Humidity Definitions MET
There are a number of terms that you need to be aware of when it comes to humidity. Firstly, there are four terms which define (in different ways) how much water vapour is actually in the air:
Absolute Humidity: The mass of water in a given volume of air, measured in g/m³. Humidity Mixing Ratio: The mass of water vapour in a given mass of dry air, measured in g/kg. Specific Mixing Ratio: The mass of water vapour in a given total mass of air, measured in g/kg. Vapour Pressure: The component of total pressure due to water vapour, measured in hPahPa —Hectopascal.
Then, there are two terms which define the maximum amount of water vapour a parcel of air can take. These both have the word "saturation" in them in some form:
Saturation Mixing Ratio: The mass of water vapour that a parcel of air would hold at saturation, measured in g/kg. Saturated Vapour Pressure: The pressure of water vapour that a parcel of air would hold at saturation, measured in hPahPa —Hectopascal.
And finally, two other concepts:
Dew Point: The temperature at which the air will become saturated if cooled. Relative Humidity: The percentage of water vapour in the air compared to the amount at saturation. Think of this as "how full the tank is."