Mixing Ratio and Specific Humidity Calculator
Find water vapor mixing ratio and specific humidity from air temperature, relative humidity, and station pressure.
💦 What is Mixing Ratio and Specific Humidity?
Mixing ratio (w) is the mass of water vapor contained in a parcel of air divided by the mass of dry air in that same parcel, usually expressed in grams of vapor per kilogram of dry air. Specific humidity (q) is a closely related quantity, the mass of water vapor divided by the total mass of moist air (dry air plus vapor). Both describe the actual amount of moisture present, rather than how close the air is to saturation, which is what relative humidity measures.
Meteorologists and atmospheric scientists rely on mixing ratio and specific humidity constantly, to track moisture transport in weather models where these are the standard prognostic variables, to compute equivalent potential temperature and moist static energy for severe weather forecasting, to quantify how much water vapor a rising air parcel carries into a thunderstorm updraft, and to compare humidity levels across different altitudes and pressures on a physically consistent basis.
A common point of confusion is treating these as interchangeable with relative humidity. They are not, relative humidity depends on temperature and changes throughout the day even with no moisture added or removed, while mixing ratio and specific humidity stay essentially constant during dry adiabatic ascent or descent, since no vapor is gained or lost. This makes them far more useful for tracking moisture as an air parcel moves vertically through the atmosphere.
This calculator computes vapor pressure from temperature and relative humidity using the Tetens formula, then derives mixing ratio and specific humidity from vapor pressure and station pressure, showing every intermediate step. The results feed directly into the potential temperature and equivalent potential temperature calculators on this site, since equivalent potential temperature requires mixing ratio as an input.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Warm Afternoon at Sea Level
T = 25°C, RH = 60%, P = 1000 hPa
Example 2 — Humid Tropical Evening at a Mountain Station
T = 30°C, RH = 80%, P = 950 hPa
Example 3 — Dry Winter Air at Standard Pressure
T = 15°C, RH = 40%, P = 1013 hPa
❓ Frequently Asked Questions
🔗 Related Calculators
What is mixing ratio in meteorology?
Mixing ratio (w) is the mass of water vapor per unit mass of dry air in a parcel of moist air, usually expressed in grams of vapor per kilogram of dry air (g/kg). It is computed as w = 0.622*e / (P-e), where e is vapor pressure and P is total air pressure, both in the same units.
What is specific humidity and how is it different from mixing ratio?
Specific humidity (q) is the mass of water vapor per unit mass of total moist air (dry air plus the vapor itself), computed as q = w / (1+w). Mixing ratio uses dry air alone in the denominator while specific humidity uses total moist air, so q is always slightly smaller than w, though the two are nearly equal when w is small.
How do you calculate mixing ratio from temperature and relative humidity?
First find vapor pressure e = (RH/100) times the Tetens saturation vapor pressure es(T) = 6.112*exp(17.67*T/(T+243.5)) in hectopascals. Then apply w = 0.622*e / (P-e), where P is the station pressure in hectopascals. The result w is in kilograms of vapor per kilogram of dry air; multiply by 1000 for grams per kilogram.
Why does the formula use 0.622?
0.622 is the ratio of the molar mass of water (about 18.02 g/mol) to the molar mass of dry air (about 28.97 g/mol). It converts the ratio of partial pressures (vapor to dry air) into a ratio of masses, since water vapor molecules are lighter than the average air molecule.
Does pressure or altitude affect mixing ratio?
Yes. For the same vapor pressure e, a lower total pressure P (higher altitude) produces a slightly higher mixing ratio, because w depends on e relative to (P-e). At typical surface pressures this effect is small, but it becomes more noticeable at mountain stations or in the free atmosphere.
Why are mixing ratio and specific humidity almost the same number?
Because w is typically small, usually under about 30 g/kg even in the most humid tropical air, the (1+w) term in q = w/(1+w) is very close to 1. The two quantities only diverge meaningfully at very high mixing ratios, which are rare in Earth's atmosphere.
Why are mixing ratio and specific humidity useful compared to relative humidity?
Both are conservative quantities during dry adiabatic motion, meaning they stay constant as an air parcel rises or sinks without condensation, unlike relative humidity which changes purely because temperature changes with altitude. This makes them the preferred humidity variables in numerical weather models and thermodynamic diagrams.
What is vapor pressure and how is it computed here?
Vapor pressure (e) is the partial pressure exerted by water vapor molecules in the air. This calculator computes it as e = (RH/100) times the saturation vapor pressure at the current temperature, using the Tetens approximation, the same formula used by the dew point and relative humidity calculators on this site.
How is mixing ratio used in forecasting severe weather?
Mixing ratio is a direct input to equivalent potential temperature and CAPE calculations, both of which measure how much energy is available for thunderstorm development. Higher low-level mixing ratios generally mean more latent heat can be released if the air rises and condenses, increasing instability.
What input range does this calculator accept?
This calculator accepts air temperatures from -20 to 50 degrees Celsius, relative humidity from 1 to 100 percent, and station pressure from 300 to 1050 hectopascals, covering conditions from mountain stations to sea level.