Specific humidity
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"
Meaning
Specific humidity is the mass of water vapour in a unit mass of air. It is measured in grams per kilogram (g/kg). It stays the same when air expands, warms or cools, as long as no water is added or taken away. That makes it the best measure for comparing the real moisture content of air at different places and heights.
Example
A parcel of moist air rising from the Arabian Sea up the Western Ghats keeps the same specific humidity as it climbs and cools, until condensation begins. Over the same climb its relative humidity rises steadily.
Don't confuse with
- Relative humidity: it is a percentage of the most vapour the air could hold. It falls when air warms and rises when air cools, even though the actual vapour is unchanged.
- Absolute humidity: it is grams of vapour per cubic metre of air. It changes when air expands or is squeezed, because the volume changes.