Mixing Ratio Of Air Calculator - Water Vapor From Temperature, Dew Point, Pressure

A mixing ratio of air calculator finds how much water vapor a parcel of air holds compared with the dry air around it, reporting the mass of water per mass of dry air. You enter the air temperature, the dew point, and the station pressure, and the calculator returns the actual mixing ratio in grams per kilogram, the saturation mixing ratio at the same temperature and pressure, and the relative humidity. At 20 degC with a 12 degC dew point and 1013.25 hPa pressure, the actual mixing ratio is about 8.7 g/kg while the saturation value is about 14.7 g/kg.

• • Meteorology and weather briefing: Forecasters read the mixing ratio to compare moisture between air masses without the pressure distortion that relative humidity introduces.
• • HVAC and building science: Engineers use the mixing ratio to size dehumidification loads and to track the moisture a ventilation stream carries into a space.
• • Cloud and fog physics: Students see when rising air reaches saturation, because the saturation mixing ratio falls as the temperature drops along a lifting path.
• • Aviation and density altitude: Pilots and dispatchers estimate water vapor content so they can correct air density for humid conditions at a given pressure.

The mixing ratio is conserved when an air parcel rises or sinks without mixing, which is why it is the natural moisture variable in adiabatic processes. Relative humidity, by contrast, swings as the temperature changes even when no water is added or removed.

Because the result is a ratio of masses, it does not change when pressure changes unless condensation actually occurs. That stability makes the mixing ratio the preferred unit on skew-T diagrams and in numerical weather prediction outputs.

If you need the water vapor mass per unit volume rather than per mass of dry air, the absolute humidity calculator gives the companion humidity measure.

The calculator converts each temperature to Celsius, evaluates the saturation vapor pressure at both the air temperature and the dew point using Tetens' equation, then applies the mixing-ratio relation w = epsilon * e / (P - e) with epsilon = 0.622. The saturation mixing ratio uses the air temperature; the actual mixing ratio uses the dew point, which is the temperature where the air is already saturated.

• T: Temperature in Celsius after any unit conversion from Fahrenheit or Kelvin.
• e_s: Saturation vapor pressure of water at temperature T, in hPa, from Tetens' equation.
• e: Actual vapor pressure; at the dew point the air is saturated, so e equals the saturation vapor pressure evaluated at the dew point.
• P: Station pressure in hPa; larger pressure dilutes the same vapor pressure, lowering the mixing ratio.
• epsilon: The ratio of water-vapor molar mass to dry-air molar mass, 0.622, which converts vapor pressure into a mass ratio.

The saturation mixing ratio falls steeply as temperature drops, so cooling air toward its dew point raises relative humidity even though the actual mixing ratio stays fixed until condensation begins.

Pressure enters the denominator as (P - e). At low station pressure, such as on a mountain top, the same vapor pressure corresponds to a larger mixing ratio, which is why humid air feels heavier at altitude than the relative humidity alone suggests.

According to NWS Mixing Ratio Worksheet, the US National Weather Service mixing-ratio worksheet defines the mixing ratio as the mass of water vapor per mass of dry air, computed with the Tetens-derived vapor pressure.

According to Mixing Ratio (Wikipedia), the mixing ratio is defined as the ratio of the mass of water vapor to the mass of dry air in a sample, the quantity this calculator reports in g/kg.

When you only have temperature and relative humidity, invert the process with the dew point calculator to recover the dew point this calculator needs.

Four ideas are enough to read every number the mixing ratio of air calculator returns.

These four concepts also explain why mixing ratio and relative humidity tell different stories: the ratio measures water per dry air, while relative humidity measures water per available capacity.

To see the saturation vapor pressure behind Tetens' equation on its own, the vapor pressure calculator evaluates the same formula at any temperature.

Five short steps move you from a temperature, dew point, and pressure reading to a defensible mixing ratio.

1. 1 Enter the air temperature and unit: Type the dry-bulb air temperature and choose Celsius, Fahrenheit, or Kelvin in the adjacent box.
2. 2 Enter the dew point and unit: Type the dew point temperature and choose its unit; it must be at or below the air temperature.
3. 3 Enter the station pressure and unit: Type the absolute station pressure and choose hPa, mbar, kPa, atm, mmHg, inHg, or psi.
4. 4 Read the actual and saturation mixing ratio: The calculator returns both values in g/kg; the actual ratio is the current moisture, the saturation ratio is the ceiling.
5. 5 Read the relative humidity: The third result is relative humidity, the ratio of the two mixing ratios expressed as a percentage.

A weather station reports 25 degC, a 15 degC dew point, and 1013.25 hPa. The calculator returns an actual mixing ratio of 10.6474 g/kg, a saturation mixing ratio of 20.0728 g/kg, and a relative humidity of 53.83%, meaning the air is a little more than half saturated.

For plant and greenhouse work the next step is the vapor pressure deficit calculator, which turns the saturation and actual vapor pressures into the deficit plants feel.

A dedicated mixing ratio of air calculator removes the unit conversions and algebra that get hand calculations wrong.

• • Reports both actual and saturation ratio: Seeing the actual and saturation mixing ratio side by side makes the headroom to saturation obvious at a glance.
• • Pressure-corrected result: Because the formula divides by (P - e), the result is already corrected for station pressure, so mountain and sea-level readings are comparable.
• • Avoids the Fahrenheit-to-Celsius trap: Choosing the unit converts temperatures to Celsius internally, preventing the common error of feeding Fahrenheit into Tetens' equation.
• • Links mixing ratio to relative humidity: Returning relative humidity in the same panel shows how the two moisture measures relate for the exact inputs entered.
• • Consistent with skew-T practice: The g/kg output matches the unit used on thermodynamic diagrams, so the figure drops straight into a meteorology lesson.

The calculator is best for single-point checks where one set of T, dew point, and pressure produces one moisture profile. For a full lifting-condensation path, the same outputs feed into stability work.

Because this calculator already reports relative humidity, the relative humidity calculator explains how that percentage relates to the mixing ratios shown here.

Three inputs drive the answer, and two approximation limits tell you when to trust the model less.

• • Tetens' equation fits saturation vapor pressure over water between about -40 and 50 degC; results below freezing should be treated as approximate because ice saturation differs.
• • The formula assumes an ideal gas mixture at low pressure, so it is reliable near the surface but should not be pushed to extreme pressures or very high altitudes without correction.

According to NWS Mixing Ratio Worksheet, the US National Weather Service mixing-ratio worksheet gives w = epsilon * e / (P - e) with epsilon = 0.622, the exact form this calculator evaluates from the dew-point vapor pressure.

For the evaporative cooling side of the same moisture balance, the wet-bulb calculator finds the wet-bulb temperature from temperature and humidity.