Dew point from temperature and humidity — the number that tells you how muggy it will feel, which relative humidity alone cannot.
The dew point is the temperature to which air must be cooled, at constant pressure, before the water vapour in it begins to condense. It is a direct measure of how much moisture the air actually contains — and that is what makes it more useful than relative humidity.
This calculator uses the Magnus formula with the Alduchov-Eskridge coefficients:
γ = ln(RH/100) + (17.625·T)/(243.04 + T)
Td = (243.04·γ)/(17.625 − γ)
Relative humidity is a percentage of capacity, and capacity changes with temperature — warm air holds far more water than cold air. So 70% relative humidity means completely different things at 5 °C and at 30 °C. The number moves through the day even when nothing about the air has changed: as the temperature rises from dawn to afternoon, relative humidity falls automatically, because the same amount of moisture is now a smaller fraction of a larger capacity.
Dew point does not do this. It is an absolute measure. A dew point of 22 °C is oppressive whether the air temperature is 26 °C or 35 °C, and it stays 22 °C all day unless an actual air mass change brings different moisture in. This is exactly why forecasters track it.
| Dew point | How it feels |
|---|---|
| Below 10 °C (50 °F) | Dry and comfortable |
| 10–13 °C (50–55 °F) | Comfortable |
| 13–16 °C (55–60 °F) | Becoming noticeable |
| 16–18 °C (60–65 °F) | Slightly humid |
| 18–21 °C (65–70 °F) | Humid and uncomfortable |
| 21–24 °C (70–75 °F) | Very humid, oppressive |
| Above 24 °C (75 °F) | Extremely oppressive, dangerous with heat |
Once you know this scale you can read a forecast far more usefully. A dew point above 21 °C means sweat will not evaporate well regardless of what the thermometer says.
The gap between air temperature and dew point — the spread — is a practical forecasting tool in itself. A small spread means the air is close to saturation, so fog, cloud and dew are likely. Aviation weather reports quote temperature and dew point together precisely so pilots can judge fog risk.
The spread also predicts the overnight low reasonably well. Air rarely cools much below its dew point, because condensation releases latent heat and resists further cooling. If the evening dew point is 15 °C, the overnight minimum is unlikely to go far below that.
Condensation on windows and walls. A surface colder than the room's dew point will collect moisture. That is why single-glazed windows stream in winter, and why cold spots behind furniture on an external wall grow mould — the surface is below the dew point of the room air.
Aircraft and car windscreens. Demisting works by either warming the glass above the dew point or drying the air to lower it. Air conditioning does the second, which is why it clears a windscreen faster than heat alone.
Storage and workshops. Condensation on tools and stored goods happens when they are colder than the surrounding air's dew point — common when cold items are brought into a warm humid space.
Below freezing the same calculation gives the frost point, where water vapour deposits directly as ice crystals without passing through a liquid stage.
Dew point is an absolute measure of moisture content; relative humidity is a percentage of what the air could hold at its current temperature. Because capacity rises with temperature, relative humidity changes through the day even when the moisture content is constant — it falls as the afternoon warms and rises again at night. Dew point stays put. That is why forecasters use it to describe how muggy conditions will feel.
Below about 13 °C (55 °F) feels comfortable to most people, and below 10 °C feels pleasantly dry. Between 16 and 18 °C it becomes noticeable, above 18 °C it feels humid, and above 21 °C it is genuinely oppressive. Beyond about 24 °C sweat evaporates so poorly that heat stress becomes a real risk regardless of the air temperature.
Because the glass surface is colder than the dew point of the air inside the room, so moisture condenses on it. Single glazing is much colder than the room and streams readily; double and triple glazing keep the inner surface warmer and usually stay clear. The fixes are to lower indoor humidity through ventilation or extraction, or to raise the surface temperature with better glazing. The same mechanism is behind mould in cold corners.
No. The dew point can equal the air temperature — that is 100% relative humidity, when fog or dew is forming — but it cannot exceed it. If it did, the air would be holding more water than it physically can, and the excess would immediately condense out. When a calculation appears to give a higher dew point, it is a measurement or rounding error.
Air rarely cools much below its dew point overnight, because as it reaches saturation, condensation begins and releases latent heat that resists further cooling. So an evening dew point of 15 °C suggests an overnight minimum not far below that. It is a rough guide rather than a rule — wind, cloud cover and an incoming air mass all interfere — but forecasters use it as a sanity check on a minimum temperature forecast.
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