Mixing Ratio of Air Calculator

The mixing ratio of air is the mass of water vapor divided by the mass of dry air in the same parcel. Meteorologists normally report it in grams of vapor per kilogram of dry air. Unlike relative humidity, mixing ratio does not change merely because an unsaturated parcel warms or cools at constant pressure without gaining or losing moisture. That makes it a practical measure for tracing atmospheric moisture, comparing air masses, and performing weather calculations. This calculator begins with saturation vapor pressure, which is the vapor pressure that would exist if the air were saturated at the entered temperature. It uses the Bolton-style exponential approximation over liquid water. Relative humidity scales that saturation value to obtain actual vapor pressure. The mixing ratio then follows from the ratio of water vapor pressure to dry-air partial pressure, multiplied by the molecular-weight ratio of water vapor to dry air. Results are converted from kilograms per kilogram to grams per kilogram for readability. Atmospheric pressure matters because total pressure is shared by dry air and water vapor. At the same temperature and relative humidity, lower-pressure air has a larger mixing ratio because the vapor represents a greater fraction of the remaining dry-air pressure. Enter station pressure, not pressure corrected to sea level, when evaluating observations from an elevated weather station. Standard pressure of 1013.25 hPa is a useful default near sea level. The calculator also reports specific humidity. Mixing ratio compares vapor mass with dry-air mass, while specific humidity compares vapor mass with the total mass of moist air. The two values are close at ordinary atmospheric moisture levels, but they are not identical. Vapor pressure and saturation vapor pressure are shown so the intermediate values can be checked against a weather report, psychrometric table, or classroom calculation. Applications include aviation weather, convective forecasting, cloud-base analysis, ventilation studies, agricultural meteorology, radiosonde interpretation, and moisture balances. Temperature should represent dry-bulb air temperature and relative humidity should come from a calibrated observation. The formula assumes water-vapor equilibrium over liquid water; specialized ice-saturation formulas may be more appropriate well below freezing. It also assumes the pressure and humidity describe the same parcel at the same time. Mixing ratio should not be confused with the ratio used to combine two air streams. This tool calculates the composition of one moist-air parcel from atmospheric observations. To blend two streams, first calculate or measure each humidity ratio, then combine them with a dry-air mass-weighted balance. Within its intended range, this calculator provides an immediate and transparent measure of atmospheric water-vapor content.