Definition
A model of the atmosphere adopted internationally that defines standard values of pressure, temperature, density, and lapse rate at sea level and with increasing altitude. At sea level the standard values are 29.92 inches of mercury (1013.2 hPa), 15°C (59°F), and a temperature lapse rate of approximately 2°C per 1,000 feet up to the tropopause. ISA is used as the reference baseline for calibrating altimeters, calculating aircraft performance, and comparing actual atmospheric conditions to a known standard.
Plain English
An agreed-upon set of 'average' atmospheric conditions used as a reference. It tells you what pressure, temperature, and air density should be at any altitude on a typical day. Real conditions are then compared to this standard to figure out how the airplane will actually perform.
Context Anchor
Seen in aircraft performance charts, climb planning, takeoff and landing calculations, and discussions of how actual weather differs from standard conditions.
Derivation
The word 'standard' here means 'an agreed reference point,' not 'normal' or 'average for today.' ISA is a fixed model — the atmosphere on any given day is rarely exactly standard, but the model gives everyone a common baseline to compare against.
Why Pilots Care
Enables accurate performance predictions and safe operating decisions regardless of actual local weather.
Analogy
ISA is like using the same ruler every time you measure something. The ruler is not the object itself, but it gives everyone a common reference.
Grounding Statement
If the actual day is much hotter than ISA, the airplane may need more runway and may climb more slowly than the chart’s basic ISA numbers suggest.
Intuition Check
Do not read standard as meaning “what the weather is usually like today.” In ISA, standard means “an agreed reference condition used for comparison.”
Example Sentence 1
On a standard day at sea level, the altimeter setting is 29.92 inches of mercury and the temperature is 15°C — that's ISA.
Example Sentence 2
At 5,000 feet in ISA, the temperature is about 5 degrees Celsius.