Definition
The mass of water vapor present in a unit mass of moist air, usually expressed in grams of water vapor per kilogram of air. Because it is a ratio of masses, specific humidity does not change as the air expands or is compressed, only when water vapor is added or removed.
Plain English
How much actual water vapor is mixed into a given amount of air, measured by weight. The number stays the same whether the air is squeezed or allowed to spread out, unless moisture is added or taken away.
Context Anchor
Seen in aviation weather study when describing the moisture content of an air mass.
Derivation
‘Specific’ in physics means ‘per unit mass.’ ‘Humidity’ comes from the Latin humidus, meaning moist. So the term literally means ‘moisture per unit of mass’ — which is exactly what it measures.
Why Pilots Care
High specific humidity lowers air density, lengthening takeoff rolls and reducing climb performance.
Grounding Statement
Imagine sealing one kilogram of air in a bag. If you weigh just the water vapor inside, that weight (in grams) is the specific humidity. Squeezing or stretching the bag doesn’t change it — only adding or removing moisture does.
Intuition Check
Do not read “specific humidity” as simply “how humid it feels.” It means the actual weight of water vapor in a given weight of air, not comfort level and not percent saturation.
Example Sentence 1
As the warm, moist air mass moved north, its specific humidity remained nearly constant even as the air cooled and the relative humidity climbed.
Example Sentence 2
At high specific humidity the reduced air density also lowered our rate of climb after takeoff.