Does the iPhone have a humidity sensor?

No, and it is worth being plain about it

No iPhone has ever shipped with a humidity sensor. Not the current models, not any previous generation. The same is true of every mainstream Android phone sold today.

That means every humidity figure you have ever seen on a phone came from the network, not from the device. Including in this app.

Why people assume otherwise

Because phones genuinely are full of environmental sensors, and one of them is closely related.

The barometer is the one that misleads. It is a real atmospheric sensor giving a real local reading, so it is natural to assume its neighbours exist too. They do not.

Those internal thermistors are worth a note as well: they measure the phone's own components, which run warm. They are not air thermometers, and apps that have tried to present them as such have produced readings several degrees high.

Why no manufacturer includes one

Not cost — capacitive humidity sensors are inexpensive. The problems are physical:

• A phone is a heat source. The processor and battery warm the device above ambient. Since relative humidity depends on temperature, a sensor inside a warm phone reads the humidity of warmed air, not of the room.
• It has to breathe. A humidity sensor needs air exchange with the outside, which fights directly against water resistance.
• Pockets and bags. Most of the time the phone is enclosed against a body at 37°C in nearly saturated air. The reading would be of your pocket.
• Contamination. Polymer films drift when exposed to sweat, cosmetics and solvents — exactly what a phone lives in.

Samsung tried it in 2013 with the Galaxy S4 and Note 3 and quietly dropped it, which tells you how well it worked in practice.

So what is an app showing you?

A weather service's estimate of outdoor conditions at your coordinates, built from observation stations and model output. The nearest real instrument might be a mile away or twenty.

That is a legitimate number and a useful one — it is exactly what you want for whether a coating will cure, whether there will be fog, or whether it will feel muggy this evening.

What it is not is a reading of your living room. The honest framing, and the one this app uses, is that it reports the air outside — and that everything derived from it is exact given that input. Where the data comes from goes into the sourcing properly.

If you need the indoor number

Buy an external sensor. A Bluetooth or HomeKit hygrometer costs very little, pairs with the phone, and does the sensing in the accessory where it belongs — away from the heat, and in the air you actually care about. An app reading that accessory is reporting a real measurement; the sensor is simply not inside the phone.

Short of one, the number can be estimated rather than measured: outdoor air sets a floor the room cannot fall below once warmed, and how the room is used decides how far above it sits. That method, and its limits, in full. It is worth knowing what it can and cannot settle before relying on it.

Then calibrate it with the salt test, because consumer hygrometers are frequently several percent out of the box and drift further from there.

What your phone can genuinely tell you

• The outdoor humidity, dew point and water content at your location, from a weather service.
• The barometric pressure, from a real sensor.
• Everything derived from an outdoor humidity reading — condensation thresholds, fog likelihood, drying conditions.
• An estimate of an indoor range, if the app says plainly that is what it is.
• A real indoor reading, if you own a sensor and the app is reading it.

That is a genuinely useful set. What none of it is, is the phone sensing the room — and an app that blurs a network figure or an estimate into a reading is telling you something it cannot know.