Dew Point Calculator
Find the dew point — the temperature at which air becomes saturated and dew, fog or condensation forms — from temperature and humidity.
How it works
γ = ln(RH/100) + (a·T)/(b + T) · Td = (b·γ)/(a − γ) · a = 17.62, b = 243.12 °C
The dew point is the temperature to which air must cool, at constant pressure, before it becomes fully saturated and water vapour starts to condense as dew, fog or cloud. Unlike relative humidity, it does not change as the air warms or cools during the day, which makes it a far better single measure of how "muggy" the air actually feels. A small gap between temperature and dew point means the air is near saturation — the reason fog forms on clear, calm nights as the ground cools toward the dew point.
Worked example
At 25 °C and 60% humidity, γ = ln(0.60) + (17.62 × 25)/(243.12 + 25) = −0.511 + 1.643 = 1.132. Then Td = (243.12 × 1.132)/(17.62 − 1.132) ≈ 16.7 °C — so cooling this air below about 17 °C, such as overnight, would produce dew.
Sources: Dew point — Wikipedia
Frequently asked questions
What is the difference between dew point and relative humidity?
Relative humidity is the percentage of moisture the air holds relative to its maximum at the current temperature, so it swings widely as the air warms and cools. Dew point is an absolute measure of moisture content that stays put — a dew point of 20 °C always feels muggy, whatever the air temperature.
What dew point feels comfortable?
Most people find a dew point below about 13 °C pleasant and dry, 16–20 °C noticeably humid, and above 21 °C sticky and uncomfortable. Dew points above 24 °C feel oppressive and raise heat-stress risk.
Why does condensation form on a cold drink?
The glass chills the surrounding air below its dew point, so water vapour in that air condenses onto the glass as droplets. The same mechanism produces morning dew on grass and mist on a cold window.
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This calculator is for educational purposes. Double-check important results before acting on them.