Hygrometer
The answer in one sentence
The water did not change. The temperature did.
Relative humidity is a fraction: the water actually present, divided by the most the air could hold at its current temperature. Overnight the numerator sits still and the denominator collapses, so the fraction climbs.
That is the entire mechanism, and once it clicks a surprising number of other things become obvious.
Watching it happen
| Time | Temperature | Could hold | Actually holds | Relative humidity |
|---|---|---|---|---|
| 4pm | 86°F / 30°C | 30.3 g/m³ | 13.2 g/m³ | 42% |
| 8pm | 77°F / 25°C | 23.0 g/m³ | 13.2 g/m³ | 56% |
| Midnight | 68°F / 20°C | 17.2 g/m³ | 13.2 g/m³ | 76% |
| 4am | 63°F / 17°C | 14.4 g/m³ | 13.2 g/m³ | 91% |
| 5:30am | 60°F / 15.6°C | 13.3 g/m³ | 13.2 g/m³ | 100% |
The "actually holds" column never moves. The reading goes from 42% to 100% anyway.
At the bottom of that table the two middle columns meet, which is the definition of the dew point — and it is why the dew point of that air was 60°F all along, at four in the afternoon just as much as at dawn.
Why the temperature falls at all
After sunset the ground radiates infrared energy to the sky and nothing replaces it. The ground cools, the air in contact with it cools, and the process continues until the sun comes back.
Which is why the minimum is at dawn rather than midnight — cooling runs the whole night, so the coldest moment is the last one before sunrise. Same reason condensation peaks then rather than in the small hours.
Where it stops
The percentage cannot exceed 100%, and something has to give. As the temperature reaches the dew point, the air begins handing water back:
- Onto surfaces, as dew — or frost, below freezing.
- Into the air itself as droplets, which is mist and fog.
And this is what puts a floor under the overnight low. Condensing water releases latent heat almost as fast as the ground can lose it, so cooling nearly stops once the dew point is reached. A night with a 70°F dew point will not fall much below 70°F, however clear the sky.
What this explains
Once you see it, the same mechanism accounts for a lot:
- Why forecasts quote the dew point. It is the number that stayed still and therefore actually describes the air.
- Why washing left out overnight comes in damper. The air's headroom shrank to nothing — the laundry case.
- Why paint applied at 4pm can fail by 9pm. The label's temperature range was satisfied; the dew point was the real deadline.
- Why heated indoor air is so dry in winter. Same arithmetic in reverse — warm cold air and its percentage collapses.
- Why deserts get dew. Enormous overnight cooling can reach even a very low dew point.
Seeing it for yourself
The humidity forecast draws the hourly relative humidity and dew point together. The percentage rises through the night; the dew point sits flat. Nothing makes the point faster than watching the two rows disagree.