Dew point is the temperature to which air must be cooled, under the usual stated pressure/moisture assumptions, to become saturated. Relative humidity compares the air's water-vapor pressure with saturation at its current temperature. Cooling air can raise its relative humidity without adding water vapor, which is why a percentage alone does not describe the same conditions at every temperature.
Why condensation appears on a window
If the inside glass surface is colder than the adjacent air's dew point, water can condense on that surface. The relevant comparison is surface temperature versus local dew point, not just the outdoor forecast or the thermostat's room reading. Cold bridges, airflow and local moisture conditions can make different parts of a window behave differently.
| Indoor-air dew point | Inside glass temperature | Interpretation |
|---|---|---|
| 12°C | 16°C | Glass is above the stated dew point; this simple comparison does not predict condensation. |
| 12°C | 9°C | Glass is below the stated dew point; condensation is possible under those local conditions. |
The table is a physical explanation, not a survey of a building or a guarantee about every surface. Sensor accuracy and where you measure matter.
Why 50% humidity can mean different moisture conditions
Compare two illustrative air readings: 20°C at 50% relative humidity and 30°C at 50%. The warmer air has a higher saturation vapor pressure, so equal percentages do not imply equal actual vapor pressure or equal dew point. Using a standard approximation, their dew points are about 9°C and 18°C respectively. Rounded values depend on the formula and measurement accuracy.
Now cool the first air sample while holding its moisture content approximately unchanged. Its relative humidity rises as it approaches saturation near its dew point. This is a useful mental experiment; it does not say an occupied room maintains perfectly constant moisture or pressure.
What to record when investigating a damp surface
- Air temperature and relative humidity near the problem, with the sensor location and time.
- Surface temperature where practical, recognizing instrument limitations.
- Whether moisture follows weather, cooking, showers, drying clothes or another activity.
- Whether it is on an interior surface, between panes or near a possible leak.
Condensation, a plumbing leak and water between sealed glazing panes are different problems. These observations can help describe the pattern to an appropriate building professional; the humidity percentage alone does not diagnose construction defects or health conditions.
What the weather report tells you
Outdoor dew point helps describe atmospheric moisture, but indoor air can have a different temperature and moisture history. Bringing outside air indoors and heating it changes relative humidity; moisture sources and ventilation also change the situation. Use measurements at the relevant boundary rather than assuming the outdoor number directly describes your window.
A compact comparison
Relative humidity is a percentage. Dew point is a temperature. Surface temperature is another temperature to compare with dew point. Keep their roles separate, and record °C or °F consistently. To see another case where a reference changes the meaning of a number, read temperature anomalies and baselines.