Humidity Ratio Calculator
Compute humidity ratio using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.
Formula
w = 0.622 * e / (P - e)
Where w is humidity ratio in kg/kg, e is actual vapor pressure in hPa, P is total pressure. 0.622 is ratio of molecular weights of water to dry air.
Worked Examples
Example 1: Standard Office Air
Problem: Calculate humidity ratio for air at 25 C, 60% RH, at 1013.25 hPa.
Solution: es = 31.67 hPa, e = 0.60*31.67 = 19.00 hPa w = 0.622*19.00/(1013.25-19.00) w = 11.89 g/kg Dew point = 16.69 C
Result: Humidity Ratio: 11.890 g/kg | Dew Point: 16.69 C
Example 2: Cold Winter Air
Problem: Find humidity ratio for air at -10 C, 80% RH, at 1000 hPa.
Solution: es=2.863 hPa, e=2.290 hPa w=0.622*2.290/(1000-2.290) w=1.428 g/kg Very low shows why winter air feels dry indoors
Result: Humidity Ratio: 1.428 g/kg | Very Dry
Frequently Asked Questions
What is the humidity ratio and how is it defined?
The humidity ratio also called mixing ratio is defined as the mass of water vapor per unit mass of dry air in a given volume of moist air. It is typically expressed in grams of water vapor per kilogram of dry air or as a dimensionless kg/kg ratio. The humidity ratio is calculated using w = 0.622 times e divided by (P minus e) where e is actual vapor pressure and P is total atmospheric pressure. Unlike relative humidity the humidity ratio is an absolute measure of moisture content that does not change with temperature alone. It is fundamental in psychrometrics and atmospheric thermodynamics.
How does humidity ratio differ from specific humidity and relative humidity?
Humidity ratio is mass of water vapor per unit mass of dry air while specific humidity is mass of water vapor per unit mass of total moist air including the vapor itself. The relationship is q = w/(1+w) where q is specific humidity and w is humidity ratio. For typical atmospheric conditions the numerical difference is small usually less than 2 percent. Relative humidity is the ratio of actual to saturation vapor pressure as a percentage. Relative humidity changes with temperature even without moisture changes while humidity ratio remains constant during dry adiabatic processes.
Why is humidity ratio important in HVAC and engineering?
The humidity ratio is the primary moisture variable in HVAC system design and psychrometric analysis because it directly quantifies actual water content of air. Engineers use it to calculate moisture that must be added or removed to achieve desired indoor conditions. On psychrometric charts humidity ratio is plotted on the vertical axis making it easy to visualize air conditioning processes. Cooling coils humidifiers and dehumidifiers are sized based on humidity ratio differences between supply and return air. Moist air enthalpy depends directly on humidity ratio for calculating heating and cooling loads.
How does atmospheric pressure affect the humidity ratio?
Atmospheric pressure has a direct inverse relationship with humidity ratio for a given vapor pressure. At lower pressures or higher altitudes the same vapor pressure produces a higher humidity ratio because the denominator P minus e becomes smaller. For example air with vapor pressure of 12 hPa has humidity ratio of about 7.5 g/kg at sea level but about 8.8 g/kg at 850 hPa. This pressure dependence is why meteorologists often prefer specific humidity or vapor pressure when comparing moisture across different altitudes or pressure levels.
What is the saturation humidity ratio and how is it used?
The saturation humidity ratio is the maximum water vapor air can hold at a given temperature and pressure. It increases approximately exponentially with temperature roughly doubling for every 10 Celsius increase. The ratio of actual to saturation humidity ratio gives the degree of saturation which is slightly different from relative humidity. Meteorologists use the difference between saturation and actual humidity ratios to assess proximity to condensation. The saturation humidity ratio at 30 Celsius and sea level is about 27 g/kg while at 0 Celsius it is only about 3.8 g/kg.
How is the humidity ratio measured in practice?
Humidity ratio is not measured directly but calculated from other measured quantities. The most common approach measures temperature and relative humidity using electronic sensors then computes vapor pressure and humidity ratio. Sling psychrometers measure dry-bulb and wet-bulb temperatures from which humidity ratio can be determined. Chilled mirror hygrometers measure dew point directly providing another calculation path. For laboratory work gravimetric methods that directly weigh moisture from a known air volume provide the most reliable results.