What is the dew point?
Enter the air temperature and the relative humidity to get the dew point. Or enter the dew point to find the humidity or the temperature.
- Dew point
- 48.7 °F
At 68 °F and 50% relative humidity, the dew point is 48.7 °F.
Dew point: 48.7 °F. At 68 °F and 50% relative humidity, the dew point is 48.7 °F.
How to calculate
Computes the dew point from air temperature and relative humidity with the Magnus formula, or the relative humidity or air temperature from the other two.
Example with the default inputs (Air temperature 67.9999999999999 °F, Relative humidity 50%): At 68 °F and 50% relative humidity, the dew point is 48.7 °F.
Formula: Td = b·γ ÷ (a − γ), where γ = ln(RH ÷ 100) + a·T ÷ (b + T), a = 17.625 and b = 243.04 °C, with T and Td in °C (Magnus formula, Alduchov and Eskridge 1996).
- Saturation is over liquid water, also below 0 °C. Over ice, the frost point is a little higher than this dew point.
- The Magnus coefficients a = 17.625 and b = 243.04 °C fit the saturation vapor pressure from −40 °C to 50 °C, so the air temperature and the dew point must both be in that range.
- Relative humidity is between 0% (not included) and 100%.
Worked examples
Each example is checked against the calculator on every build.
- Air temperature 68 °F, Relative humidity 50% gives Dew point 48.7 °F.Source: hand calculation in content.mdx: γ = ln 0.5 + 17.625 × 20 ÷ 263.04 = 0.64695; Td = 243.04 × 0.64695 ÷ (17.625 − 0.64695) = 9.26 °C (Alduchov and Eskridge 1996 coefficients)
- Air temperature 86 °F, Relative humidity 70% gives Dew point 75.1 °F.Source: hand calculation in content.mdx: γ = ln 0.7 + 17.625 × 30 ÷ 273.04 = 1.57985; Td = 243.04 × 1.57985 ÷ (17.625 − 1.57985) = 23.93 °C
- Air temperature 77 °F, Dew point 77 °F gives Relative humidity 100%.Source: definition of the dew point (WMO No. 8): at T = Td the air is saturated, RH = 100%
- Relative humidity 40%, Dew point 41 °F gives Air temperature 66 °F.Source: hand calculation in content.mdx: s = 17.625 × 5 ÷ 248.04 − ln 0.4 = 1.27158; T = 243.04 × 1.27158 ÷ (17.625 − 1.27158) = 18.90 °C
How it works
The calculator uses the Magnus formula for the saturation vapor pressure over water, e(T) = 6.1094 × exp(a × T ÷ (b + T)) hPa, with the coefficients of Alduchov and Eskridge (1996): a = 17.625 and b = 243.04 °C. T is in degrees Celsius.
Relative humidity is the actual vapor pressure divided by the saturation vapor pressure at the air temperature. The dew point Td is the temperature whose saturation vapor pressure equals the actual vapor pressure. The 6.1094 cancels, and this gives:
- γ = ln(RH ÷ 100) + a × T ÷ (b + T)
- Td = b × γ ÷ (a − γ)
The same equation, rearranged, gives the other two values:
- RH = 100 × exp(a × Td ÷ (b + Td) − a × T ÷ (b + T))
- T = b × s ÷ (a − s), where s = a × Td ÷ (b + Td) − ln(RH ÷ 100)
Temperatures in °F or K are converted to °C first: °C = (°F − 32) ÷ 1.8 and °C = K − 273.15.
Assumptions
- Saturation is over liquid water, also below 0 °C.
- The air temperature and the dew point are both from −40 °C to 50 °C (−40 °F to 122 °F), where the coefficients hold. A dew point below −40 °C (very dry, cold air) has no answer.
- Relative humidity is above 0% and at most 100%. If the numbers you enter need more than 100% humidity (a dew point above the air temperature), there is no answer.
- The air pressure does not matter here: the ratio of vapor pressures is the same at any pressure (a small pressure correction is left out).
Worked examples by hand
20 °C at 50% humidity. γ = ln 0.5 + 17.625 × 20 ÷ 263.04 = −0.69315 + 1.34010 = 0.64695. Td = 243.04 × 0.64695 ÷ (17.625 − 0.64695) = 157.235 ÷ 16.97805 = 9.26 °C (48.7 °F).
30 °C at 70% humidity. γ = ln 0.7 + 17.625 × 30 ÷ 273.04 = −0.35667 + 1.93653 = 1.57985. Td = 243.04 × 1.57985 ÷ (17.625 − 1.57985) = 23.93 °C (75.1 °F).
25 °C with a dew point of 25 °C. The two Magnus terms are equal, so RH = 100 × exp(0) = 100%.
The air temperature at 40% humidity with a 5 °C dew point. s = 17.625 × 5 ÷ 248.04 − ln 0.4 = 0.35529 + 0.91629 = 1.27158. T = 243.04 × 1.27158 ÷ (17.625 − 1.27158) = 18.90 °C (66.0 °F).
Other questions people ask
What is the dew point?
The dew point is the temperature the air must cool to, at the same pressure and moisture content, before it is saturated and water starts to condense as dew, fog or cloud. The higher the dew point, the more water vapor is in the air.
How is the dew point calculated?
With the Magnus formula: γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T), then Td = 243.04 × γ ÷ (17.625 − γ), with temperatures in °C. At 20 °C and 50% humidity, the dew point is 9.26 °C (48.7 °F).
Can the dew point be higher than the air temperature?
No. When the air cools to its dew point it is saturated (100% relative humidity), and extra vapor condenses. So the dew point is at most the air temperature, and equal to it only at 100% humidity.
What dew point feels humid?
The US National Weather Service uses this rough summer guide: a dew point of 55 °F (13 °C) or lower feels dry and comfortable, 55 to 65 °F (13 to 18 °C) starts to feel sticky, and 65 °F (18 °C) or higher feels oppressive. Relative humidity alone can mislead, because the same humidity holds much more water on a hot day than on a cold one.
How accurate is the Magnus formula?
With the Alduchov and Eskridge (1996) coefficients, the saturation vapor pressure it uses is within about 0.4% of reference values from −40 °C to 50 °C. Outside that range, for the air temperature or the dew point, the calculator gives no answer.