Hygrometer
Route 1 — buy a capacitive hygrometer
The straightforward answer, and right for nearly everyone.
A digital hygrometer with a capacitive sensor costs very little, reads in seconds, needs no maintenance and is accurate to a few percent. Buy one, put it in the right place, salt-test it once, and you are done.
The salt test is the part people skip and the part that matters, because consumer devices are frequently several percent out of the box and drift about a percent a year after that.
Route 2 — build a psychrometer
Two thermometers, one with a shoelace or cotton wick over the bulb dipped in distilled water, and air moving across both. Whirl them on a string or point a small fan at them for a minute or two, then read both.
The wet one reads lower because evaporation is taking heat away, and how much lower depends on how readily the air accepts water. The gap — the wet-bulb depression — gives the humidity via a psychrometric table or the formula in the relative humidity formula guide.
Why bother: nothing here is calibrated, so nothing can drift. It is a thermodynamic relationship between two temperatures, which is why psychrometers were the professional standard for a century and still appear in calibration procedures. Accuracy is comparable to a good capacitive sensor, provided the wick is clean, genuinely wet, and properly ventilated.
Route 3 — derive it
If you already have the air temperature and any one of these, you do not need to measure anything:
| If you have | Then |
|---|---|
| Dew point | RH = 100 × eₛ(dew point) / eₛ(air temperature) |
| Absolute humidity in g/m³ | Convert to vapour pressure, then divide by eₛ(T) |
| Mixing ratio in g/kg | Same, but needs the station pressure too |
| Wet-bulb temperature | Psychrometric relation, then divide by eₛ(T) |
The relative humidity calculator does all four. This is the route that applies when a forecast has given you a dew point but not a percentage, and it is exactly what the app does internally when a forecast hour arrives without a humidity field.
Placement, which matters more than the instrument
A hygrometer measures the air at the sensor. Humidity varies enough within a single room that placement can easily contribute more error than the specification does.
Indoors, avoid: exterior walls, window recesses, directly above radiators, in the path of vents, and anywhere in direct sun. Put it where the air you care about actually is — inside the humidor, next to the instrument, at head height in the room.
Outdoors, shade and ventilation are essential. A sensor in the sun measures its own heated enclosure; since relative humidity depends on temperature, a few degrees of solar heating produces a large humidity error. This is the entire purpose of the louvred white screens at weather stations, and it is the main difference between a hobby station's numbers and an official observation's.
What none of these give you
An outdoor reading from a weather service — which is what an app shows — is measuring air that may be miles away. That is fine for outdoor questions, where the air mass is shared, and useless for indoor ones.
Conversely, an instrument in your hallway tells you nothing about whether it will be foggy in the morning. The two are different measurements answering different questions, and neither substitutes for the other.