Plain English Meaning

The real air outside almost never matches the textbook 'standard' values for temperature and pressure. When it doesn't, the air is called nonstandard, and the aircraft will perform differently than the charts predict.

Full Definition

Atmospheric conditions at a given altitude that differ from the International Standard Atmosphere (ISA) values, which define standard sea level conditions as 59 °F (15 °C) and 29.92 inches of mercury (1013.2 hPa), with a temperature lapse rate of about 2 °C per 1,000 feet of altitude gain. Any deviation from these reference values, whether warmer, colder, higher, or lower in pressure, constitutes nonstandard temperature and pressure and affects density altitude, aircraft performance, and .