Dew Point, Explained Simply
Relative humidity keeps changing its mind: 90% at dawn, 45% by afternoon, same air. The dew point is the opposite — a single temperature that tells you, honestly, how much water the air is carrying. Once you can read it, condensation, muggy nights and mouldy corners stop being mysteries.
What the dew point actually is
Air holds water as invisible vapour, and how much it can hold depends on temperature: warm air has room for a lot, cold air for little. Take a parcel of air and cool it down without adding or removing water. At some temperature, it hits its limit — it can no longer hold all the vapour it carries, and the surplus condenses into liquid. That temperature is the dew point.
The name is literal. Overnight, grass cools below the dew point of the surrounding air, and dew appears. Nothing was “added” to the air; it was simply cooled past its limit.
The beauty of the dew point is that it depends only on how much water is in the air, not on the current temperature. Air at 20 °C and 60% RH has a dew point of roughly 12 °C. Heat that air to 25 °C and RH drops to about 44% — but the dew point is still 12 °C, because the water didn’t go anywhere.
Why cold surfaces get wet
Here is the rule that explains most damp problems at home: any surface colder than the dew point of the air touching it will get wet.
Take that same air at 20 °C with a 12 °C dew point. The air in the middle of the room is fine. But the air film touching a single-glazed window at 8 °C on a cold morning gets chilled below 12 °C — and water condenses right there, on the glass. Same story on a poorly insulated wall behind a wardrobe, or on cold water pipes in summer. The surface isn’t “attracting” moisture; it is simply the coldest thing the air touches. That is the real cause of condensation on windows, and it is why mould loves cold corners of warm, humid rooms.
Dew point vs relative humidity
Relative humidity is a percentage of a moving target — it swings wildly as temperature changes, even when the water content doesn’t. The dew point ignores the theatrics:
| The air right now | RH | Dew point |
|---|---|---|
| 30 °C, feels “dry” | 40% | ~15 °C |
| 15 °C, feels “damp” | 90% | ~13 °C |
The “dry” afternoon air actually carries slightly more water than the “damp” cool air. If you make ventilation decisions on RH, this table is the trap you fall into — the full story is in relative vs absolute humidity.
As a comfort compass: dew points below about 13 °C feel dry and pleasant, 16–18 °C starts to feel humid, and above 20 °C is the sticky, tropical-night territory where sweat stops evaporating properly.
Using dew point at home
Two practical habits follow directly from the physics. First, when deciding whether to air, compare the water content of outdoor air with indoor air — dew point or absolute humidity in g/m³ (computed with the Magnus formula), never RH alone. Opening up for outdoor air with a higher dew point than your rooms imports water, whatever the percentage says.
Second, when you see condensation, read it as a message: either the air carries too much water (lower the dew point by airing at the right moment) or the surface is too cold (insulate, or let air circulate behind furniture). Wiping the glass treats the symptom.
Let the app watch for you
Dew points are easy to reason with and tedious to look up. Open/Shut does the comparison continuously — outdoor versus indoor water content, in absolute terms — and simply tells you when opening your windows will dry your home instead of dampening it.