The air around you is always carrying invisible water, called water vapour. Warm air can hold a lot of it, like a big sponge. But here is the key: cold air can only hold a little. So if you cool a patch of air down, its sponge shrinks, and at some point it simply cannot hold all the water it has. That tipping-point temperature is called the dew point. Cool the air past it and the leftover water has nowhere to go, so it turns back into tiny liquid droplets. Overnight the ground gets cold and chills the air just above it. If that air reaches its dew point, water condenses straight onto the cool grass and leaves — and in the morning you find everything covered in sparkling dew. Slide the temperature down in the simulator and watch droplets appear the instant the air fills up.
Most people think dew falls from the sky like a fine rain. In fact dew condenses straight out of the air against cold surfaces, which is why grass under a cover stays dry while the grass around it gets soaked.
What's actually happening
It feels like dew must fall from the sky, the way rain does — a fine moisture settling down out of the night. For a long time that is exactly what people assumed. But you can disprove it with a sheet of plastic: lay one on the lawn before a clear evening and in the morning the grass underneath is dry while the grass around it is soaked. Whatever made the dew did not come from above. It came out of the air sitting right against the cold ground.
The secret is that air is a sponge for water, and the size of that sponge depends on temperature. Warm air can hold a surprising amount of invisible water vapour; cool it down and its capacity collapses. The measure of how full the sponge is is relative humidity: 50 per cent means the air is holding half the water it possibly could at that temperature. Now cool that same air without adding or removing any water. The amount of vapour stays put, but the capacity shrinks, so the relative humidity climbs — 50, then 70, then 90 per cent. The temperature at which it hits 100 per cent, completely full, is the dew point. Push past it and the air can no longer keep all its water in gas form, and the surplus condenses into liquid.
On a clear, calm night the ground is brilliant at losing heat: with no cloud blanket, it radiates warmth straight out to space and grows colder than the air. The thin layer of air touching the cold grass gets chilled too, and if it reaches its dew point, water condenses directly onto every blade, every leaf, every car bonnet. That is dew — not fallen, but squeezed out of the air on the spot. The very same physics, lifted into the sky, makes everything from your breath fogging on a winter morning to the clouds overhead. Cool damp air to its dew point and the water always comes back.
The dew point is simply the temperature where air gets too cold to hold its water, so the surplus condenses out as dew.
- 1Fill a dry glass with ice and a little water and leave it on the table for a few minutes.
- 2Watch the outside of the glass: droplets appear from nowhere, because the chilled glass cools the air against it below its dew point.
- 3Wipe it dry and breathe on it instead — your warm, damp breath hits the cool surface and fogs it for the same reason.
Common questions
No. Dew condenses out of the air touching cold surfaces. Cover a patch of grass overnight and it stays dry, proving the water comes from the air against the cold ground, not from above.
Relative humidity is how full the air is as a percentage of its capacity at the current temperature; the dew point is the fixed temperature at which that air would become 100 per cent full. The dew point tracks the actual amount of water, so forecasters prefer it.
By the Clausius–Clapeyron relation, the saturation vapour pressure rises steeply with temperature, roughly doubling for every 10 degrees Celsius. Cooler air therefore reaches saturation with much less water.