How this instrument works
Absolute humidity is a density — grams of water vapor suspended in each cubic metre of air — and it does not care what the temperature happens to be. Relative humidity, the percentage a household hygrometer displays, is a ratio instead: how close the air sits to its current saturation ceiling. A 50% RH reading marks a different mass of water at different temperatures, because that ceiling itself moves. Absolute humidity, AH here, is the figure that stays comparable when you set a warm room against a cold one.
The formula runs two physical facts together. Warm air can hold exponentially more vapor before it saturates — the Clausius-Clapeyron relation from thermodynamics — and David Bolton fit that curve in 1980 to the compact term 6.112·e^(17.67T/(T+243.5)), the saturation vapor pressure in hPa for any Celsius reading. Multiplying by relative humidity scales that ceiling down to the fraction actually present. Pressure then becomes density through the ideal gas law: divide by water vapor's specific gas constant, 461.5 J/(kg·K), and by the absolute temperature. Folding those unit conversions into one factor gives 2.1674, and 273.15 turns Celsius into kelvin.
One consequence catches people out: the formula never asks for barometric pressure, because a mass of vapor per cubic metre depends only on vapor pressure and temperature, not on how much other gas is crowded in beside it. Specific humidity and the mixing ratio, which describe vapor relative to dry air rather than to volume, do need total pressure — absolute humidity does not. Bolton's fit stays honest to roughly 0.1% between minus 35°C and plus 35°C, which covers essentially every inhabited climate; feed it a relative humidity above 100% and it is telling you the air has already shed its excess as fog, frost or dew, not that more vapor somehow fits.
- Enter the thermometer reading into Air temperature; its unit menu also accepts °F or K if that is what you have.
- Enter the hygrometer's percentage into Relative humidity, % — any value from 0 up to 100.
- Read the result in Absolute humidity, g ⁄ m³ — the mass of water vapor packed into each cubic metre of that air.
- Hold Relative humidity, % fixed and change Air temperature to see how sharply the same percentage swings in actual moisture.
- Switch Air temperature to °F or K to confirm the reading matches a thermostat or a lab instrument quoted in different units.
Worked example — 25°C air at 50% relative humidity
Enter 25 for Air temperature and 50 for Relative humidity, %. The exponent comes first: 17.67 × 25 ⁄ (25 + 243.5) = 1.645251, so e^1.645251 = 5.182313, and 6.112 × 5.182313 gives a saturation vapor pressure of 31.6743 hPa — the ceiling this parcel could hold before fog began forming. Fifty percent of that ceiling, converted to density by 2.1674 ⁄ 298.15, returns Absolute humidity = 11.5128 g/m³, about eleven and a half grams of water riding in the air of an ordinary phone box.
Cool that same parcel to 0°C and push it all the way to saturation — 100% relative humidity — and Absolute humidity drops to 4.85 g/m³, under half of what the warmer room held at only fifty percent. That is the arithmetic behind the winter dry-air complaint: heating a bubble of freezing, saturated outdoor air up to room temperature adds no water at all, so its relative humidity collapses even though its absolute humidity never changed.
Questions
How is absolute humidity different from relative humidity?
Absolute humidity is a mass density — grams of water vapor per cubic metre of air — while relative humidity is a percentage measuring distance to saturation. Air at 25°C and 50% RH holds about 11.51 g/m³ of water vapor, while saturated air at 0°C, 100% RH, holds only 4.85 g/m³. The same percentage, or even a higher one, can describe far less actual water once the temperature drops.
Why does the formula use an exponential term for temperature?
Because the amount of vapor air can hold before it saturates rises exponentially with temperature — the Clausius-Clapeyron relation from thermodynamics. David Bolton fit that curve in 1980 to the compact expression 6.112·e^(17.67T/(T+243.5)), accurate to within about 0.1% between -35°C and 35°C. It is the same approximation built into most digital hygrometers and weather-station firmware, so this instrument's readout lines up with theirs.
What does the constant 2.1674 actually represent?
It converts vapor pressure into vapor density by way of the ideal gas law. Water vapor's specific gas constant is 461.5 J/(kg·K); dividing 1000 by that value, to land in grams rather than kilograms and hPa rather than pascals, gives 2.1674. Multiply saturation vapor pressure by relative humidity and by 2.1674, then divide by the absolute temperature in kelvin, and a pressure figure becomes the density this instrument reports.
Does absolute humidity depend on atmospheric pressure?
No, and that surprises people. Absolute humidity is mass of water vapor per unit volume, which the ideal gas law ties only to vapor pressure and temperature — total barometric pressure never enters the sum. Specific humidity and the mixing ratio are different: both express vapor relative to the surrounding dry air, so they do need total pressure. Altitude shifts those two figures; it leaves absolute humidity, as computed here, untouched.
Who actually needs absolute humidity instead of relative humidity?
Anyone moving or removing a specific mass of water. An HVAC engineer sizing a dehumidifier multiplies absolute humidity by airflow to get grams removed per hour; a museum conservator tracks it to keep a gallery's real moisture content steady while temperature drifts overnight; a controlled-environment grower dosing a tent works in the same units. Relative humidity alone cannot answer how much water is present, because it says nothing about temperature.
Can relative humidity exceed 100% and still make sense here?
Not physically. Readings above 100% describe supersaturated air, and that state does not persist — excess vapor condenses out as fog, dew or frost the moment it forms, so real measurements stay at or below saturation. If a sensor briefly reports above 100%, treat it as noise rather than a genuine value, and keep Relative humidity, % at or under 100 for a physically meaningful result.