Comparison International Standard Atmosphere space diving

The International Standard Atmosphere (ISA) is a static atmospheric model of how the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

, and

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

of the

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

change over a wide range of

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

s or

elevation The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...

s. It has been established to provide a common reference for temperature and pressure and consists of tables of values at various altitudes, plus some formulas by which those values were derived. The

International Organization for Standardization The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...

(ISO) publishes the ISA as an

international standard An international standard is a technical standard developed by one or more international standards organizations. International standards are available for consideration and use worldwide. The most prominent such organization is the International O ...

, ISO 2533:1975.

,
Standard Atmosphere
', ISO 2533:1975, 1975. Other

standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...

s, such as the

International Civil Aviation Organization The International Civil Aviation Organization (ICAO ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sch ...

(ICAO) and the

United States Government The Federal Government of the United States of America (U.S. federal government or U.S. government) is the Federation#Federal governments, national government of the United States. The U.S. federal government is composed of three distinct ...

, publish extensions or subsets of the same atmospheric model under their own standards-making authority.

Description

The ISA

mathematical model A mathematical model is an abstract and concrete, abstract description of a concrete system using mathematics, mathematical concepts and language of mathematics, language. The process of developing a mathematical model is termed ''mathematical m ...

divides the atmosphere into layers with an assumed linear distribution of

absolute temperature Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expres ...

''T'' against geopotential altitude ''h''.Gyatt, Graham (2006-01-14)
"The Standard Atmosphere"
. A mathematical model of the 1976 U.S. Standard Atmosphere. The other two values (pressure ''P'' and density ''ρ'') are computed by simultaneously solving the equations resulting from: * the vertical pressure gradient resulting from

hydrostatic balance In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...

, which relates the rate of change of pressure with geopotential altitude: ::

\frac = - \rho g

, and * the

ideal gas law The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stat ...

in molar form, which relates

, and

: ::

\ P = \rho R_T

at each geopotential altitude, where ''g'' is the standard acceleration of gravity, and ''R''_specific is the

specific gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment pe ...

for dry air (287.0528J⋅kg⁻¹⋅K⁻¹). The solution is given by the

barometric formula The barometric formula is a formula used to model how the air pressure (or air density) changes with altitude. Pressure equations There are two equations for computing pressure as a function of height. The first equation is applicable to the ...

. Air density must be calculated in order to solve for the pressure, and is used in calculating

dynamic pressure In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5 :q = \frac\rho\, u^2 where (in SI units): * is the dynamic pressure in pascals ...

for moving vehicles.

Dynamic viscosity Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

is an empirical function of temperature, and

kinematic viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

is calculated by dividing dynamic viscosity by the density. Thus the standard consists of a tabulation of values at various altitudes, plus some formulas by which those values were derived. To accommodate the lowest points on Earth, the model starts at a base geopotential altitude of ''below sea level'', with standard temperature set at 19 °C. With a temperature

lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'' (in its "becoming less" sense, not its "interruption" sense). In dry air, ...

of −6.5 °C (-11.7 °F) per km (roughly −2 °C (-3.6 °F) per 1,000 ft), the table interpolates to the standard mean sea level values of temperature, (1 atm) pressure, and a density of . The

tropospheric The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the planetary s ...

tabulation continues to , where the temperature has fallen to , the pressure to , and the density to . Between 11 km and 20 km, the temperature remains constant.Batchelor, G. K., ''An Introduction to Fluid Dynamics'', Cambridge Univ. Press, 1967. :

given per kilometer of ''geopotential altitude (A positive lapse rate (λ > 0) means temperature decreases with height)'' In the above table, '' geopotential altitude'' is calculated from a mathematical model that adjusts the altitude to include the variation of gravity with height, while ''geometric altitude'' is the standard direct vertical distance above

mean sea level A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

(MSL). * The equation that relates the two altitudes are (where z is the ''geometric altitude'', h is the '' geopotential altitude'', and r₀ = 6,356,766 m in this model): ::

\ z = \frac

Note that the Lapse Rates cited in the table are given as °C per kilometer of geopotential altitude, not geometric altitude. The ISA model is based on average conditions at mid latitudes, as determined by the ISO's TC 20/SC 6 technical committee. It has been revised from time to time since the middle of the 20th century.

Use at non-standard day conditions

The ISA models a hypothetical standard day to allow a reproducible engineering reference for calculation and testing of engine and vehicle performance at various altitudes. It ''does not'' provide a rigorous meteorological model of actual atmospheric conditions (for example, changes in barometric pressure due to wind conditions). Neither does it account for

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...

effects; air is assumed to be dry and clean and of constant composition. Humidity effects are accounted for in vehicle or engine analysis by adding water vapor to the

thermodynamic state In thermodynamics, a thermodynamic state of a system is its condition at a specific time; that is, fully identified by values of a suitable set of parameters known as state variables, state parameters or thermodynamic variables. Once such a set ...

of the air after obtaining the pressure and density from the standard atmosphere model. Non-standard (hot or cold) days are modeled by adding a specified temperature delta to the standard temperature at altitude, but pressure is taken as the standard day value. Density and viscosity are recalculated at the resultant temperature and pressure using the ideal gas equation of state. Hot day, Cold day, Tropical, and Polar temperature profiles with altitude have been defined for use as performance references, such as

United States Department of Defense The United States Department of Defense (DoD, USDOD, or DOD) is an United States federal executive departments, executive department of the federal government of the United States, U.S. federal government charged with coordinating and superv ...

MIL-STD-210C, and its successor MIL-HDBK-310.

ICAO Standard Atmosphere

The International Civil Aviation Organization (ICAO) published their "ICAO Standard Atmosphere" as Doc 7488-CD in 1993. It has the same model as the ISA, but extends the altitude coverage to 80 kilometers (262,500 feet). The ICAO Standard Atmosphere, like the ISA, does not contain water vapor. Some of the values defined by ICAO are: Aviation standards and flying rules are based on the International Standard Atmosphere.

Airspeed indicator The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometres per hour (km/h), knots (kn or kt), miles per hour (MPH) and/or metres per second (m/s). The recommendation by ICAO is to u ...

s are calibrated on the assumption that they are operating at sea level in the International Standard Atmosphere where the air density is 1.225 kg/m³. Physical properties of the ICAO Standard Atmosphere are:

Other standard atmospheres

The U.S. Standard Atmosphere is a set of models that define values for atmospheric temperature, density, pressure and other properties over a wide range of altitudes. The first model, based on an existing international standard, was published in 1958 by the U.S. Committee on Extension to the Standard Atmosphere,U.S. Extension to the ICAO Standard Atmosphere, U.S. Government Printing Office, Washington, D.C., 1958 and was updated in 1962,U.S. Standard Atmosphere, 1962, U.S. Government Printing Office, Washington, D.C., 1962 1966,U.S. Standard Atmosphere Supplements, 1966, U.S. Government Printing Office, Washington, D.C., 1966 and 1976.U.S. Standard Atmosphere
1976, U.S. Government Printing Office, Washington, D.C., 1976 (Linked file is 17 MB) The U.S. Standard Atmosphere, International Standard Atmosphere and WMO (World Meteorological Organization) standard atmospheres are the same as the ISO International Standard Atmosphere for altitudes up to 32 km.

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

"U.S. Standard Atmosphere 1976"

NRLMSISE-00 NRLMSISE-00 is an empirical, global reference atmospheric model of the Earth from ground to space. It models the temperatures and densities of the atmosphere's components. A primary use of this model is to aid predictions of satellite orbital de ...

is a newer model of the Earth's atmosphere from ground to space, developed by the

US Naval Research Laboratory The United States Naval Research Laboratory (NRL) is the corporate research laboratory for the United States Navy and the United States Marine Corps. Located in Washington, DC, it was founded in 1923 and conducts basic scientific research, appl ...

taking actual satellite drag data into account. A primary use of this model is to aid predictions of

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

orbital decay due to

atmospheric drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or b ...

. The COSPAR International Reference Atmosphere (CIRA) 2012 and the ISO 14222 Earth Atmosphere Density standard both recommend NRLMSISE-00 for composition uses. JB2008 is a newer model of the Earth's atmosphere from 120 km to 2000 km, developed by the US Air Force Space Command and Space Environment Technologies taking into account realistic

solar irradiance Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ( ...

s and time evolution of geomagnetic storms. It is most useful for calculating satellite orbital decay due to

. Both CIRA 2012 and ISO 14222 recommend JB2008 for mass density in drag uses.

References

*
NASA JPL Reference Notes
* ICAO, ''Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet))'', Doc 7488-CD, Third Edition, 1993, .

External links