Humidity is the measurement most likely to be quietly wrong on your weather station, because the sensors that read it drift over time and are easily thrown off by dust, pollutants and age. A hygrometer reading that’s ten percent out doesn’t just give you a wrong humidity number — it feeds into your dew point calculation and your comfort readings too, so the error spreads. The reassuring part is that you can check any hygrometer’s accuracy at home, for essentially nothing, using a trick called the salt test.
The science behind it is elegant. When ordinary table salt — sodium chloride — is mixed with a little water into a damp slurry and sealed in a small enclosed space, it does something remarkably consistent: it holds the air in that space at very close to 75% relative humidity, almost regardless of temperature. This is a known physical property of saturated salt solutions, and it gives you a reliable reference point without any special equipment. If your hygrometer reads 75% in that environment, it’s accurate. Whatever it reads instead tells you its error.
To run the test, you’ll need table salt, water, a small open container like a bottle cap or shot glass, your hygrometer, and a sealable clear container or a zip-lock freezer bag big enough to hold both. Put a tablespoon or so of salt in the small container and add just enough water to dampen it into a thick paste — you want it wet but not flooded, with no standing water pooling on top. Place that container and your hygrometer together inside the larger sealed bag or box, making sure they’re close but the sensor isn’t touching the salt. Seal it up as airtight as you can.
Now the hard part: leave it alone. The enclosed air needs time to equilibrate to that 75% level, and the sensor needs time to respond, so give it a solid six to eight hours, and ideally overnight. Keep the whole thing somewhere with a stable temperature, away from sunlight, radiators or draughts, because swinging temperatures will disturb the reading. Resist opening the bag to peek — every time you do, you flush out the conditioned air and have to start the clock again.
When the time’s up, read the hygrometer through the clear container if you can, or open it and read quickly. A result between about 73% and 77% is well within the tolerance of most consumer sensors and means your hygrometer is doing fine. If it reads, say, 68%, then it’s under-reading by roughly seven points, and you can apply a correction. Many digital weather stations let you enter a humidity offset in their calibration settings — in this example you’d add about seven percent. If your unit has no offset option, at least now you know the error and can mentally adjust its readings, or factor it in when the humidity figure matters.
It’s worth running this test on a new hygrometer when you first get it, and then once a year afterward, because the drift is gradual and you won’t notice it creeping in from day to day. If you have several humidity sensors, you can test them all in the same bag at once and see how they compare — it’s often illuminating to discover that two sensors you trusted equally are several points apart. A little salt, a bag, and a patient afternoon is all it takes to turn your humidity readings from "probably about right" into "verified," and that confidence is worth a great deal when you’re trying to understand your local weather.