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:
- Temperature (T) of the air in degrees Celsius (°C)
- Relative humidity (RH) as a percentage
- 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).
Full download access to any calculation is available to users with a paid or awarded subscription (XLC Pro).
Subscriptions are free to contributors to the site, alternatively they can be purchased.
Click here for information on subscriptions.