Humid Air water content calculations



Humid Air water content calculations

Humid air water content, or absolute humidity, refers to the mass of water vapor present in a given volume of air. To calculate the water content of humid air, you need the following information:

  1. Temperature (T) of the air in degrees Celsius (°C)
  2. Relative humidity (RH) as a percentage
  3. Air pressure (P) in pascals (Pa)

Once you have this information, you can use the following steps to calculate the water content of humid air:

Step 1: Calculate the saturation vapor pressure (e_s) at the given temperature using the August-Roche-Magnus formula:

e_s = 6.112 * exp((17.67 * T) / (T + 243.5))


  • e_s = saturation vapor pressure in hectopascals (hPa)
  • exp() = the exponential function
  • T = temperature in °C

Step 2: Calculate the actual vapor pressure (e_a) using the relative humidity:

e_a = (RH / 100) * e_s


  • e_a = actual vapor pressure in hPa
  • RH = relative humidity as a percentage

Step 3: Convert the actual vapor pressure to pascals:

e_a_Pa = e_a * 100


  • e_a_Pa = actual vapor pressure in pascals (Pa)

Step 4: Calculate the specific humidity (q) using the actual vapor pressure and air pressure:

q = 0.622 * (e_a_Pa / (P - e_a_Pa))


  • q = specific humidity (dimensionless)
  • P = air pressure in pascals (Pa)

Step 5: Calculate the absolute humidity (AH) using the specific humidity, air pressure, and temperature:

AH = (q * P) / (R_v * (T + 273.15))


  • AH = absolute humidity in grams of water vapor per cubic meter of air (g/m³)
  • R_v = specific gas constant for water vapor, approximately 461.5 J/(kg·K)

This will give you the water content of humid air in terms of absolute humidity (g/m³).

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Yogesh Kumar (kumaryk)
25 Apr 2023
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Comments: 1
JohnDoyle[Admin] 2 years ago
A nice debut calculation! I have awarded a three month XLC Pro subscription by way of thanks.
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