What the Dew Point Is
The dew point is the temperature to which air must be cooled, at constant pressure, for it to become saturated with water vapour. Cool the air below that point and water condenses out — as dew on grass, mist on a window, or fog in the air itself.
Unlike relative humidity, the dew point is an absolute measure of how much moisture the air contains. That makes it far more useful for judging comfort. Air at 70% relative humidity feels pleasant at 15°C and oppressive at 30°C, because the same percentage represents very different quantities of water.
The Formula
The Magnus-Tetens approximation is the standard working formula, accurate to about 0.4°C between −45°C and 60°C:
γ = ln(RH / 100) + (17.625 × T) / (243.04 + T)
Td = (243.04 × γ) / (17.625 − γ)
with T and Td in Celsius. A useful rule of thumb for quick mental work: for every 1°C the dew point sits below the air temperature, relative humidity drops by roughly 5 percentage points from 100%.
Comfort Scale
| Dew point | How it feels |
|---|---|
| Below 10°C (50°F) | Dry, comfortable, possibly a little parched |
| 10–13°C (50–55°F) | Very comfortable |
| 13–16°C (55–60°F) | Comfortable; humidity barely noticeable |
| 16–18°C (60–65°F) | Slightly humid, becoming noticeable |
| 18–21°C (65–70°F) | Humid and uncomfortable for many |
| 21–24°C (70–75°F) | Very humid, oppressive |
| Above 24°C (75°F) | Extremely humid, potentially dangerous with heat |
Meteorologists routinely use dew point rather than relative humidity precisely because these bands mean the same thing at every air temperature.
Dew Point Chart
Dew point in °C for a given air temperature and relative humidity:
| RH | 10°C | 15°C | 20°C | 25°C | 30°C | 35°C |
|---|---|---|---|---|---|---|
| 30% | −6.0 | −2.0 | 1.9 | 6.2 | 10.5 | 14.8 |
| 40% | −2.9 | 1.5 | 6.0 | 10.5 | 14.9 | 19.4 |
| 50% | 0.1 | 4.7 | 9.3 | 13.9 | 18.4 | 23.0 |
| 60% | 2.6 | 7.3 | 12.0 | 16.7 | 21.4 | 26.1 |
| 70% | 4.8 | 9.6 | 14.4 | 19.1 | 23.9 | 28.7 |
| 80% | 6.7 | 11.6 | 16.4 | 21.3 | 26.2 | 31.0 |
| 90% | 8.4 | 13.4 | 18.3 | 23.2 | 28.2 | 33.1 |
| 100% | 10.0 | 15.0 | 20.0 | 25.0 | 30.0 | 35.0 |
At 100% humidity the dew point equals the air temperature by definition — the air can hold no more.
Practical Uses
- Condensation and mould. Any surface colder than the dew point collects water. Cold-water pipes, single-glazed windows and uninsulated exterior walls are the usual sites, and persistent condensation leads to mould.
- Fog forecasting. When the spread between air temperature and dew point falls below about 2.5°C, fog becomes likely. Pilots watch this figure closely.
- Painting and coating. Most paint and industrial coating specifications require the surface to be at least 3°C above the dew point, or moisture ruins adhesion.
- Aviation. The temperature-dew point spread also determines cloud base height: roughly 400 feet per °C of spread.
- Comfort and health. Indoor dew points above 16°C encourage dust mites and mould; below about 2°C the air feels uncomfortably dry and static builds.
Dew Point, Frost Point and Wet Bulb
| Measure | Definition |
|---|---|
| Dew point | Temperature at which water vapour condenses to liquid |
| Frost point | The same, but below freezing, where vapour deposits directly as ice |
| Wet-bulb temperature | The lowest temperature reachable by evaporative cooling; always between the dew point and the air temperature |
Frequently Asked Questions
Can the dew point be higher than the air temperature?
No. That would mean more than 100% relative humidity. The two are equal at saturation and the dew point is lower at all other times.
Why does the same humidity feel different in summer and winter?
Because warm air holds far more water. 60% relative humidity at 30°C corresponds to a dew point of 21°C and feels oppressive; the same 60% at 5°C is a dew point of −2°C and feels dry.
What is a comfortable indoor dew point?
Roughly 10–13°C (50–55°F). That corresponds to about 40–50% relative humidity at normal room temperature, which is also the range that minimises both mould growth and respiratory irritation.
Why do my windows fog up?
Because the glass surface is colder than the indoor dew point. Either raise the glass temperature with better glazing or lower the indoor dew point with ventilation or dehumidification.