Humidity Mixing Ratio Calculator

How it is calculated

The tool computes w = 0.622 × e ÷ (p − e), where e is the water vapour pressure and p is the total air pressure, both in hPa (the same number as millibars). The constant 0.622 is the ratio of the molar masses of water and dry air. The result is mass of vapour per mass of dry air, shown twice: in g/kg (w × 1000, two decimals) and in kg/kg (four decimals). Both pressures must be above zero, and the total pressure must be larger than the vapour pressure, otherwise no result appears. The tool does not work out the vapour pressure from temperature or relative humidity. You must supply it, for example from a psychrometric table. Pressures in other units, such as kPa or inHg, must be converted to hPa first.

Common mistakes

• Entering pressure in kPa or inHg. Sea-level pressure typed as 101.3 (kPa) is read as 101.3 hPa, and with a vapour pressure of 17.5 the tool shows 129.89 g/kg, about ten times too high.
• Entering relative humidity in the vapour-pressure field. A value such as 60 (for 60 %) is read as 60 hPa, which gives about 40 g/kg rather than a realistic figure.
• Using the same value for e and p, or an e larger than p. The tool returns no result because the denominator would be zero or negative.
• Confusing mixing ratio with specific humidity. This tool divides by the mass of dry air, while specific humidity divides by moist air, so the two differ slightly.
• Reading the kg/kg line as g/kg. The two lines are the same quantity, and 0.0109 kg/kg equals 10.93 g/kg.