Dew Point, Humidity & Wet-Bulb Calculator

About the Dew Point, Humidity & Wet-Bulb Calculator

The dew point is the temperature air must be cooled to, at constant pressure, before it is saturated and water condenses — as dew on grass, fog, or droplets on a cold glass. Relative humidity changes whenever the temperature changes, but the dew point measures the moisture actually in the air. That makes it the better guide to how muggy it feels, and to when windows, walls or freshly painted steel will get wet.

Enter the air temperature with one humidity reading: relative humidity, the dew point, or the dry- and wet-bulb temperatures of a psychrometer. The calculator then works out the rest, in °C or °F:

• dew point, and the frost point below 0 °C;
• relative humidity over water, and over ice below freezing;
• vapour pressure and the vapour-pressure deficit;
• absolute humidity, mixing ratio and specific humidity;
• the wet-bulb temperature;
• an estimate of the cumulus cloud base;
• a summer comfort rating.

It uses the World Meteorological Organization formulas (WMO-No. 8, Annex 4.B) and shows every step. You can also allow for altitude or use a measured air pressure.

Common uses

• Weather and science homework: converting between relative humidity, dew point, vapour pressure and mixing ratio, with the working shown.
• Reading a sling or aspirated psychrometer, including an iced wet bulb in winter.
• Checking condensation risk before painting, coating, laying floors or insulating, and finding when windows or cold walls will sweat.
• Greenhouse and indoor growing: the vapour-pressure deficit (VPD) from temperature and RH.
• Pilots, glider pilots and photographers: a quick cumulus cloud-base estimate, and how close the air is to fog or dew.

The formulas

The saturation vapour pressure over water uses the Magnus form with the WMO coefficients (from Sonntag): e_w(t) = 6.112·exp[17.62·t ÷ (243.12 + t)] hPa, with t in °C, for −45 to 60 °C. Over ice it is e_i(t) = 6.112·exp[22.46·t ÷ (272.62 + t)], for −65 to 0 °C. Real (moist) air holds slightly more vapour than pure water vapour would; WMO multiplies by f(p) = 1.0016 + 3.15·10⁻⁶·p − 0.074 ÷ p, about 1.0047 at sea level.

The actual vapour pressure e′ comes from your reading. It is RH × e′_w(t), or the saturation value at the dew point. For a psychrometer it is e′ = e′_w(t_w) − 6.53·10⁻⁴·(1 + 0.000944·t_w)·p·(t − t_w). Everything else follows from e′:

• the dew point is the Magnus formula run backwards;
• the mixing ratio is r = 621.98·e′ ÷ (p − e′) g/kg;
• absolute humidity is e′ ÷ (R_v·T), with R_v = 461.52 J/(kg·K);
• the wet bulb is the temperature that satisfies the psychrometer equation, found numerically.

You can switch to the Alduchov & Eskridge coefficients, 6.1094, 17.625 and 243.04 (6.1121, 22.587 and 273.86 over ice), which many textbooks use. That option applies no moist-air factor, and the two methods agree within a few hundredths of a degree.

Dew point, frost point and humidity below freezing

Below 0 °C water vapour can condense as supercooled droplets or deposit directly as ice. WMO defines the dew point and relative humidity with respect to water even then, and so does this calculator. When the dew point is below freezing it also gives the frost point: the temperature at which frost forms. The frost point is always a little higher than the dew point, because ice holds on to water molecules more tightly than liquid water does.

For −10 °C air at 80 % RH, the dew point is −12.8 °C and the frost point is −11.4 °C. That same air is at 88 % relative humidity over ice. Above 100 % over ice, frost grows on surfaces even though the air is not saturated over water.

Wet bulb and cloud base

The wet-bulb temperature is what a thermometer wrapped in a wet, well-ventilated wick reads: evaporation cools it until it balances the heat from the air. It always lies between the dew point and the air temperature, and all three meet when the air is saturated. Here it is solved from the WMO psychrometer equation at your pressure. Within its range, Stull’s one-line formula is shown as a check; it is fitted at sea level, for 5–99 % RH and −20 to 50 °C, and is accurate to about −1 to +0.65 °C.

Rising air cools by about 9.8 °C per kilometre. Its dew point falls more slowly, so a rising parcel saturates at the lifting condensation level — the base of fair-weather cumulus clouds. The calculator uses Romps’s exact formula, which has an uncertainty of about 5 m, and also shows Espy’s rule of thumb: 125 m per °C of difference between temperature and dew point (410 ft per °C, or about 228 ft per °F).

Comfort and condensation

The comfort rating follows the summer guide of the US National Weather Service office in La Crosse, Wisconsin:

• a dew point at or below 55 °F (12.8 °C) feels dry and comfortable;
• 55 to 65 °F (12.8–18.3 °C) is becoming sticky, with muggy evenings;
• 65 °F (18.3 °C) or higher is oppressive.

A surface condenses water when it is at or below the dew point. Below 0 °C it collects frost when it is at or below the frost point. Coating work needs a safety margin. For example, US Unified Facilities Guide Specification 09 90 00 does not allow coating when the surface is less than 3 °C (5 °F) above the dew point. Check your product’s data sheet for its own limit.

Sources

• WMO, Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8), Part I, Annex 4.A and Annex 4.B (humidity formulae). Read from the copy published by the US National Weather Service.
• O. A. Alduchov & R. E. Eskridge, “Improved Magnus form approximation of saturation vapor pressure”, J. Appl. Meteor. 35, 601–609; DOE report DOE/ER/61011-T6, Tables II and V.
• R. Stull, “Wet-bulb temperature from relative humidity and air temperature”, J. Appl. Meteor. Climatol. 50, 2267–2269; R. Stull, Practical Meteorology, eq. 4.19.
• D. M. Romps, “Exact expression for the lifting condensation level”, J. Atmos. Sci. 74, 3891–3900.
• NWS La Crosse, Dew Point vs Humidity.
• UFGS 09 90 00, §1.10.1, coating application conditions.
• The U.S. Standard Atmosphere (troposphere) for the pressure at an elevation.

Limitations

• The formulas are for air near the ground. WMO’s moist-air factor is meant for pressures around 1,000 hPa, so it is not suitable for upper-air soundings.
• The psychrometer formula is WMO’s, for a well-ventilated (Assmann-type or sling) psychrometer. An unventilated wet bulb reads too warm and needs a different coefficient.
• Pressure from elevation uses the standard atmosphere; real pressure changes with the weather, by up to a few per cent.
• The cloud-base estimate assumes air rises from the surface through a well-mixed layer, as under afternoon cumulus. It does not apply to fog, low stratus or clouds along fronts.
• The comfort rating is a broad summer guide; how humid air feels also depends on temperature, wind, sun and the person.

Privacy

Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.