Calculate dew point temperature from air temperature and relative humidity using the Magnus formula, or solve for relative humidity from dew point.
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Dew point
16.7 °C
Dew point (°C)
16.7
Relative humidity
60.0%
Temp–dew spread
8.3 °C
Comfort
Humid
Also in Physics
Science — Physics
The dew point is the temperature at which air becomes saturated with water vapour at constant pressure, causing condensation to form. It is a more reliable measure of perceived humidity than relative humidity alone, because it does not change with temperature. Meteorologists, HVAC engineers, and pilots rely on dew point to assess moisture conditions.
The August–Roche–Magnus approximation gives the dew point from air temperature T (°C) and relative humidity RH (%): γ = (aT)/(b+T) + ln(RH/100); Td = bγ/(a−γ), with constants a = 17.625 and b = 243.04 °C. This formula is accurate to ±0.35 °C for temperatures between −40 °C and 60 °C. The reverse calculation — RH from T and Td — inverts the same equation.
A dew point below 10 °C is comfortable; 10–16 °C is slightly humid; 16–21 °C is noticeable; 21–24 °C is very uncomfortable; above 24 °C can be dangerous, as sweat cannot evaporate efficiently. The "dew point depression" (air temperature minus dew point) indicates how far the air is from saturation: a small spread means high relative humidity.
Frequently asked questions
Relative humidity (RH) expresses current moisture as a percentage of the air's maximum capacity at that temperature. Because warm air holds more moisture, RH changes with temperature even if the actual water content is constant. Dew point is an absolute measure of moisture content and does not change with temperature alone, making it more useful for comfort assessment.
No — by definition, the dew point cannot exceed the air temperature. When dew point equals air temperature, relative humidity is 100% and the air is saturated. This is why the calculator returns null if a dew point above air temperature is entered.
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