Definition
The weight of a unit volume of air at a stated temperature and pressure. At standard sea-level conditions (15°C and 29.92 inches of mercury), the specific weight of dry air is approximately 0.0765 pounds per cubic foot.
Plain English
How much a given amount of air weighs. A cubic foot of air at sea level on a standard day weighs about 0.0765 pounds — a little over an ounce.
Context Anchor
Seen in discussions of aircraft performance, weather, and why air becomes less able to support flight as it gets hotter or as altitude increases.
Derivation
‘Specific’ here comes from the Latin specificus, meaning ‘of a particular kind or measure.’ In physics it signals a property measured per unit (per cubic foot, per pound, etc.) so different substances can be compared on equal terms.
Why Pilots Care
Lower specific weight reduces lift, engine power, and propeller efficiency, directly lengthening takeoff rolls and climb performance.
Analogy
Think of filling the same-sized box with two different materials. A box full of feathers weighs less than the same box full of sand. Air also has different weight in the same-sized space depending on its condition.
Grounding Statement
Air has real weight: the column of air above a typical car at sea level weighs roughly the same as a small adult.
Intuition Check
“Specific” does not mean “exact” here. It means weight measured for a set amount of space.
Example Sentence 1
On a hot, high-altitude day the specific weight of air drops, which is why the aircraft needs more runway to reach flying speed.
Example Sentence 2
When the specific weight of air drops on a hot afternoon, the airplane needs more runway to reach rotation speed.