Hygrometer
Two questions, not one
The confusion comes from treating these as two ways of saying the same thing. They answer different questions:
- Relative humidity asks how close is this air to full? It is a ratio, and its denominator changes with temperature.
- Dew point asks how much water is in this air? It is expressed as a temperature, but it behaves like a quantity.
A glass that is 70% full tells you nothing about how much water it holds unless you know the size of the glass. Relative humidity is the 70%. Temperature sets the size of the glass.
The same percentage, two different airs
| January | July | |
|---|---|---|
| Air temperature | 35°F / 1.7°C | 85°F / 29.4°C |
| Relative humidity | 70% | 70% |
| Dew point | 26°F / −3.2°C | 74°F / 23.4°C |
| Water in the air | 3.8 g/m³ | 20.6 g/m³ |
| How it feels | Dry. Static, chapped lips. | Oppressive. |
Identical readings. More than five times the water in one than the other. Anybody quoting the 70% alone has told you almost nothing.
Why one moves all day and the other does not
Relative humidity swings enormously between dawn and mid-afternoon on an ordinary day — 95% to 45% is unremarkable. Almost none of that is water arriving or leaving.
What changes is the temperature, and with it the amount of water the air could hold. The numerator stays put; the denominator roughly doubles for every 10°C of warming. So the percentage falls through the morning and climbs again after sunset, tracing out a curve that is really just the temperature curve upside down.
The dew point, meanwhile, sits nearly flat. When it does move, something real has happened — a front, a wind shift, air arriving off the sea instead of off the land. That is why it is the more informative line on a dew point graph.
Which to use for what
| Question | Use | Why |
|---|---|---|
| Will it feel muggy? | Dew point | Comfort tracks the water present, not the ratio. |
| Will this surface fog? | Dew point | It is a direct threshold temperature. |
| Is today more humid than last week? | Dew point | Only it is comparable across temperatures. |
| How cold will tonight get? | Dew point | Air will not cool far below it. |
| How fast will this dry? | Relative humidity | Evaporation rate depends on how close to full the air is. |
| Can mould establish here? | Relative humidity | The thresholds are defined at surfaces as a percentage. |
| Why is there so much static? | Relative humidity | Surface conductivity depends on the ratio. |
The honest summary
Neither number is wrong, and the pairing is the point. A percentage without a temperature is ambiguous, and a dew point without the percentage does not tell you how fast things dry.
Both come from the same measurement, so there is never a reason to have only one — which is why the app puts them together, along with the grams per cubic metre that makes the comparison concrete.