Hygrometer
Forty-nine guides, grouped by what you are trying to work out. If you have a number and want to know what it means, start with the dew point set. If something is wet that should not be, start with condensation.
Guides
The answer in the first paragraph, then the explanation. Most of it is about the air outside, whose properties are the same wherever that air goes — including once it comes indoors and warms up.
Dew point
The one humidity figure that means the same thing in every city, and the reason it barely moves while the percentage swings all day.
- What is dew point The one humidity figure that means the same thing everywhere — and the part most definitions leave out.
- Reading a dew point graph What a 24-hour dew point curve tells you that a temperature line cannot — and what the gap between them predicts.
- Dew point vs humidity The same percentage in January and July describes two completely different airs. Here is why, in grams.
- Comfortable dew point Where comfortable ends and sticky begins — and why the threshold is a dew point, not a percentage.
- Dew point today Today's figure, and why it is the one number that does not move while everything else does.
- High dew point 65, 70 and 75°F — what each threshold stops working, rather than just how it feels.
- How to measure dew point One instrument measures it; everything else calculates it. Which is which, and how to do the arithmetic yourself.
Relative and absolute humidity
Why a percentage on its own describes two completely different airs in January and July, and what the grams-per-cubic-metre figure adds.
- Relative vs absolute 80% in winter really is drier than 40% in summer. Here is the arithmetic that proves it.
- What is relative humidity What the percentage measures — and, more importantly, what it is a percentage of.
- Relative humidity formula The definition, the approximation, and why conflating the two is where most explanations go wrong.
- What is absolute humidity Grams of water per cubic metre, explained — and when it is the number you actually want.
- Converting between them Both directions, with the working — for when you need it in a spreadsheet rather than once in a browser.
- Relative humidity chart One percentage, every temperature, and the grams of water each cell really represents.
- Specific humidity Per kilogram of total air, not dry air. The difference is small, and this is when it matters.
- Mixing ratio The one humidity figure that survives being heated or carried up a mountain.
Condensation, dew and fog
Water appearing on glass, grass and metal. All of it is one mechanism, and all of it is predictable the evening before.
- Condensation on windows Inside, outside, or between the panes — three different causes, and only two of them are about the air.
- Condensation outside It looks like a fault. It is the opposite — proof the glazing is doing its job.
- Stopping condensation There are exactly two levers. Every remedy is one of them — ranked by how far it moves the number.
- Why windows fog up One comparison explains all of it — houses, cars, glasses and camera lenses alike.
- Why dew forms Why the grass gets soaked on a night the air never got cold.
- When dew forms The overnight clock — when it starts, when it peaks, and when it burns off.
- Morning condensation The coldest moment of the night is dawn, not midnight. That one fact explains the whole pattern.
- Skylight condensation It faces straight up at the sky. That geometry alone makes it the coldest glass in the building.
- Will it be foggy The gap between temperature and dew point is the entire fog forecast. What each size of gap means.
- Condensation and mould Mould needs hours, not water. Why duration beats the peak, and why the frame goes before the glass.
Hygrometers and measuring humidity
What the instrument is, how each kind works, how far off it probably is, and how to check.
- What is a hygrometer Any instrument that measures humidity — and there are more kinds than the word suggests.
- What it measures One measurement, four more numbers for free — and the one thing it cannot tell you.
- How humidity is measured Five mechanisms cover every humidity measurement ever made. Which one is behind your number.
- How it works Each mechanism explained, with the way each one eventually goes wrong.
- What it is used for The uses, sorted by the only question that matters: does this need an instrument in the room?
- Measuring relative humidity Three routes that work, what each costs in accuracy, and where to put the thing.
- Types of hygrometers Six mechanisms in one table: accuracy, drift, response time and what each is for.
- How accurate they are Typical error by type — and why the sensor is rarely the thing that is wrong.
- The salt test 75% from a tablespoon of salt. The procedure, and the three mistakes that ruin it.
- Without a hygrometer Three methods that work, one that does not, and how accurate each really is.
- iPhone humidity sensor No, and neither does any mainstream phone. What every humidity app is actually reading.
- Psychrometer vs hygrometer All psychrometers are hygrometers. Why this particular kind refuses to drift.
Comfort, health and mould
The thresholds people actually ask about, and where the numbers behind them come from.
- Indoor humidity range 30–50% is the target. Here is how to work out where your room actually is.
- Comfortable humidity 30–50%, and the standards behind it — plus why the answer changes with temperature.
- Humidity and mould 80% at the surface, not 60% in the room — and why duration matters as much as level.
- Preventing mould The number to stay under, and why duration matters more than the peak ever does.
- Humidity and sleep 30–50% is the standard answer. The overnight dew point is the more useful one.
- Dry air symptoms Why they all arrive in the same week each year — and it is not the heating drying the air.
- Static and humidity Below 40% the water film on surfaces stops forming, and charge has nowhere to go.
- Humidity at night The water didn't change. The temperature did. The most satisfying answer in the whole subject.
The app, and your iPhone
What the phone can and cannot tell you, where this app's figures come from, and what is on the widgets.
- Humidity widget Every widget size, and what each one fits. The point is rarely opening the app.
- Best humidity app Every app shares one constraint. What to compare once you have accepted it.
- Apple Weather humidity Where the humidity tile actually is — and the four figures it does not carry.
- Reading the dial The five bands, and what the four numbers underneath the dial are actually for.
- Where the data comes from Apple Weather, stations and models — with the error sources named rather than waited for.