Calculate the dew point temperature from air temperature and relative humidity, using the Magnus-Tetens approximation formula.
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The dew point is the specific temperature air would need to cool down to (at constant pressure and moisture content) for it to become fully saturated with water vapor, causing dew, fog, or condensation to begin forming. Unlike relative humidity, which is relative to the current temperature and can be misleading on its own, the dew point is widely considered a more reliable measure of how humid the air actually feels to the human body, since a higher dew point always means more actual moisture in the air regardless of the current temperature. This tool calculates it using the Magnus-Tetens approximation, a well-established formula in meteorology that relates temperature, relative humidity, and dew point using natural logarithms and is commonly used in weather-station software worldwide for its good accuracy across ordinary atmospheric conditions. Meteorologists commonly favor it over relative humidity alone precisely because it stays consistent regardless of what the current air temperature happens to be.
Weather forecasts commonly report relative humidity as a percentage, but that number is more confusing than it first appears: 50% relative humidity feels completely different on a cool spring morning than it does on a hot summer afternoon. The dew point solves this problem by measuring humidity in a way that doesn't depend on the current temperature, which is why meteorologists often consider it the more reliable figure for judging how humid the air will actually feel.
Technically, the dew point is the temperature air would need to cool to, at constant pressure and moisture content, in order to become fully saturated with water vapor — the point where dew, fog, or condensation begins to form. Because this depends only on how much actual moisture is in the air, not on the current air temperature, two days with wildly different relative-humidity percentages can have the same dew point and feel similarly humid, while two days with the same relative humidity percentage but different temperatures can feel very different.
This calculator uses the Magnus-Tetens approximation to compute dew point from temperature and relative humidity. It's a well-established formula in meteorology, built on logarithmic relationships between saturation vapor pressure and temperature, and it's widely used in weather-station software and forecasting tools because it strikes a good balance between accuracy and simplicity across the range of temperatures and humidity levels people typically encounter day to day.
As a practical guide, meteorologists generally consider dew points below about 16°C comfortable and dry-feeling, the 16-18°C range still comfortable with slightly noticeable moisture, 18-21°C starting to feel humid and sticky, and anything above 21°C increasingly uncomfortable to oppressive. One useful mathematical property worth knowing: the dew point can never exceed the actual air temperature, since it represents the temperature air must cool to in order to reach saturation — the two values only become equal when relative humidity hits exactly 100%, which is exactly the condition present in fog.
Because relative humidity is relative to the current air temperature (so 50% humidity can feel very different at 20°C versus 35°C), while dew point directly reflects the actual amount of moisture in the air regardless of temperature, making it a more consistent indicator of how humid the air will actually feel.
Dew points below about 16°C are generally considered comfortable and dry-feeling, 16-18°C is still comfortable but slightly noticeable, 18-21°C starts to feel humid and sticky, and above 21°C is widely considered uncomfortably to oppressively humid.
No — the dew point can never exceed the current air temperature, since it represents the temperature the air would need to cool to in order to reach full saturation; the two values become equal only when relative humidity reaches exactly 100%.
That's the condition for fog or dew formation, since it means the air is already fully saturated with moisture (100% relative humidity) with no further cooling needed for condensation to begin.