Dew Point Calculator

The dew point comes from the saturation vapor pressure of water, and the World Meteorological Organization's Guide to Instruments and Methods of Observation (WMO-No. 8) gives the formula weather services use for it:

ew(t) = 6.112 x exp(17.62 x t / (243.12 + t)) hPa, with t in °C e = RH / 100 x ew(t) td = 243.12 x ln(e / 6.112) / (17.62 - ln(e / 6.112))

The first line is how much water vapor air at temperature t could hold, as a pressure. The second is how much it actually holds. The third runs the first backwards: the temperature at which the vapor already in the air would be enough to saturate it. WMO states the formula for -45 to 60 °C over water.

A worked example at 77 °F (25 °C) and 60% relative humidity. ew(25) = 6.112 x exp(440.5 / 268.12) = 31.60 hPa. The air holds 60% of that, e = 18.96 hPa. ln(18.96 / 6.112) = 1.1321, so td = 243.12 x 1.1321 / (17.62 - 1.1321) = 16.69 °C, which is 62.0 °F. Cool this air by 15 °F and dew forms on whatever is that cold: a glass of iced water, a car window at night, the cold side of a duct.

The same air has a wet-bulb temperature of 67.1 °F, a heat index of 77.2 °F, and holds 11.9 grams of water per kilogram of dry air (83.4 grains per pound), or 13.8 grams per cubic meter. Each of those answers a different question, and the page computes all of them from the one pair of inputs.

Air is not pure water vapor, and WMO's full version multiplies each saturation pressure by an enhancement factor, f(p) = 1.0016 + 3.15 x 10^-6 x p - 0.074 / p with p in hPa, which is 1.0047 at sea level. It cancels out of the dew point and the relative humidity, because both sides of the ratio carry it. That is why elevation does not change either of them. It does change the humidity ratio and the wet bulb, which depend on the total pressure, so the page asks for an elevation and takes the pressure from the 1976 U.S. Standard Atmosphere.

Below freezing there are two answers. The dew point is the temperature at which the vapor would saturate over liquid water; the frost point is the same over ice, from WMO's second formula with 22.46 and 272.62 in place of 17.62 and 243.12. Ice holds its molecules more tightly than supercooled water, so the frost point is the warmer of the two: at 23 °F and 70% relative humidity the dew point is 14.7 °F and the frost point 16.6 °F. Frost on a windshield forms at the frost point. Weather reports still give the dew point, and WMO has relative humidity reported with respect to water at every temperature.

The common mistake is reading relative humidity as a measure of how much water is in the air. It is a ratio against a ceiling that moves with temperature. The National Weather Service office at La Crosse puts it this way: "a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity." The dew point does not move when the afternoon warms up; the relative humidity falls. That is why forecasters talk about the dew point in summer, and why this calculator leads with it.

The comfort reading on the result is that office's, quoted rather than paraphrased: a summer dew point of 55 °F or less is "dry and comfortable", between 55 and 65 °F is "becoming "sticky" with muggy evenings", and 65 °F or more is "lots of moisture in the air, becoming oppressive".

What this does not model. Supersaturated air, which the atmosphere does produce above ice but which a dew point cannot describe; air that is not at the pressure of the standard atmosphere for its elevation, which matters only for the humidity ratio and the wet bulb; and the gap between a real instrument and the formula. Other calculators use other published constants for the same Magnus form, so a second page can disagree in the first decimal. This one uses WMO's.