How to measure humidity without a hygrometer

Method 1 — two thermometers (the good one)

This is not a workaround. It is a psychrometer, and it was the professional standard for a century.

What you need: two thermometers reading the same value in the same conditions, a shoelace or strip of cotton, and distilled water if you have it.

What to do: wrap the wick around one bulb and wet it thoroughly. Get air moving across both — whirl them on a string, or aim a fan at them — for a minute or two, until the wet reading stops falling. Read both.

What it gives you: the wet bulb reads lower, because evaporation is carrying heat away. How much lower depends on how readily the air accepts more water. Feed both temperatures into the relative humidity calculator, or use a psychrometric table.

Accuracy: ±2–3% with a clean wick and proper ventilation, which is as good as most consumer hygrometers. And nothing drifts, because nothing is calibrated — it is a thermodynamic relationship between two temperatures.

Method 2 — the ice-water glass (rough dew point)

What to do: put water and a thermometer in a metal or thin glass container, add ice slowly while stirring, and watch the outside. Note the temperature at the moment condensation first appears on the outer surface.

What it gives you: approximately the dew point of the room air. Which, with the air temperature, gives the relative humidity.

Accuracy: a few degrees, and biased. The outer surface is always slightly warmer than the water inside, so it fogs a little late — meaning the reading tends to come out low. Add ice slowly to keep the gap small.

Rough, but it needs nothing you do not already own, and it measures the right physical quantity rather than guessing from symptoms.

Method 3 — a weather service (for outdoor air only)

For anything about outdoor conditions, someone has already measured it with a properly sited, maintained instrument. That reading is available for your coordinates, and everything else derives from it exactly.

This is what humidity today does, and what the app does. The honest framing matters: it is a real measurement, just not one made where you are standing, and distance to the nearest station is its main limitation.

It answers outdoor questions well. It also gets you most of the way to an indoor one, because the outdoor water content sets the floor a heated room sits above — enough to estimate the likely range, though not to read it.

What does not work

Physical signs. Sticking doors, clumping salt, slow-drying laundry, static, hair behaviour — all genuinely respond to humidity, and all are directional only. They tell you it has gone up. They cannot tell you whether it is 60% or 80%, and the thresholds that matter for mould or a humidor sit inside that gap.

Pine cones and seaweed. They do respond. They are slow, they lag badly in both directions, and they are unquantified. Charming, not instruments.

Phone apps claiming a local sensor reading. There is no sensor. No mainstream phone has one, so an unqualified indoor figure either came over the network and describes outdoor air, or is an estimate being passed off as a measurement. An app reading a sensor you own is a different thing, and so is one that labels its estimate as an estimate — the objection is to the blurring, not to the arithmetic.

Which to use