Table of Contents
What Is Mixing Ratio?
The mixing ratio (also called humidity ratio) is the mass of water vapor per unit mass of dry air, typically expressed in grams per kilogram (g/kg). It is a fundamental humidity variable in meteorology and HVAC engineering because, unlike relative humidity, it does not change with temperature as long as no moisture is added or removed. This makes it ideal for tracking moisture content through heating and cooling processes.
The mixing ratio is closely related to other humidity measures: specific humidity (mass of vapor per total air mass), vapor pressure (partial pressure of water vapor), and dew point temperature (temperature at which air becomes saturated). The saturation mixing ratio increases exponentially with temperature, approximately doubling for every 10 degrees C increase.
Mixing Ratio Formulas
Where w is mixing ratio, e is vapor pressure, P is total atmospheric pressure, and es is saturation vapor pressure (Magnus formula).
Values at Various Conditions (P = 1013.25 hPa)
| Temperature | RH | Mixing Ratio | Dew Point |
|---|---|---|---|
| 0°C | 100% | 3.8 g/kg | 0°C |
| 20°C | 50% | 7.3 g/kg | 9.3°C |
| 25°C | 50% | 9.9 g/kg | 13.9°C |
| 30°C | 80% | 21.5 g/kg | 26.2°C |
| 35°C | 100% | 36.5 g/kg | 35°C |
Frequently Asked Questions
How does mixing ratio differ from relative humidity?
Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure (a percentage). It changes with temperature even if no moisture is added. Mixing ratio is the actual mass of water per mass of dry air, which stays constant during heating or cooling without condensation. This makes mixing ratio more useful for energy calculations and psychrometric analysis.
What is the saturation mixing ratio?
The saturation mixing ratio is the maximum amount of water vapor air can hold at a given temperature and pressure. It increases rapidly with temperature: at 0 C it is about 3.8 g/kg, at 20 C about 14.7 g/kg, and at 40 C about 48.9 g/kg. When the actual mixing ratio equals the saturation value, RH is 100% and condensation (dew, fog, clouds) can occur.
How is this used in weather forecasting?
Meteorologists use mixing ratio to track moisture in air masses as they move and change altitude. When a parcel of air rises and cools, its saturation mixing ratio decreases while its actual mixing ratio stays constant. When the two are equal, clouds form. The lifting condensation level is the altitude where this occurs, and it can be calculated from surface temperature and dew point.