Hygrometer
The energy balance, not the clock
Midnight is the middle of the night by the clock and means nothing thermally. What matters is that surfaces have been losing heat since sunset and nothing puts any back until sunrise.
So the curve does not bottom out halfway through. It falls the whole way, decelerating as it goes, and reaches its minimum at or just after sunrise — with the deepest cold in the last hour before the sun clears the horizon.
Condensation follows the surface temperature, so it follows the same curve. Every surface that is going to get wet is at its wettest right at the point most people are waking up.
The dew point did not move
The other half of the explanation is what did not happen: the air did not get more humid overnight.
Water content stays essentially constant unless a different air mass arrives. What changed is only the temperature, which is why the relative humidity climbed towards 100% while the dew point stayed flat.
So nothing pushed the dew point up to meet the glass. The glass came down to meet the dew point.
| Time | Air temperature | Dew point | Glass temperature | Wet? |
|---|---|---|---|---|
| 9pm | 62°F | 52°F | 60°F | No |
| Midnight | 57°F | 52°F | 54°F | No |
| 3am | 54°F | 52°F | 51°F | Yes |
| 5:30am | 52°F | 52°F | 49°F | Heavily |
| 8am | 58°F | 52°F | 57°F | Clearing |
The dew point column never changes. The glass runs a few degrees below the air all night because it is radiating to the sky, and it crosses the threshold somewhere in the small hours.
Why it clears so fast
Radiation cools a surface slowly — it took eight hours to lose those degrees. Direct sunlight delivers several hundred watts per square metre and reverses it in minutes.
That asymmetry is why morning condensation feels like it appears out of nowhere and then vanishes: the formation took all night and you slept through it; the clearing takes an hour and happens while you watch.
Shaded surfaces are the exception. Anything the sun does not reach stays wet far longer, which is why the north side of a building, a shaded deck or the grass under a hedge can still be soaked at lunchtime.
What it means for the things that care
The length of the wet window matters more than the fact of it:
- Fungal disease. Spores need continuous surface moisture for several hours, so a deposit that forms at 10pm is far more consequential than one forming at 4am — why the mildew keeps coming back.
- Finishes and coatings. A coat laid down in the afternoon may still be curing when the dew arrives, which is the actual deadline on the can.
- Anything you have to drive. The windscreen and the road are both at their worst at exactly commuting time.
Predicting tomorrow morning
Three inputs, all in an ordinary forecast: a clear sky, light wind, and a small gap between the overnight low and the dew point. The humidity forecast shows the last one hour by hour, so the convergence is something you can see the evening before rather than discover on the way to the car.