Moisture/Humidity » Mixing Ratio (w) Definition
Definition: Mixing Ratio
In a sample of moist air, the mixing ratio (w) is the ratio of the mass of water vapor (Mv) to the mass of dry air (Md):
w = Mv / Md
The mixing ratio is expressed in parts per thousand, usually grams of water vapor per kilogram of dry air.
The mixing ratio differs from the saturation mixing ratio in that it measures the actual amount of water vapor present, while the saturation mixing ratio measures the amount of water vapor that would be present at saturation.
Note for very precise physical computations, the "specific humidity" (q) is often a preferable quantity to use. Specific humidity is the mass of water vapor per mass of moist air:
q = Mv / (Mv + Md)
However, for synoptic forecasting purposes, the mixing ratio is sufficiently representative and is easier to evaluate.