Guide

What is a hygrometer used for?

Two families of use. Indoor ones — humidors, instruments, reptile enclosures, damp diagnosis — need a physical sensor in that space. Outdoor ones — whether coatings will cure, whether fog or frost is coming, whether laundry will dry, whether optics will mist — need a reading of the air outside, which is what a weather service can give.

The split that matters

Most lists of hygrometer uses run them together, which is unhelpful, because the single most important question is: does this use need a sensor in that specific air, or a reading of the outdoor air?

Get that wrong and you either buy an instrument you did not need or trust an app for something it cannot know.

The same instrument, two entirely different jobs.
Needs a sensor in the space	Needs an outdoor reading
Humidors and cigar storage	Whether paint, stain or sealer will cure overnight
Wooden instruments and their cases	Whether there will be fog on the morning drive
Reptile and amphibian enclosures	Whether tonight brings dew or frost
Wine cellars and archives	Whether laundry hung out will actually dry
Diagnosing damp in a specific room	Whether a lens or visor will mist
Server rooms and clean rooms	Whether it will feel muggy this evening
Deciding on a dehumidifier	Whether ventilating will dry a room or dampen it
Curing chambers and drying rooms	Whether concrete will cure or sweat

The left column

These all share one property: the air in question is enclosed and its humidity is different from outside. A humidor is held at 70% while the room is at 45%. A reptile enclosure is deliberately tropical. A room with a damp problem differs from the room next door.

Nothing outside that box can tell you what is inside it. These need a calibrated instrument in the space, and — because consumer hygrometers drift — one that has been checked with the salt test.

The right column

These share the opposite property: the air in question is the outdoor air, and its properties are the same wherever that air mass extends. The dew point over your garden is the dew point over your neighbour's.

That is what makes them answerable without an instrument. A weather service operating real hygrometers in the region can tell you what the outdoor air is carrying, and the derived figures — dew point, grams per cubic metre, the condensation threshold — follow from it exactly.

This is the column the app covers, and it is why the app can be honest about having no sensor: it does not need one for these questions.

The interesting middle case

Ventilation sits in both columns, which is why it causes so much confusion.

Whether opening a window will dry a room out depends on comparing the indoor dew point with the outdoor one. You need the first from an instrument in the room and the second from a weather service — and most advice on the subject supplies neither, which is why "just ventilate more" works brilliantly in January and can backfire on a humid August night.

What to buy, if anything

If your question is in the left column, buy an instrument, put it where the air you care about actually is, and calibrate it. If it is in the right column, you do not need hardware — you need the outdoor dew point, and that is free.

If someone is selling you an app that claims to measure the humidity of the room you are standing in, it cannot.

Common questions

Do I need a hygrometer at home?

Only if you have a specific indoor question — a humidor, an instrument collection, a damp problem you are trying to diagnose, or a room you suspect is too dry in winter. For general curiosity about the weather, an outdoor reading is more useful and costs nothing.

Where should I put a hygrometer?

In the air you actually care about, away from direct sun, heat sources, draughts and exterior walls. Humidity varies sharply within a single room, so a sensor beside a radiator or against cold glass is measuring a microclimate rather than the room.

What is a hygrometer used for in a humidor?

To hold the interior near 68–72% relative humidity, which keeps cigars from drying out or growing mould. It is a sealed-box application and needs a calibrated instrument inside the box — no external or app reading can substitute.

Do greenhouses need a hygrometer?

They benefit from one, because the length of time leaves stay wet drives fungal disease. That said, the outdoor dew point is often the more actionable figure, since it sets what the overnight conditions inside will trend towards.

Is a weather app a substitute for a hygrometer?

For outdoor questions, largely yes. For indoor questions, no — an app reports outdoor conditions from a weather service and knows nothing about your building. The two answer different questions and neither replaces the other.

Sources

WMO Guide to Instruments and Methods of Observation (WMO-No. 8)

Published 12 August 2026