How far from "standard" is today's air?
The International Standard Atmosphere assumes 15 °C at sea level, cooling 1.98 °C per 1,000 ft up to the tropopause. Every performance chart is built on it — and today's air never matches it. Enter an altitude to get the ISA temperature, add your OAT to get the deviation ("ISA+10"), and see what it does to your density altitude.
Use pressure altitude for chart work (set 29.92 or use the pressure altitude calculator); indicated altitude is close enough for a quick estimate.
Educational estimates only — not for flight planning or dispatch. Fly your POH numbers.
| Altitude | ISA temp | Pressure | Notes |
|---|---|---|---|
| Sea level | +15 °C / 59 °F | 29.92" / 1013 hPa | The reference everything starts from |
| 5,000 ft | +5.1 °C | 24.90" | |
| 10,000 ft | −4.8 °C | 20.58" | |
| 18,000 ft | −20.7 °C | 14.95" | ≈ half the atmosphere below you |
| 30,000 ft | −44.4 °C | 8.89" | |
| 36,089 ft | −56.5 °C | 6.68" | Tropopause — temperature stops falling |
Exam shortcut: ISA temp = 15 − (2 × altitude in thousands). At 8,000 ft: 15 − 16 = −1 °C. The precise lapse is 1.98 °C/1,000 ft, which is why this calculator shows −0.8 °C — close enough that exams accept both.
Why deviation matters: every degree warmer than ISA adds ~120 ft of density altitude. ISA+15 at an 8,000 ft pressure altitude means your aircraft performs like it's at ~9,800 ft. Turbine flight planning uses ISA deviation directly — cruise tables are published for ISA, ISA+10, ISA+20.
Start at 15 °C and subtract about 2 °C per 1,000 ft (precisely 1.98 °C) up to the tropopause near 36,000 ft, where it holds at −56.5 °C. At 10,000 ft: 15 − 20 = about −5 °C.
The outside air is 10 °C warmer than the standard atmosphere predicts for that altitude. ISA+10 at 6,000 ft means an OAT of about +13 °C instead of the standard +3 °C — and roughly 1,200 ft of extra density altitude.
Yes — it's the temperature half of density altitude. Warmer than ISA means thinner air: longer takeoff rolls, weaker climb, reduced engine power, and higher true airspeed for the same indicated. Colder than ISA improves all of them.
Strictly, pressure altitude — ISA is defined against the standard 29.92 datum, and performance charts expect it. Below the transition altitude the difference is usually a few hundred feet, so indicated altitude gives a fine estimate when you don't have the altimeter setting handy.
DensityAlt is a set of free, no-signup aviation calculators. Explore the rest: