Hygrometer
The chain, end to end
- Instruments measure. Weather stations run capacitive humidity sensors inside radiation shields, reporting at regular intervals. These are real hygrometers, maintained and calibrated.
- Models fill the gaps. Numerical weather prediction assimilates those observations, along with satellite and radiosonde data, and produces a gridded field covering everywhere — including the large spaces between stations.
- Apple Weather aggregates. It combines observation and model data into a service that can answer for arbitrary coordinates.
- The app requests your point. Given your location, it receives temperature, relative humidity and, where available, a dew point.
- The device computes the rest. Dew point if not supplied, absolute humidity, mixing ratio, comfort band and condensation margin are all calculated on the phone from that input.
Steps 1 to 4 are somebody else's measurement. Step 5 is exact arithmetic on whatever arrived — and that arithmetic is what this website's calculators run too, ported from the same source and tested against it.
Where the error actually is
Worth ranking honestly, because the sources are not equal:
| Source | Typical size | Comment |
|---|---|---|
| Distance to the nearest station | Largest by far | Humidity varies over hundreds of metres. A valley, a lake, a car park and a wood genuinely differ. |
| Model interpolation | Significant | Grid spacing is kilometres; local terrain is finer than that. |
| Station sensor accuracy | ±2–3% RH | Small in comparison, and well controlled. |
| The formulas | Negligible | Well under 0.1%. Never your problem. |
The ordering is the point. People assume the arithmetic or the sensor is the weak link; it is almost always the fact that the nearest real observation is somewhere else.
What that means for reading the number
- Trends are more reliable than absolutes. The direction and the overnight pattern are usually right even when the exact figure is a few points off.
- The dew point travels better than the percentage. It is a property of the air mass, so it varies less over distance than relative humidity, which also depends on the local temperature.
- Nearby readings can legitimately disagree. Two providers, or a provider and your own instrument, may both be correct about different air.
The website's live pages use a different source
Worth stating so nothing here is misleading. The humidity today and forecast pages on this site read Open-Meteo, not Apple Weather, because the site runs in a browser with no WeatherKit entitlement.
Same physics on top, different underlying observations. Small differences between this site and the app are expected and are about the weather model, not about the calculation.
Why there is no indoor figure
Because there is nothing to measure it with. No iPhone has a humidity sensor, and a weather service knows nothing about your building.
Indoor humidity is a product of the structure, the heating, the ventilation, the occupants and whatever is drying — none of which is visible from outside. Claiming to report it would be inventing a number, so the app does not, and this site's tests fail the build if any page implies otherwise.
What the outdoor reading genuinely gives you is the air that arrives when you open a window, the threshold every cold surface has to stay above, and whether anything outdoors will dry today. That is a real answer to real questions, and it does not require pretending.