Atmosphere¶
International Standard Atmosphere (ISA) model and airspeed conversions.
Provides temperature, pressure, density, and speed of sound as functions of geometric altitude, plus conversions between calibrated airspeed (CAS), true airspeed (TAS), and Mach number using compressible flow equations.
Used internally by aircraft speed schedules and wind field pressure-level conversions.
International Standard Atmosphere (ISA) model and airspeed conversions.
Provides the 1976 US Standard Atmosphere (identical to ICAO ISA below 32 km) and compressible-flow airspeed conversions accurate through FL510+.
All public functions accept and return pint.Quantity objects.
Altitude convention¶
The ISA temperature and pressure profiles in this module are defined on
geopotential altitude H, not geometric altitude z. The barotropic
relation P(H) = P₀·(T(H)/T₀)^(-g₀/(L·R)) follows from hydrostatic
equilibrium under the standard-gravity assumption g(H) ≡ g₀, which
holds by definition for geopotential altitude. In aviation practice
pressure altitude — what altimeters report when set to 29.92 inHg /
1013.25 hPa — is operationally equivalent to geopotential altitude, so
mission-planning altitudes (FL, indicated MSL above the transition
altitude) can be passed directly.
If you have a geometric altitude z (e.g. from GPS / WGS-84), convert
first via H = R_E · z / (R_E + z) with R_E ≈ 6 356 766 m (the
ICAO/US-1976 reference value). At z = 15.5 km (≈ FL510) the
geopotential correction is roughly -0.24 % in altitude, which
propagates to about -0.5 % in pressure — small but non-negligible
for high-altitude work that mixes GPS altitudes with pressure-altitude
formulas.
- temperature_at(altitude)[source]¶
ISA static temperature at altitude.
Covers the troposphere (lapse rate −6.5 K/km) and the lower stratosphere (isothermal at 216.65 K above 11 km).
- cas_to_tas(cas, altitude)[source]¶
Convert calibrated airspeed to true airspeed at altitude (ISA).
Uses the compressible (isentropic) form, not the incompressible approximation. Accurate for subsonic flight through FL510+.
- tas_to_cas(tas, altitude)[source]¶
Convert true airspeed to calibrated airspeed at altitude (ISA).
Inverse of
cas_to_tas().