International Standard Atmosphere

Module 1

1.1 Atmosphere

1.2 ISA Deviation

1.1 Atmosphere

Layer of atmosphere – Notes

1. Atmospheric Composition

The Earth's atmosphere is primarily composed of about 78 -79% nitrogen and 20-21% oxygen, with the remaining 1% consisting of argon and trace gases like carbon dioxide, neon, and water vapor. These gases, along with dust particles, are vital for life, with nitrogen being essential for plant growth, oxygen for respiration, and carbon dioxide for photosynthesis and trapping heat.

Gas	Volume (%)
Nitrogen (N₂)	78.08
Oxygen (O₂)	20.95
Argon (Ar)	0.93
Carbon Dioxide (CO₂)	~0.04
Water Vapor	Variable (0–4%)

2.Main layers of atmosphere

The five layers of the Earth's atmosphere, from lowest to highest, are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.The atmosphere is layered, corresponding with how the atmosphere’s temperature changes with altitude.

3. International Standard Atmosphere (ISA)

The International Standard Atmosphere (ISA) is a reference model describing how temperature, pressure, density, and viscosity vary with altitude in Earth’s atmosphere. It provides standardized tables and formulas for these values. Published as ISO 2533:1975, the ISA is also adopted and extended by organizations such as ICAO and the U.S. government for aviation and engineering use.

Parameter	Symbol	Value
Temperature	T₀	288.15 K (15°C)
Pressure	p₀	101325 Pa
Density	ρ₀	1.225 kg/m³
Speed of Sound	a₀	340.3 m/s
Gravity	g₀	9.80665 m/s²

4. Gradient Layer (Lapse-Rate Layer)

Temperature changes linearly with altitude:
T = T₀ + L (h − h₀)

Where:
T = temperature at altitude h
T₀ = temperature at reference altitude h₀
L = lapse rate (K/m or K/km)

Positive L → temperature increases with altitude
Negative L → temperature decreases with altitude

5. Isothermal Layer

Temperature remains constant with altitude:
T = T₀

Pressure formula (Isothermal Layer)
P = P₀ · exp [ − g(h − h₀) / (R·T₀) ]

Density formula (Isothermal Layer)
ρ = ρ₀ · exp [ − g(h − h₀) / (R·T₀) ]

Here, pressure and density decay exponentially with altitude.

6. Temperature Behaviour as change in Layers

Layer	Type	Lapse Rate	Temperature Behavior
Troposphere (0–11 km)	Gradient	–6.5 K/km	Decreasing
Lower Stratosphere (11–20 km)	Isothermal	0	Constant (216.65 K)
Middle Stratosphere (20–32 km)	Gradient	+1 K/km	Increasing
Upper Stratosphere (32–47 km)	Gradient	+2.8 K/km	Increasing
Mesosphere (47–51 km)	Isothermal	0	Constant
Mesosphere (51–71 km)	Gradient	–2.8 K/km	Decreasing

7. Layers, Altitudes & Typical Flying Vehicles

Atmospheric Layer	Approx. Altitude Range	Flying Vehicles / Systems Operating in This Layer
Troposphere	0 – 12 km	• Commercial Airliners (A320, B737, B787) • Helicopters • General Aviation Aircraft (Cessna, Piper) • Hot Air Balloons • UAVs / Drones (Quadcopters, MALE) • Weather Balloons (initial ascent)
Stratosphere	12 – 50 km	• High-Altitude Long Endurance (HALE) UAVs • Supersonic Aircraft (SR-71 Blackbird) • Weather Balloons (float region ~30–35 km) • Military Reconnaissance Aircraft (U-2) • Stratospheric Airships (Loon, HAPS platforms)
Mesosphere	50 – 85 km	• Sounding Rockets (sub-orbital scientific missions) • Meteor Trails (no aircraft can fly—too thin air)
Thermosphere	85 – 600 km	• Space Shuttles (during re-entry) • International Space Station (ISS operates ~400 km) • Low Earth Orbit (LEO) Satellites • Aurora Phenomena (natural, not vehicles)
Exosphere	600 – 10,000 km	• High-Earth-Orbit (HEO) Satellites • GPS Satellites (~20,000 km region is transitional) • Spacecraft traveling to Moon/planets

8. Atmospheric Layers — Altitude, Flying Vehicles & Temperature Profile

The table below lists the major layers of Earth's atmosphere with approximate altitude ranges and typical flying vehicles / systems that operate there. The diagram to the right (or below on narrow screens) shows a representative temperature profile versus altitude — note that the exact temperatures vary with time, latitude, and solar activity; this diagram gives the typical trend (decrease in the troposphere, increase in the stratosphere, decrease in the mesosphere, then strong increase in the thermosphere).

Atmospheric Layer	Approx. Altitude Range (km)	Common Flying Vehicles / Systems
Troposphere	0 – 12	Commercial airliners, helicopters, general aviation, hot-air balloons, most drones, weather balloons (initial ascent)
Stratosphere	12 – 50	High-altitude UAVs (HALE), reconnaissance aircraft (e.g., U-2), stratospheric airships / long-endurance platforms, some high-altitude research flights
Mesosphere	50 – 85	Sounding rockets and sub-orbital probes; conventional aircraft cannot operate (air too thin)
Thermosphere	85 – 600	Low Earth Orbit (LEO) spacecraft during re-entry, International Space Station (~400 km). Many LEO satellites operate in/above this region.
Exosphere	600 – 10,000	High-Earth-Orbit satellites, transfer orbits, spacecraft traveling beyond Earth (trans-lunar, interplanetary)

Note: the SVG shows a representative temperature trend — actual temperatures depend on latitude, local weather, and solar activity. The thermosphere's temperature rises strongly (but density is extremely low).

ISA Calculator (Table-Based)

ISA Calculator (0–20 km, Table-Based)

Geopotential Altitude h (km):

Property	Symbol	Value

1.2 ISA Deviation

ISA Deviation

It is essential to present performance data at temperatures other than the ISA temperature for all flight levels within the performance-spectrum envelope. If this were to be attempted for the actual or forecast temperatures, it would usually be impracticable and in some instances impossible.

To overcome the presentation difficulty and retain the coverage or range required, it is necessary to use ISA deviation. This is simply the algebraic difference between the actual (or forecast) temperature and the ISA temperature for the flight level under consideration. It is calculated by subtracting the ISA temperature from the actual (or forecast) temperature for that particular altitude.

ISA Deviation = Ambient temperature − Standard Temperature

JSA Deviation

As an alternative to ISA deviation some aircraft manuals use the Jet Standard Atmosphere (JSA) Deviation that assumes a temperature lapse rate of 2◦/1000 ft and that the atmosphere has no tropopause, the temperature is, therefore, assumed to continue decreasing at this rate beyond 36 090 ft.

Height and Altitude

Three parameters are used for vertical referencing of position in aviation. They are the airfield surface level, mean sea level (MSL) and the standard pressure level of 1013.2 hPa. It would be convenient if the performance data could be related to the aerodrome elevation because this is fixed and published in the Aeronautical Information Publication. However, this is impractical because of the vast range that would have to be covered. Mean sea level and pressure altitude are the only permissible references for assessing altitude for the purposes of aircraft performance calculations, provided that the one selected by the manufacturers for the Flight Manual is used consistently throughout the manual.

Pressure Altitude

In Aeroplane Flight Manuals (AFMs) the word altitude refers strictly to pressure altitude, which can be defined as the vertical distance from the 1013.2 hPa pressure level. Therefore, aerodrome and obstacle elevations must be converted to pressure altitude before they can be used in performance graphs. Many large aerodromes provide the aerodrome pressure altitude as part of their hourly weather reports.

A/F Pressure Altitude = Aerodrome elevation in ft + [(1013.2 hPa − QNH) × 27 ft] Aerodrome Pressure Altitude = (1013.2 hPa − QFE) × 27 ft

To correct an altitude for the temperature errors of the altimeter use the following formula:

Altitude Correction = 4 × ISA Deviation × Indicated Altitude ÷ 1000

Density Altitude

The performance data for small piston/propeller-driven aeroplanes is calculated using density altitude, which is pressure altitude corrected for nonstandard temperature. It is the altitude in the standard atmosphere at which the prevailing density occurs and can be calculated by using the formula:

Density Altitude = Pressure Altitude + (118.8 × ISA Deviation)

Integrated ISA, Pressure Altitude & Density Altitude Calculator

Geopotential Altitude (km):
Ambient Pressure (Pa):
Outside Air Temperature (°C):

Quantity	Symbol	Value

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