DryWindowDew Point Explained: Why Surfaces Get Wet When It Isn't Raining
If you have ever come back to a freshly painted gate in the evening and found it covered in tiny water droplets, you have met the dew point. It is the single most useful – and most ignored – number in any forecast.
What the dew point actually is
Air always contains some water vapour. Warm air can hold more than cold air. If you take a parcel of air and cool it down without adding or removing any moisture, at some temperature it will become completely saturated: it cannot hold any more vapour, and the excess condenses into liquid water. That temperature is the dew point.
So a dew point of 9°C means: if anything cools to 9°C, water will start to form on it. Your car windscreen, a cold can from the fridge, a metal railing at dusk – they all collect water once their surface temperature falls to the dew point of the surrounding air.
Dew point versus relative humidity
Relative humidity (RH) is a percentage: how much water vapour the air holds compared with the maximum it could hold at its current temperature. Because that maximum changes with temperature, RH changes through the day even if the actual amount of moisture stays the same. On a typical day RH might be 95% at dawn and 60% in the afternoon, purely because the air warmed up.
The dew point, by contrast, describes the absolute amount of moisture and tends to stay fairly steady over a day. The useful number is the gap between the air temperature and the dew point – sometimes called the dew point spread or depression. When the gap is small, condensation is close; when it is large, surfaces stay dry.
| Temperature minus dew point | What it means |
|---|---|
| Less than 1°C | Fog, mist or dew is likely. Surfaces are wet or will become wet. |
| 1–3°C | High risk: shaded or metal surfaces may already be at the dew point. |
| 3–5°C | Acceptable for most coatings if the surface is not in shade or exposed to a clear night sky. |
| More than 5°C | Comfortable margin. Good drying conditions if it is not too cold. |
Why surfaces are colder than the air
Forecasts report the air temperature about two metres above the ground, in the shade. Real surfaces behave differently. On a clear evening, roofs, car panels, decking and metal radiate heat to the sky and can drop several degrees below the air temperature. That is why a car can be covered in frost while the forecast says 3°C. A surface that is 4°C colder than the air will reach the dew point even when the forecast spread looks safe.
This is the reason coating manufacturers specify that the surface must be at least 3°C above the dew point. DryWindow applies a 3–4°C margin to the forecast air temperature, which builds in some protection, but in the evening and in shaded spots it is worth checking the surface itself with an inexpensive infrared thermometer.
What condensation does to your work
- Water-based paint can develop surfactant leaching – sticky, brownish streaks that run down the wall – when moisture reaches it before it has cured.
- Oil-based gloss can lose its shine and turn cloudy (blooming).
- Silicone and other sealants may fail to bond to a surface with a thin film of moisture on it.
- Ceramic coatings and sealants on cars can flash unevenly, leave high spots and cure with reduced water-beading.
- Washing on the line stops drying and starts reabsorbing moisture in the evening.
Using the dew point in practice
When planning any outdoor job, look at three times: when you start, when you finish, and the evening after. If the spread is comfortable at midday but narrows to 1°C by 6pm, a job that is not dry by late afternoon is at risk. In the UK, autumn mornings are often damp until 10 or 11am, and the safe drying period may end by 3 or 4pm.
The DryWindow tool shows every hour on a colour grid. Hover over or tap any square to see the temperature, humidity, dew point, rain chance and wind for that hour, along with the reason it was rejected if it does not pass – including "condensation risk" whenever the temperature is too close to the dew point.
Quick summary
The dew point tells you how much moisture is in the air. The gap between the temperature and the dew point tells you how close you are to surfaces getting wet. For almost every outdoor coating job, keep that gap at 3°C or more – and remember that surfaces cool faster than the air after sunset.