The standard atmosphere and pressure

The ISA reference model, how an altimeter actually works, and why temperature and pressure changes can put you lower than the dial says.

13 min read · Weather

After this lesson you can

  • State the sea-level values of the International Standard Atmosphere and its lapse rate.
  • Compute pressure altitude from field elevation and the altimeter setting.
  • Tell indicated, true, absolute, pressure, and density altitude apart in a real scenario.
  • Apply the high-to-low and hot-to-cold corrections to avoid a dangerously low true altitude.

Weather begins with the atmosphere itself: a fluid ocean of air whose pressure and temperature change constantly, in every direction. Before any of the weather products in this module will make sense, you need a fixed reference point to compare that real, messy atmosphere against, and you need to understand the one instrument that turns raw air pressure into the number you call "altitude."

That reference point is the International Standard Atmosphere (ISA). It never changes, which is exactly the point: it lets every altimeter, every performance chart, and every controller be reading from the same page, even though the real atmosphere outside your window almost never matches it exactly.

The rest of this lesson

  1. 1.The atmosphere and the ISA reference
  2. 2.Five altitudes, one instrument
  3. 3.Reading and setting the altimeter
  4. 4.A cross-country in altimeter settings
  5. 5.High to low, look out below
  6. 6.Density altitude, in brief
  7. 7.Why the atmosphere moves

These 7 sections, their figures, the checks along the way and the end-of-lesson quiz come with the PPL track. Paid once, yours for good.

See what the PPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.