What the dew point tells you
The National Weather Service defines the dew point as the temperature to which air must be cooled to reach saturation, with the air pressure and the amount of moisture held constant. A higher dew point means more moisture in the air. Relative humidity, by contrast, compares the moisture in the air with the most it could hold at its current temperature, so it changes whenever the temperature changes, even if the moisture stays the same. The dew point does not.
That is why the same dew point can come with very different humidity readings. Air with a dew point of 10 °C has about 52 percent relative humidity at 20 °C, but about 29 percent at 30 °C, as the table in the calculator shows for any temperature you enter.
The Magnus formula
The calculator first works out the saturation vapor pressure, the highest vapor pressure the air can have at its temperature, with the Magnus formula. The actual vapor pressure is the relative humidity times that value. The dew point is the temperature at which the saturation vapor pressure would equal the actual vapor pressure, which the formula gives in closed form:
Td = 243.04 γ ÷ (17.625 − γ)
T is the air temperature and Td the dew point, both in degrees Celsius; Fahrenheit values are converted first. The constants 6.1094, 17.625 and 243.04 come from O. A. Alduchov and R. E. Eskridge, "Improved Magnus' form approximation of saturation vapor pressure" (NOAA National Climatic Data Center). They found these constants stay within 0.384 percent of three standard reference formulations from −40 to 50 °C, which is the range this calculator treats as valid. Relative humidity as the ratio of actual to saturation vapor pressure is the definition the National Weather Service uses in its vapor pressure calculator.
How to use the dew point calculator
- 1Choose °C or °F, and whether you want the dew point or the relative humidity.
- 2Enter the air temperature and the relative humidity, or the air temperature and the dew point. The result updates as you type.
- 3Read the result with the vapor pressures, and check the table to see how the dew point changes with humidity at your temperature. Copy the result or a link to it.
Frequently asked questions
What is the dew point?
The National Weather Service defines the dew point as the temperature to which air must be cooled to reach saturation, assuming the air pressure and the moisture content stay the same. A higher dew point means more moisture in the air. When the air cools to its dew point, the relative humidity reaches 100 percent.
Can the dew point be higher than the air temperature?
No. The dew point is at most equal to the air temperature, and the two are equal when the relative humidity is 100 percent. If you enter a dew point above the temperature, the calculator shows an error.
Which formula does the calculator use?
The Magnus formula for the saturation vapor pressure over water, with the constants 6.1094 hPa, 17.625 and 243.04 °C from Alduchov and Eskridge. The dew point is the temperature at which the saturation vapor pressure equals the actual vapor pressure of the air, so the formula can be solved for it directly.
How accurate is the result?
Alduchov and Eskridge report that their constants stay within 0.384 percent of three reference formulations of the saturation vapor pressure for temperatures from −40 to 50 °C (−40 to 122 °F). Outside that range the calculator still gives a result but marks it as an estimate. The result can only be as good as your thermometer and hygrometer readings.
What about dew points below freezing?
The formula describes vapor over liquid water. Below 0 °C (32 °F) the result is the dew point over water, not the frost point over ice, which uses a different set of constants.