Hygrometer
The timeline
| When | What is happening |
|---|---|
| Sunset | Surfaces stop receiving solar energy and begin radiating heat to the sky. Still warmer than the air. |
| 1–2 hours after | Surfaces fall through the air temperature and keep going. Nothing visible yet. |
| 2–4 hours after | The coldest, most exposed surfaces — car roofs, open lawn — cross the dew point. Dew begins. |
| Through the night | The deposit accumulates. More surfaces join as they cool further. |
| Hour before sunrise | Peak. Surface temperatures are at their minimum for the whole night. |
| Sunrise + 1–2 hours | Direct sun warms surfaces past the dew point and the water evaporates back into the air. |
| Shaded areas | Can stay wet for hours longer, or all day in winter. |
Why it peaks at dawn and not at midnight
This is the part that catches people out. The intuition is that the middle of the night is the coldest point, and it is not.
Surfaces lose heat continuously from sunset onwards and keep losing it until something replaces it. Nothing does until the sun comes back. So the temperature minimum arrives at or just after sunrise, and the dew deposit is still growing right up to that moment.
Exactly the same reasoning explains why window condensation is worst first thing rather than at 2am.
What starts it early or late
- Clear sky from sunset — dew starts early. If cloud clears at 2am, it starts at 2am.
- Wind dropping — a night that is breezy until midnight and then calm will produce dew from midnight.
- High dew point — less surface cooling is needed, so the threshold is crossed sooner.
- Dry air — a wide dew point spread may never be crossed at all.
Telling the evening before
Three things, all in an ordinary forecast:
- Will the sky be clear? No clear sky, no radiative cooling, no dew.
- Will the wind be light? Anything above a gentle breeze mixes the surface layer and prevents it.
- How big is the gap between the overnight low and the dew point? Under about 8°F and the surface will get there easily. Over about 15°F and it will not.
The humidity forecast shows that third one hour by hour, which is what turns "probably" into a decision — whether to cover the beds, whether the hay will be dry enough, or whether a coat laid down at four in the afternoon will survive the night.
When it becomes fog instead
If the two lines do not merely converge but meet — if the air itself reaches saturation rather than just the surfaces — then water condenses throughout the air rather than only on things.
That is fog, and it usually forms in the same pre-dawn window for the same reason.
The leaf-wetness window
For anything growing, the number that matters is not whether dew formed but how long surfaces stayed wet. Fungal spores need a continuous period of surface moisture to establish, typically several hours.
So a dew that forms at 10pm and burns off at 8am gives ten hours of leaf wetness; one that forms at 4am gives four. That difference is what decides whether mildew takes hold, and it is why evening watering can be counterproductive — it starts the clock hours early.