ISA calculator

Is it warmer or colder than standard today?

ISA calculator

Set the altitude and the outside air temperature. The ISA deviation works itself out.

• Hot day at sea level, 35 °C
• 2 km, +5 °C
• FL300, −40 °C
• FL390, −56.5 °C
• 19 km, −60 °C

Memory hook: Actual minus ISA.

Takeaway: ISA deviation = actual − ISA. ISA falls 1.98 °C per 1000 ft (6.5 °C/km) from 15 °C at sea level, and stays −56.5 °C from 11 to 20 km.

What exactly is in the ICAO standard atmosphere?

ISA and JSA

Every number of the ISA, then the jet standard atmosphere. Tap Next.

1. Why a standard atmosphere? The real atmosphere changes every day. To design and test aircraft, compare how they perform and calibrate altimeters, everyone needs the same imaginary atmosphere. ICAO's version is the International Standard Atmosphere (ISA).
2. At mean sea level The ISA starts with dry air (no water vapour) at mean sea level. Temperature 15 °C (288.15 K), pressure 1013.25 hPa, density 1225 g/m³, and gravity 980.665 cm/s².
3. Up to 11 km Going up, the temperature falls 6.5 °C per km (1.98 °C per 1000 ft). This goes on up to 11 km, which is 36 090 ft. That height is the ISA tropopause, the top of the weather layer.
4. From 11 to 20 km Above the tropopause the temperature stops falling. It stays at −56.5 °C from 11 km up to 20 km (65 617 ft).
5. From 20 to 32 km Higher still, it warms again by 1 °C per km (0.3 °C per 1000 ft), reaching −44.5 °C at 32 km (104 987 ft).
6. Jet standard atmosphere (JSA) The ISA suits aircraft below about 30 000 ft. For very high flying, space flight and engine design the JSA is used instead. It starts at 15 °C at sea level and cools a steady 2 °C per 1000 ft, with no tropopause: the dashed line just keeps going. ISA deviation = actual − ISA. Warmer than ISA is +, colder is −.

Memory hook: 15 · 1013.25 · 6.5 · 11 · −56.5 · 20.

Takeaway: The ISA is an agreed, made-up atmosphere. At sea level it is dry air at 15 °C, 1013.25 hPa and 1225 g/m³. It cools 6.5 °C per km up to 11 km, stays at −56.5 °C up to 20 km, then warms 1 °C per km up to 32 km. The JSA simply cools 2 °C per 1000 ft, with no tropopause.

Can you work out ISA temperatures without a calculator?

ISA sums

The lecture's shortcut and its exam-style questions. Tap Next.

1. One yardstick for everyone If every airfield calibrated altimeters to its own air, aircraft from different places would read different altitudes at the same height. ICAO's ISA is one imaginary atmosphere everyone uses: dry air, 15 °C, 1013.25 hPa and 1225 g/m³ at sea level, falling 1.98 °C per 1000 ft (6.5 °C/km) to −56.5 °C at 11 km (36,090 ft), constant to 20 km, then warming slowly.
2. 15 minus twice the thousands No calculators in the exam, so round 1.98 to 2: ISA temperature = 15 − 2 × (altitude in thousands of feet). Check: at 2000 ft, 15 − 4 = 11 °C.
3. Never below −56.5 At 45,000 ft the shortcut gives 15 − 90 = −75 °C, but in the ISA the temperature stops falling at the tropopause.
4. The FL330 question It is −48 °C at FL300 and the tropopause is at FL330. What is the temperature at FL350? Count down 2 °C per 1000 ft to the tropopause, then stop.
5. Count the gap It is −9 °C at FL130. What will it be at FL80, with the ISA lapse rate? The gap is 5000 ft, so 10 °C warmer lower down.
6. Actual minus ISA 20 °C at sea level: +5 °C deviation. 1002.25 hPa at sea level: −11 hPa. −45 °C at 6 km, where ISA is 15 − 6.5 × 6 = −24 °C: deviation −21 °C.
7. The lecture's last question It is 3 °C at FL70. At what altitude will it be 11 °C, with the ISA lapse rate?

Memory hook: ISA °C = 15 − 2 × (feet ÷ 1000), but never colder than −56.5.

Takeaway: Why ISA exists: one agreed atmosphere to calibrate instruments and compare the real day against. In the exam, round 1.98 to 2 °C per 1000 ft: ISA = 15 − 2 × (height in thousands of feet). Stop at −56.5 °C above the tropopause. Deviation = actual − ISA.