; how much it takes up depends on the ratio, not on the total water present. The instrument is built around what it can actually feel.
The four you get for free
Add a temperature — which every hygrometer also has — and four more figures fall out without any extra hardware:
| Figure | What it tells you | Measured or derived? |
|---|---|---|
| Relative humidity | How close the air is to full | Measured |
| Dew point | The temperature at which it would saturate | Derived |
| Absolute humidity | Grams of water per cubic metre | Derived |
| Mixing ratio | Grams per kilogram of dry air | Derived |
| Condensation threshold | The temperature below which surfaces get wet | Derived — it is the dew point |
None of that is a shortcoming. The relationships are exact physics, and the approximation used to evaluate them contributes far less error than the sensor does. A device showing five figures from one sensor is being efficient, not misleading.
Why the percentage alone is not enough
The measurement is relative humidity, and relative humidity is a ratio to a moving target. 70% at 35°F and 70% at 85°F describe air with more than five times the difference in water content.
So a hygrometer that shows only a percentage is giving you the raw reading and leaving you to do the interpretation. That is why the derived figures matter: the dew point and the grams per cubic metre are the ones that mean the same thing every time you look at them.
What it cannot measure
Two things worth being clear about.
It cannot measure air it is not in. A hygrometer reports the conditions at the sensor, and humidity varies sharply over short distances — between rooms, between the middle of a room and a cold corner, between a lawn and a car park. This matters for what accuracy actually means in practice.
And no phone contains one. When a weather app shows a humidity figure, it is reporting a weather service's interpolation from instruments elsewhere — which is worth understanding before trusting any app's number for the room you are standing in.