What is the standard atmospheric lapse rate and why does it matter for density altitude calculations?
Why →
The International Standard Atmosphere (ISA) defines a lapse rate of approximately 2°C (3.5°F) per 1,000 feet of altitude gain. When actual temperature is WARMER than standard (ISA + X), density altitude is higher than pressure altitude. Air is less dense, aircraft performance degrades. A hot summer day at a high-elevation airport (e.g., 95°F at 5,000 feet elevation) can produce a density altitude of 8,000–9,000 feet.The trap →
Choice B reverses the direction. Cooler than standard means denser air and LOWER density altitude (better performance), not higher. Choice C is true when temperature exactly equals ISA standard. Pressure altitude does equal density altitude at standard temperature. But this distracts from the core point: any deviation from standard temperature changes density altitude.SOURCE → PHAK Chapter 11, Aircraft PerformanceCHECKED APR 21ACS III.B.K2MED