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 →Cooler than standard means denser air and LOWER density altitude, not higher. The "always the same value" option is the sneaky one: pressure altitude does equal density altitude at exactly standard temperature, but "always" is what makes it wrong.

Field note →Quick mental check: if your thermometer reads higher than what ISA predicts for your elevation (ISA at sea level = 15°C, minus 2°C per 1,000 feet), density altitude is higher than your field elevation. For every 10°F above standard temperature, density altitude increases roughly 600–700 feet.