Mixing Ratio and Specific Humidity
Specific humidty is a measurement of humidity that is not influenced by changes in air volume. It is the ratio of the mass of the water vapor in the air parcel to the total mass of air.
Specific Humidity = mass of water vapor/total mass of air
The hightest average specific humidities occur in the tropics, with values decreasing as you move towards the poles. The figure below shows how specific humidity varies with latitude.
The amount of water vapor in a certain volume of air can be defined as the ratio of the mass mv of water vapor to the mass of dry air. This is the mixing ration, r.
r = mv/md
Units are usually grams per kilogram of dry air, however in numerical excercises it must be expresed as kg of water vapor per kg of dry air, a dimensionless number. The value of r varies depending on latitude, with values around a few g/kg in the mid latitudes to around 20g/kg in the tropics. The mixing ration of an air parcel is constant if neither condensation nor evaporation takes place.
The mass of water vapor, mv, in a unit mass of air (dry air plus water vapor) is called the Specific Humidity, q:
Average specific humidty for each latitude. Values decrease from the tropics towards to the polar regions.
w and q are nearly equivalent due to the value of w being only a few percent.
Specific humidity and mixing ratio of an air parcel both remain constant as long as water vapor is not added to or taken away from the parcel.
