Almost every temperature, altitude and performance question in the DGCA Aviation Meteorology paper rests on one reference: the ISA standard atmosphere. If you know its values by heart and can work out ISA deviation quickly, a whole group of exam questions becomes easy marks. This guide gives you the numbers, the method and worked examples for the DGCA CPL exam.
What is the ISA standard atmosphere?
The International Standard Atmosphere (ISA) is an agreed model of how temperature, pressure and density change with height in an "average" atmosphere. It is defined by the International Civil Aviation Organization (ICAO) so that altimeters, aircraft performance charts and flight levels all use the same reference.
The real atmosphere is almost never exactly ISA. That is the whole point: ISA gives you a fixed yardstick, and you then measure how far the real day differs from it. That difference is the ISA deviation.
ISA values you must remember for the DGCA exam
These are the figures the exam expects you to know without looking them up.
- Mean sea level (MSL) temperature: +15°C
- MSL pressure: 1013.25 hPa
- MSL density: 1.225 kg/m³
- Lapse rate in the troposphere: 1.98°C per 1,000 ft (6.5°C per km)
- Tropopause height: 11 km, which is about 36,090 ft
- Temperature at the tropopause: −56.5°C
- Above the tropopause: temperature stays constant at −56.5°C up to 20 km (about 65,617 ft)
The ISA atmosphere is also assumed to be dry air. Many exam questions round the ISA lapse rate to 2°C per 1,000 ft. Read the options carefully: if the answers are close together, use 1.98°C; if they are well spread, 2°C is fine.
ISA pressure levels worth knowing
Pressure does not fall at a steady rate with height; it falls faster near the surface and more slowly higher up. Near sea level, 1 hPa is roughly 27 ft (many textbooks round this to 30 ft for quick working). Some useful ISA reference levels:
- 850 hPa: about 4,800 ft
- 700 hPa: about 9,900 ft (close to FL100)
- 500 hPa: about 18,300 ft (pressure has roughly halved)
- 300 hPa: about 30,000 ft (close to FL300)
- 200 hPa: about 38,700 ft
How to find ISA temperature at a flight level
The ISA temperature at any level below the tropopause is the sea-level value minus the lapse rate for the height.
- Write the flight level as thousands of feet (FL150 = 15,000 ft = 15).
- Multiply by 1.98 (or 2).
- Subtract the result from 15.
ISA temperature at FL150
15 − (1.98 × 15) = 15 − 29.7 = −14.7°C (about −15°C)
Above 36,090 ft, do not keep subtracting. The ISA temperature is simply −56.5°C, so at FL390 the ISA temperature is −56.5°C.
ISA deviation calculation: step by step
ISA deviation is the actual outside air temperature (OAT) minus the ISA temperature at that level. A positive answer is written ISA+ (warmer than standard); a negative answer is ISA− (colder than standard).
ISA deviation = OAT − ISA temperature
Worked example 1: warmer than ISA
Flight level: FL150
OAT: −8°C
ISA temperature: −14.7°C
ISA deviation: −8 − (−14.7) = +6.7 → about ISA +7
Worked example 2: high level
Flight level: FL330
ISA temperature: 15 − (1.98 × 33) = 15 − 65.3 = −50.3°C
OAT: −45°C
ISA deviation: −45 − (−50.3) = +5.3 → about ISA +5
Worked example 3: above the tropopause
Flight level: FL390
ISA temperature: −56.5°C (constant above 36,090 ft)
OAT: −60°C
ISA deviation: −60 − (−56.5) = −3.5 → about ISA −4
Why ISA deviation matters in flight
ISA deviation is not just an exam number. It changes how high you really are and how well the aircraft performs.
Temperature error on the altimeter
An altimeter is calibrated to ISA. In air colder than ISA, the air column is denser and "shorter", so the aircraft is lower than the altimeter shows. In warmer air it is higher. A common rule of thumb is about 4 ft of error for every 1,000 ft of height above the pressure-setting datum, for each 1°C of ISA deviation.
Altimeter reads 8,000 ft (QNH set, airfield near MSL), ISA −10
Error ≈ 4 × 8 × 10 = 320 ft
True altitude ≈ 8,000 − 320 = 7,680 ft
This is why the memory aid "from high to low, look out below" applies to temperature as well as pressure. You can revise the pressure side of this in our post on QNH, QFE and QNE.
Density altitude and performance
Density altitude is the height in the ISA standard atmosphere at which the air density equals the actual density. On a hot day the air is thinner, so the aircraft behaves as if it were higher: longer take-off run, weaker climb and less engine power. A common approximation is to add about 120 ft to the pressure altitude for every 1°C above ISA.
Airfield elevation 3,000 ft, QNH 1008 hPa, OAT 30°C
Pressure altitude ≈ 3,000 + (1013 − 1008) × 27 = 3,135 ft
ISA temperature at 3,135 ft ≈ 15 − (1.98 × 3.135) = 8.8°C
ISA deviation ≈ 30 − 8.8 = +21.2
Density altitude ≈ 3,135 + (120 × 21.2) ≈ 5,700 ft
These rules of thumb give exam-level accuracy. For real flight planning, always use the aircraft's performance charts.
Common mistakes with ISA in the DGCA exam
- Subtracting the wrong way round. ISA deviation is OAT minus ISA, not ISA minus OAT. Getting this backwards flips the sign.
- Applying the lapse rate above the tropopause. Above 36,090 ft the ISA temperature stays at −56.5°C.
- Using flight level as feet without dividing by 1,000. FL250 means 25 in the formula, not 250.
- Mixing up pressure altitude and true altitude. Pressure altitude uses 1013.25 hPa; true altitude is your real height above mean sea level.
- Forgetting negative signs. Write every temperature with its sign before you subtract.
ISA deviation is always OAT minus ISA temperature, and above the tropopause the ISA temperature stays at −56.5°C.
Frequently asked questions
What are the ISA values at mean sea level?
In the International Standard Atmosphere, mean sea level has a temperature of +15°C, a pressure of 1013.25 hPa and an air density of 1.225 kg/m³. These three values are the starting point for every ISA calculation, so learn them by heart before you attempt any temperature or altimetry question in the DGCA Meteorology paper.
How do you calculate ISA deviation?
First find the ISA temperature at your level: 15°C minus 1.98°C for every 1,000 ft, or −56.5°C above the tropopause. Then subtract that ISA temperature from the actual outside air temperature. A positive result means warmer than standard (ISA+), and a negative result means colder than standard (ISA−).
What is the ISA lapse rate in feet?
The ISA lapse rate is 1.98°C per 1,000 ft, which equals 6.5°C per kilometre. Many exam questions and quick mental calculations round it to 2°C per 1,000 ft. It applies only in the troposphere, from mean sea level up to the tropopause at about 36,090 ft, where the temperature stops falling.
At what height is the ISA tropopause?
In the ISA standard atmosphere the tropopause is at 11 km, which is about 36,090 ft, and the temperature there is −56.5°C. In the real atmosphere the tropopause is higher over the tropics, including much of India, and lower over the poles, so actual heights differ from the ISA model.
Why is the ISA standard atmosphere used in aviation?
ISA gives pilots, engineers and air traffic control one common reference. Altimeters are calibrated to it, flight levels are based on it, and aircraft performance charts are built around it. Because everyone uses the same model, you can compare the real day with ISA and correct altitudes and performance figures accordingly.
Practise ISA standard atmosphere questions
Once you can find the ISA temperature at any level and work out ISA deviation without hesitation, test yourself with free DGCA practice questions and time how quickly you solve each one. Fly with Sudharshan Aviation Academy (FSAA) covers the ISA standard atmosphere, altimetry and the rest of the syllabus in the DGCA Aviation Meteorology course, with classes in Bengaluru and online.