Hygrometer
The surprising part first
Most explanations say dew forms when the air cools to its dew point overnight. That is true sometimes, and it is not what usually happens.
What usually happens is that the grass gets colder than the air. On a clear, still night an open surface can sit 4 to 8°F below the air temperature a metre above it. So dew appears on nights when the air never came close to saturating — the surface crossed the threshold on its own.
That is why you can wake to a soaked lawn after a night the forecast called mild, and why the grass is wet when it has not rained in a week.
Radiative cooling, briefly
Every surface radiates infrared energy according to its temperature. Normally it receives about as much back from its surroundings, and nothing much happens.
A surface facing the open sky is different. It is exchanging radiation with the upper atmosphere and, through it, with space — effectively a very cold sink. The surface loses far more than it receives, and its temperature falls until conduction from the air and the ground below balances the loss.
How far it falls depends on what is in the way:
| Situation | Typical difference | Dew? |
|---|---|---|
| Open lawn, clear sky | 4–8°F colder | Very likely |
| Car roof, clear sky | 5–10°F colder | Very likely — thin metal, nothing below it |
| Under a tree | 1–2°F colder | Rarely |
| Under an overhang | Near air temperature | No |
| Overcast night | Near air temperature | No |
| Windy night | Near air temperature | No |
The car roof case is the one everybody has seen: dew or frost on the roof and bonnet, nothing on the sides. The horizontal surfaces face the sky; the vertical ones face other objects.
The three conditions
Dew needs all three, and losing any one prevents it:
- A clear sky. Cloud is a radiator pointing back at you.
- Still air. Wind mixes warmer air across the surface faster than it can cool.
- Enough water in the air. A dew point close enough to the air temperature that a few degrees of surface cooling can reach it.
The third is the one you can look up. If the gap between tonight's temperature and tonight's dew point is under about 8°F and the sky is clear, expect dew. If it is 20°F, the surface will not get close.
Predicting it
The humidity forecast shows the dew point alongside the temperature for the hours ahead, so the closing gap is visible before it happens. That is the same reading behind whether the beds need covering and whether a finish will still be curing at four in the morning.
If the two lines actually meet rather than merely converging, the whole air mass saturates and you get fog rather than dew.
Where the water comes from
Not from the ground, mostly — from the air. A heavy dew deposits something like 0.1 to 0.3 mm of water across a surface overnight. That sounds trivial until you convert it: a millimetre of depth over a square metre is a litre, so a single dewy night puts 100 to 300 millilitres onto every square metre, and a season of them adds up to tens of litres.
In some very dry regions it is a genuine water source for plants and insects, and dew-harvesting nets exploit exactly this mechanism: give the air a cold surface with a clear view of the sky, and it will hand over its water.