Wet Bulb Humidity Calculator (Psychrometer)
This wet bulb humidity calculator works from psychrometer readings. Enter the readings of a dry and wet bulb thermometer (a sling or aspirated psychrometer) and the air pressure to get relative humidity, absolute humidity in grams per cubic metre and the actual vapour pressure.
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Formula
Temperatures in C (F inputs are converted), pressure P in hPa:
- Saturation vapour pressure:
es(T) = 6.112 × exp(17.62 T ÷ (243.12 + T))hPa - Psychrometer equation:
e = es(Tw) − A × P × (Td − Tw), withA = 0.00066 × (1 + 0.00115 × Tw) - Relative humidity:
RH = 100 × e ÷ es(Td) - Absolute humidity:
AH = 216.7 × e ÷ (273.15 + Td)g/m³
Td is the dry bulb and Tw the wet bulb temperature. The wet bulb cannot read higher than the dry bulb, and it must be above 0 C (32 F): below freezing the bulb is iced and a different (ice-bulb) formula applies.
Worked example
Dry bulb 25 C, wet bulb 20 C, 1013.25 hPa:
es(20) = 23.33,A = 0.000675, soe = 23.33 − 0.000675 × 1013.25 × 5 =19.9 hPaes(25) = 31.60, soRH = 100 × 19.905 ÷ 31.60 =63.0%AH = 216.7 × 19.905 ÷ 298.15 =14.47 g/m³
Frequently asked questions
How do wet and dry bulb readings give humidity?
Water evaporating from the wet bulb cools it below the dry bulb, and the drier the air the bigger the gap. With dry 25 C and wet 20 C at sea-level pressure, the vapour pressure is 19.9 hPa against a saturation value of 31.6 hPa, which is 63.0% relative humidity.
Which formula is used?
The psychrometer equation e = es(Tw) - A × P × (Td - Tw), with A = 0.00066 × (1 + 0.00115 × Tw) per C for a ventilated instrument, and es(T) = 6.112 × exp(17.62T/(243.12+T)) hPa. Relative humidity is 100 × e / es(Td).
What is absolute humidity and how is it found?
Absolute humidity is the mass of water vapour per cubic metre of air. It is 216.7 × e / (273.15 + T) grams per m3, with e in hPa and T in C. For the 25 C and 20 C example that is 14.47 g/m3.
Why does air pressure matter?
The cooling effect of the wet bulb depends on air density. At 1000 hPa, dry 20 C and wet 10 C give 23.9% relative humidity. At high altitude, enter the local station pressure, not the sea-level corrected value. Readings need airflow of about 3 m/s past the wet bulb; still air reads too humid.
Why must the wet bulb be above 0 C (32 F)?
Below freezing the wet bulb is covered in ice, and the ice-bulb psychrometer formula uses a different latent heat and saturation curve. This calculator only uses the water-bulb formula, so the wet bulb must be above 0 C (32 F).