Hygrometer
The definition
A hygrometer is any instrument that measures the water vapour content of air. That is the whole definition, and it is deliberately broad — it says nothing about the mechanism, the units, or whether there is a display.
What has changed over the centuries is only how the measurement is made. The category has held a horsehair under tension, a pair of thermometers with a wet sock on one of them, a cooled mirror with a photodetector watching it, and a polymer film between two electrodes.
What it reports
Almost always relative humidity, as a percentage. That is what the sensing element physically responds to — a polymer film absorbs water in proportion to how close the surrounding air is to saturation, not in proportion to the absolute amount present.
Everything else is calculated from that reading plus a temperature:
- Dew point — the temperature at which the air would saturate.
- Absolute humidity — grams of water per cubic metre.
- Mixing ratio — grams per kilogram of dry air.
So a device advertising four humidity figures usually has one sensor and three formulas. That is not a criticism; the arithmetic is exact and the derived figures are genuinely more useful for some questions than the percentage is.
The main kinds
| Type | Mechanism | Where you meet it |
|---|---|---|
| Capacitive | Polymer film changes capacitance as it absorbs water | Nearly every modern device |
| Resistive | A salt or polymer changes electrical resistance | Cheap sensors, HVAC controls |
| Hair tension | Human or horse hair lengthens with humidity | Antique and decorative dial instruments |
| Psychrometer | Two thermometers, one with a wetted wick | Reference and calibration work |
| Chilled mirror | Cools a mirror until dew forms on it | Laboratory reference standard |
| Weather service data | Not an instrument — a network of them, interpolated | Forecasts, and apps like this one |
That last row is worth stating plainly. When a phone shows a humidity figure, nothing in the phone measured it — no phone has a humidity sensor. The figure comes from a weather service that operates real instruments elsewhere and interpolates to your coordinates.
Why the word covers so much
"Hygrometer" describes a purpose, not a technology, which is why it stretches from a 15th-century balance to a semiconductor die. Compare "thermometer": mercury, bimetallic strip and thermistor share nothing mechanically, but all measure temperature.
The practical consequence is that comparing two hygrometers means asking which mechanism each uses and what accuracy that mechanism gives — which is a table rather than a number.
What it is for
Broadly, two families of use: knowing whether the air will let something dry, and knowing whether something is about to get wet. What a hygrometer is used for sorts them by whether they need an instrument in the room or an outdoor reading — which is the distinction that decides whether an app can help you at all.