Standard Atmosphere API

International Standard Atmosphere (ISA) maths as an API, computed locally and deterministically. The properties endpoint gives the air temperature, pressure, density and speed of sound at any altitude from sea level to 20 km — using the standard troposphere lapse rate (T = T0 − 0.0065·h) and the isothermal lower stratosphere above 11 km — along with the density, pressure and temperature ratios relative to sea level. The density-altitude endpoint computes the density altitude — the ISA altitude with the same air density — from a pressure altitude and the actual outside-air temperature, the figure pilots use because heat and low pressure rob an aircraft of lift, engine power and propeller thrust; it also reports the ISA temperature deviation. The pressure-altitude endpoint turns a barometric reading (in hectopascals or pascals) into the pressure altitude, the ISA altitude at which the standard pressure equals your reading. Altitudes accept metres or feet, temperature °C or kelvin. Everything is computed locally and deterministically, so it is instant and private. Ideal for aviation, drone, ballooning, HVAC and meteorology app developers, flight-planning and performance tools, and physics education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is the ISA atmospheric model; for the acoustic and relativistic Doppler effect use a Doppler API.